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This paper discusses different feature selection methods and CO2 flux data sets with a varying quality‐quantity balance for the application of a Random Forest model to predict daily CO2 fluxes at 250 m spatial resolution for the Rur catchment area in western Germany between 2010 and 2018. Measurements from eddy covariance stations of different ecosystem types, remotely sensed vegetation data from MODIS, and COSMO‐REA6 reanalysis data were used to train the model and predictions were validated by a spatial and temporal validation scheme. Results show the capabilities of a backwards feature elimination to remove irrelevant variables and an importance of high‐quality‐low‐quantity flux data set to improve predictions. However, results also show that spatial prediction is more difficult than temporal prediction by reflecting the mean value accurately though underestimating the variance of CO2 fluxes. Vegetated parts of the catchment acted as a CO2 sink during the investigation period, net capturing about 237 g C m−2 y−1. Croplands, coniferous forests, deciduous forests and grasslands were all sinks on average. The highest uptake was predicted to occur in late spring and early summer, while the catchment was a CO2 source in fall and winter. In conclusion, the Random Forest model predicted a narrower distribution of CO2 fluxes, though our methodological improvements look promising in order to achieve high‐resolution net ecosystem exchange data sets at the regional scale. Plain Language Summary: Whether ecosystems absorb or emit CO2 plays a major role in the global carbon cycle and impacts climate change. This exchange is already measured at scattered stations, but creating spatially resolved data sets remains a challenge. In this paper, we used satellite images of vegetation and meteorological data to predict the CO2 exchange of the Rur catchment area near the German‐Dutch‐Belgian border for every day from 2010 to 2018. In order to assess the prediction quality, we compared actual measurements from several stations within the catchment with the predictions at the locations of these stations. Results show that our method could increase prediction quality compared to previous process‐based models, though the error remains rather high. Vegetated parts of the catchment including coniferous forests, deciduous forests, grasslands, and croplands were all CO2 sinks on average. In late spring and early summer, they were the strongest sink, but in fall and winter a CO2 source. Key Points: CO2 flux upscaling with Random Forest can be improved with a backward feature elimination and strict quality control of flux dataVegetated parts of the Rur catchment were predicted to be a CO2 sink on average, with the highest uptake in late spring and early summerThe Enhanced Vegetation Index and potential evapotranspiration are useful predictors for the regionalization of CO2 flux measurements [ABSTRACT FROM AUTHOR]

Sunlight can oxidize dissolved organic carbon (DOC) to dissolved inorganic carbon (DIC) in freshwaters. The importance of complete photooxidation, or photomineralization, as a sink for DOC remains unclear in temperate rivers, as most estimates are restricted to lakes, high latitude rivers, and coastal river plumes. In this study, we construct a model representing over 75,000 river reaches in the Connecticut River Watershed (CRW), USA, to calculate spectrally resolved photomineralization. We test the hypothesis that photomineralization is a negligible DOC sink across all reaches and flow conditions relative to DOC fluxes. Our model quantifies reaction rates and transport drivers within the river reaches for the ranges of flow conditions, incoming solar irradiance, and canopy cover shading observed throughout the year. Our model predicts average daily areal photomineralization rates ranging from 1.16 mg‐C m−2 day−1 in low flow river reaches in the winter, to 18.33 mg‐C m−2 day−1 in high flow river reaches during the summer. Even for high photomineralization fluxes, corresponding photomineralization uptake velocities are typically at least an order of magnitude smaller than those reported for other instream processes. We calculate DOC elimination by photomineralization relative to DOC fluxes through individual stream reaches as well as the entire riverine portion of the CRW. We find that relative photomineralization fluxes are highest in summer drought conditions in low order streams. In median flows and mean light intensities, for an average watershed travel distance, 3%–5% of the DOC fluxes are eliminated, indicating that photomineralization is a minor DOC sink in temperate rivers. Plain Language Summary: Rivers are an important part of the carbon cycle, moving carbon compounds from land to the ocean. Within rivers, dissolved organic carbon molecules can be broken down into inorganic carbon molecules, including the greenhouse gas carbon dioxide. Sunlight shining into rivers can cause these organic molecules to break down in a process called photomineralization, but it is not clear if this process is important compared to the total amount of organic carbon that travels through rivers every day. In this paper, we build a model for the river sections of temperate Connecticut River Watershed, which calculates photomineralization for possible river flow conditions, dissolved organic carbon concentrations, and seasons, and compares the size of the sunlight‐driven breakdown of dissolved organic carbon to the amount of dissolved organic carbon in the river. This is the first model that puts photomineralization rates in the context of a flowing temperate river network. We show that compared with the dissolved organic carbon amounts present in the river at any time of the year or any flow conditions, photomineralization is essentially an unimportant process, removing on average 3%–5% of the dissolved organic carbon through an average watershed river route. Key Points: We build a model calculating reaction and transport drivers for photomineralization in 75,000 temperate river reachesPhotomineralization is a negligible dissolved organic carbon (DOC) sink relative to DOC fluxesDOC elimination by photomineralization in temperate rivers is limited by short water residence times and canopy cover [ABSTRACT FROM AUTHOR]

Headwater stream networks contribute substantially to the global carbon dioxide terrestrial flux because of high turbulence and coupling with terrestrial environments. Heterogeneity within headwater stream networks, both spatially and temporally, makes measuring and upscaling these emissions challenging because measurements of carbon dioxide in streams are often limited to a few monitoring points. We modified a stream network model to reflect real measurements made under base flow and high flow conditions at Martha Creek in Stabler, WA in the US Pacific Northwest. We found that under high flow conditions, the stream network had much greater total carbon emissions than during low flow conditions (1.22 Mg C day−1 vs. 0.034 Mg C day−1). We attribute this increase to a larger overall stream network area (0.04 vs. 0.01 km2) and discharge (1.9 m3 s−1 vs. 0.005 m3 s−1) in November versus August. Our results demonstrate the need to understand the nonperennial stream reaches when calculating carbon emissions. We compared the stream network emissions with the terrestrial net ecosystem exchange (NEE) estimated by local eddy covariance measurements per watershed area (−5.5 Mg C day−1 in August and −2.2 Mg C day−1 in November). Daily stream emissions in November accounted for a much larger percentage of NEE than in August (54% vs. 0.62%). We concluded that the stream network can emit a large percentage of the forest NEE in the winter months, and annual estimates of stream network emissions must consider the flow regime throughout the year. Plain Language Summary: Streams contribute to the global carbon cycle because they can release carbon dioxide to the atmosphere. It can be difficult to measure these emissions because the concentration of carbon dioxide will change along a stream length and from season to season. We modeled carbon dioxide emissions from a stream during August in base flow and during November in high flow. We found that emissions increased during November with an increased stream network size and stream discharge and that this emission can be a large fraction of whole watershed net carbon emission. We determined that seasonality is an important indicator of carbon emissions in headwater streams. Key Points: Seasonality can drive variation in carbon emissions from headwater stream area with changes in flow and stream network areaStream networks can emit a large percentage of the terrestrial net ecosystem exchange, especially under high flow conditions [ABSTRACT FROM AUTHOR]

Inland waters are hotspots of greenhouse gas (GHG) cycling, with small water bodies particularly active in the production and consumption of carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O). However, wetland ponds are being replaced rapidly by small constructed reservoirs in agricultural regions, yet it is unclear whether these two water body types exhibit similar physical, chemical, and environmental controls of GHG content and fluxes. Here, we compared the content and regulatory mechanisms of all three major GHGs in 20 pairs of natural wetland ponds and constructed reservoirs in Canada's largest agricultural region. Carbon dioxide content was associated primarily with metabolic indicators in both water body types; however, primary production was paramount in reservoirs, and heterotrophic metabolism a stronger correlate in wetland ponds. Methane concentrations were correlated positively with eutrophication of the reservoirs alone, while competitive inhibition by sulfur‐reducing bacteria may have limited CH4 in both waterbody types. Contrary to expectations, N2O was undersaturated in both water body types, with wetlands being a significantly stronger and more widespread N2O sink. Varying regulatory processes are attributed to differences in age, depth, morphology, and water‐column circulation between water body types. These results suggest that natural and constructed water bodies should be modeled separately in regional GHG budgets. Plain Language Summary: Small inland water bodies, ponds and reservoirs, are hotspots of greenhouse gas (GHG) production. However, modern farming practices replace natural wetlands by artificial reservoirs, without consideration on the effects on landscape GHG fluxes. Here we measured concentrations and controls of carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) in locally‐paired wetland ponds and agricultural reservoirs and found that while GHG concentrations were similar in the two waterbody types, underlying mechanisms differed, owing to the differences in shape, depth, and age of individual water bodies. Shallower wetland ponds were typically more fully mixed, exhibited more oxygen at depth, and had higher nutrients and organic carbon concentrations reflecting their ages. Findings suggest that the balance between wetland drainage and reservoir construction may have a pronounced effect on regional budgets of GHG fluxes, particularly in the face of forthcoming climate changes. Key Points: CO2 content was associated with metabolism; primary production in reservoirs, but bacterial respiration in wetlandsCH4 concentrations were correlated positively with either eutrophication (reservoirs) or competitive inhibition of methanogenesis (both)N2O undersaturation was ubiquitous, but wetlands were stronger and more widespread N2O sinks [ABSTRACT FROM AUTHOR]

To better understand the depositional constraints on the fingerprint characteristics of methane isotopes, we present a set of carbon/hydrogen isotopic data for CH4, CO2, pore water, carbonate, and total organic carbon along a 70‐m sedimentary core. The sedimentary facies (Units I, II, and III from upper to bottom) suggested depositional environments of the present estuary, shallow marine, and floodplain‐estuary. Calculations suggested that around 86% of methane was produced through the CO2 reduction pathway. In this pathway, the hydrogen in CH4 is from ambient water, while the carbon is from dissolved inorganic carbon. Correspondingly, our study showed that the values of δDCH4 displayed similar trends with those of δDH2O and Cl− concentrations along the depth profiles. The low δDCH4 below 44.5 m corresponded to low δDH2O and low salinity during the cold and low‐sea‐level period. The values of δ13CCH4 generally synchronously changed with those of δ13CCO2. The variation trends of δ13CCH4 and δ13CCO2 were the same with δ13Ccarbonate from 10 to 70 m depth but decoupled above 10 m. The values of δ13CCH4 in Units II and III were correlated with the δ13Ccarbonates, which is related to the sedimentary processes. But decoupling of low values of δ13CCH4 and δ13CCO2 from δ13Ccarbonates in Unit I may be related to preferential microbial consumption of labile compounds with light carbon isotopic compositions, such as lipids. In short, the variations of δ13CCH4 and δDCH4 were largely related to the sedimentary process, which controlled the isotopic compositions of both water and carbonate. Plain Language Summary: As an important greenhouse gas, methane (CH4) contributes more to global warming than the same mole's carbon dioxide (CO2). The methane isotopic composition is widely used to trace sources. Coastal sediments, recording the rise and stabilization of the global sea level since the Last Glacial Maximum, host enormous volumes of CH4. Of interest is whether the depositional process could impact methane isotopic signatures, adding uncertainty to the traceability. We conducted analyses including C/H isotopic data of CH4, CO2, porewater, carbonates, and total organic carbon along a 70‐m sedimentary core from Hangzhou Bay, China. Calculations suggested that about 86% of methane was produced through the CO2 reduction pathway. In this pathway, the hydrogen in CH4 is derived from ambient water, while the carbon is from CO2, which was isotopically in equilibrium with carbonate. Our results suggested that sedimentary processes are the primary abiotic factors affecting the variations of the stable carbon and hydrogen isotopic compositions of biogenic CH4 from coastal sediment. Key Points: Higher than 80% of biogenic methane along the sediment core was produced through the CO2 reduction pathwayC and H of biogenic CH4 from ambient CO2 and H2 are isotopically in equilibrium with ambient carbonate and water, respectivelyThe sedimentary process is one of the primary abiotic factors affecting the variations of δ13CCH4 and δDCH4 from coastal sediment [ABSTRACT FROM AUTHOR]

Emergent constraints on carbon cycle feedbacks in response to warming and increasing atmospheric CO2 concentration have previously been identified in Earth system models participating in the Coupled Model Intercomparison Project (CMIP) Phase 5. Here, we examine whether two of these emergent constraints also hold for CMIP6. The spread of the sensitivity of tropical land carbon uptake to tropical warming in an idealized simulation with a 1% per year increase of atmospheric CO2 shows only a slight decrease in CMIP6 (−52 ± 35 GtC/K) compared to CMIP5 (−49 ± 40 GtC/K). For both model generations, the observed interannual variability in the growth rate of atmospheric CO2 yields a consistent emergent constraint on the sensitivity of tropical land carbon uptake with a constrained range of −37 ± 14 GtC/K for the combined ensemble (i.e., a reduction of ∼30% in the best estimate and 60% in the uncertainty range relative to the multimodel mean of the combined ensemble). A further emergent constraint is based on a relationship between CO2 fertilization and the historical increase in the CO2 seasonal cycle amplitude in high latitudes. However, this emergent constraint is not evident in CMIP6. This is in part because the historical increase in the amplitude of the CO2 seasonal cycle is more accurately simulated in CMIP6, such that the models are all now close to the observational constraint. Plain Language Summary: The statistical model of so‐called emergent constraints help to better understand the sensitivity of Earth system processes in a changing climate. Here, we analyze the robustness of two previously found emergent constraints on carbon cycle feedbacks, using models from the Coupled Model Intercomparison Project (CMIP) of Phases 5 and 6. First the decrease of carbon storage in the tropics due to increasing near‐surface air temperatures, which is found to be robust on the choise of model ensemble. Giving a constraint estimate of −52 ± 35 GtC/K for CMIP6 models, being within the range of uncertainty for the previously estimated result for CMIP5. Second, the increase of carbon storage in high latitudes due to CO2 fertilization effect, which is found to be not evident among CMIP6 models. This is in part because the historical increase in the amplitude of the CO2 seasonal cycle is more accurately simulated in CMIP6, such that the models are all now close to the observational constraint. Key Points: An emergent constraint on the sensitivity of tropical land carbon to global warming, originally derived from Coupled Model Intercomparison Project Phase 5 (CMIP5), also holds for CMIP6The combined CMIP5 + CMIP6 ensemble gives an emergent constraint on the sensitivity of tropical land carbon to global warming of −37 ± 14 GtC/KAn emergent constraint on the fertilization feedback due to rising CO2 levels, previously derived, is not evident in CMIP6 [ABSTRACT FROM AUTHOR]

Freshwater ecosystems are globally significant sources of greenhouse gases (GHGs) to the atmosphere. Previous work has indicated that GHG flux in headwater streams is dominated by terrestrially derived gases, with in situ production limited by short organic matter residence times and high dissolved oxygen concentrations due to turbulent reaeration. However, low‐gradient headwater streams that contain pool structures with longer residence times may be conducive to the in situ production of GHG. These streams, and the longitudinal heterogeneity therein, are seldom studied. We measured continuous ecosystem metabolism alongside concentrations and fluxes of carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) in a low‐gradient third order stream in the North Carolina Piedmont. From autumn to the following spring, we characterized spatial and temporal patterns of GHG along an 8 km segment in the context of channel geomorphology, hydrology, and ecosystem metabolic rates using linear mixed effects models. We found that stream metabolism was responsible for most of the CO2 flux over this period, and that in‐channel aerobic metabolism was a primary predictor of both CH4 and N2O fluxes as well. Long water residence times, limited reaeration, and substantial organic matter from terrestrial inputs foster conditions conducive to the in‐stream accumulation of CO2 and CH4 from microbial respiration. Streams like this one are common in landscapes with low topographic relief, making it likely that the high contribution of instream metabolism to GHG fluxes that we observed is a widespread yet understudied behavior of many small streams. Plain Language Summary: Stream ecosystems play a role in producing greenhouse gases (GHGs) and transporting them from groundwater to be released into the atmosphere. Some of these gases are produced through the breakdown of organic matter by microbes in the stream. We don't know how important this microbial production is compared to gases coming from soil and groundwater, but comparing it to rates of ecosystem metabolism may help us learn about it. We measured GHG and metabolism along a stream and found that metabolism is directly responsible for the production of carbon dioxide and is a good predictor of methane, meaning that microbial production in the stream is likely important. N2O production was limited by competition between microbes for nitrogen, as a result, the stream was removing N2O from the atmosphere rather than releasing it. Key Points: Patterns in carbon dioxide are almost entirely accounted for by instream metabolism except during a storm eventMethane is highly variable in space and covaries with aerobic respirationThe stream shifts from a source of nitrous oxide to a sink during peak autumn respiration, likely due to nitrogen limitation [ABSTRACT FROM AUTHOR]

The eddy covariance method is widely used to investigate fluxes of energy, water, and carbon dioxide at landscape scales, providing important information on how ecological systems function. Flux measurements quantify ecosystem responses to environmental perturbations and management strategies, including nature‐based climate‐change mitigation measures. However, due to the high cost of conventional instrumentation, most eddy covariance studies employ a single system, limiting spatial representation to the flux footprint. Insufficient replication may be limiting our understanding of ecosystem behavior. To address this limitation, we deployed eight lower‐cost eddy covariance systems in two clusters around two conventional eddy covariance systems in the Chihuahuan Desert of North America for a period of 2 years. These dryland settings characterized by large temperature variations and relatively low carbon dioxide fluxes represented a challenging setting for eddy covariance. We found very good closure of energy and water balance across all systems (within ±9% of unity). We found very good correspondence between the lower‐cost and conventional systems' fluxes of sensible heat (with concordance correlation coefficient (CCC) of ≥0.87), latent energy (evapotranspiration; CCC ≥ 0.89), and useful correspondence in the net ecosystem exchange ((NEE); with CCC ≥ 0.4) at the daily temporal resolution. Relative to the conventional systems, the low‐frequency systems were characterized by a higher level of random error, particularly in the NEE fluxes. Lower‐cost systems can enable wider deployment affording better replication and sampling of spatiotemporal variability at the expense of greater measurement noise that might be limiting for certain applications. Replicated eddy covariance observations may be useful when addressing gaps in the existing monitoring of critical and underrepresented ecosystems and for measuring areas larger than a single flux footprint. Plain Language Summary: Vegetation around the world is critical to how our planet functions. Plants regulate how sunlight, rain, and even carbon dioxide are used and recycled. In order to learn about plant responses to rain, sunlight, temperature, and climate change, scientists measure the exchanges of water, energy, and carbon between the atmosphere and the land surface. However, the instruments used for these measurements are expensive, so they are only installed in a small number of locations around the entire planet. This is a problem for two reasons: First, these instruments are more common in more wealthy countries giving a distorted view of the world. Second, often only one instrument is installed, meaning we must assume that the area measured is exactly the same as the entire landscape, when this is often not true. To solve this problem, we built a much cheaper version of the instruments, which costs just one‐quarter of the usual price. To test our new instruments, we installed them next to the conventional instruments in two locations. We found strong agreement between our cheaper instruments and the high‐cost ones. This is good because cheaper instruments could be used more widely to improve the measurement of important ecosystems. Key Points: Lower‐cost, low‐frequency eddy covariance (EC) systems yield evapotranspiration and CO2 fluxes that correspond strongly with high‐frequency EC systemsLower‐cost EC systems enable increased spatial sampling and replication, improving confidence in retrieved evapotranspiration and CO2 fluxesLower‐cost EC systems may therefore address the undersampling and spatial biases in existing EC studies and networks [ABSTRACT FROM AUTHOR]

First‐order streams flowing through peatlands receive, carry and transform large amounts of organic carbon, methane (CH4) and carbon dioxide (CO2) but remain poorly documented. The objectives of this study were to (a) identify the origins of CO2 and CH4 (thereafter C‐GHG for carbon greenhouse gases) in a peatland headwater stream, (b) determine the environmental factors driving C‐GHG export and emissions, and (c) quantify C‐GHG losses from this system and discuss its implications. Data were collected from eight sampling sites along a 3 km boreal peatland headwater stream (Eastern Canada) over the growing seasons 2019 and 2020. The studied stream was oversaturated in pCO2 [min: 2,044; max: 23,306 μatm] and pCH4 [14; 17,614 μatm]. A mass balance model showed that ∼81% of in‐stream CO2 originated from porewater seepage while the remaining 17% and 2% originated from in‐stream productivity and methane oxidation, respectively. Porewater seepage was concluded to be the primary source of CH4. Seasonal dissolved C‐GHG concentrations were negatively correlated with the peatland water table depth, suggesting an active release of carbon‐rich peat porewater during the base flow. Nevertheless, greater C‐GHG losses occurred during stormflow periods which acted as pulses with most of the C‐GHG being shunted downstream. The sum of C‐GHG export and emissions at our site was 8.08 gC m−2 y−1 with 86% being released to the atmosphere and 14% being exported downstream. Our study demonstrates that peatland headwater streams act as large sources of C‐GHG and that precipitation events and topography control the magnitude of the fluxes. Plain Language Summary: Headwater streams are the inceptive fragment of the river network where a substantial quantity of organic carbon is released from the surrounding substrate. This terrestrial organic carbon can also be transformed into greenhouse gases (GHGs) such as methane and carbon dioxide. In the stream, those GHG can be either consumed, emitted to the atmosphere, or exported downstream. However, the respective contribution of those pathways is unknown. We sampled a stream draining a rain‐fed peatland in Eastern Canada to describe and quantify its carbon GHG dynamics. We found higher concentrations in this stream than in most headwater streams but the total carbon emissions and export were comparable. Peatland streams typically have a low velocity and gentle slope which minimizes atmospheric gas release. We also found that concentrations were greater during the low flow period, potentially because carbon‐rich porewater was released from the peat. However, greater export was reported during the few days of intense rain. The significance of this work is that GHG in headwater streams are spatially and temporally variable and controlled by the surrounding ecosystems, the intensity of rain events and the stream morphology. The inclusion of this variability will improve regional and global carbon budget estimates. Key Points: Porewater seepage accounted for 81% of the carbon dioxide (CO2) present in the stream, in‐stream production and methane (CH4) oxidation contributed to 17% and 2%86.5% of the dissolved CO2 and CH4 was emitted to the atmosphere rather than exported downstream due to gases oversaturation and slow flow46% of the CO2 and CH4 exported downstream occurred during the 10% period when water discharge rates were maximum, suggesting a pulse effect [ABSTRACT FROM AUTHOR]

Lakes evade significant amounts of carbon dioxide (CO2) to the atmosphere; yet the magnitude and origin of the evasion are still poorly constrained. We quantified annual CO2 evasion and its origin (in‐lake net ecosystem production vs. lateral inputs from terrestrial ecosystems) in 14 high‐latitude lakes through high‐frequency estimates of open water CO2 flux and ecosystem metabolism and inorganic carbon mass‐balance before and after ice breakup. Annual CO2 evasion ranged from 1 to 25 g C m−2 yr−1 of which an average of 57% was evaded over a short period at ice‐breakup. Annual internal CO2 production ranged from −6 to 21 g C m−2 yr−1, of which at least half was produced over winter. The contribution of internal versus external source contribution to annual CO2 evasion varied between lakes, ranging from fully internal to fully external with most lakes having over 75% of the evasion sustained through a single source. Overall, the study stresses the large variability in magnitude and control of CO2 evasion and suggests that environmental change impacts on CO2 evasion from high‐latitude lakes are not uniform. Plain Language Summary: Lakes release significant amounts of carbon dioxide (CO2) to the atmosphere. This CO2 evasion is mainly sustained by carbon from land, either via external inputs of CO2 rich water or by internal breakdown of imported organic carbon to CO2. The extent of CO2 emission and the contribution of these two sources are poorly understood in regard to the numerous lakes at high latitudes. We studied 14 high‐latitude lakes and found that all lakes emitted CO2 on an annual basis and that 0%–100% (average 44%) of this CO2 came from breakdown of organic carbon in the lakes. The spring ice‐breakup period contributed, on average, 57% of the annual emitted CO2 despite covering only 16% of the open water season duration. Our study defines the fundamental role of lakes on the landscapes as "reactors" or "chimneys" for carbon processing and emission to the atmosphere, and how these roles change over the course of the year. Key Points: Carbon dioxide evasion during spring ice breakup dominates annual evasionCarbon dioxide evasion is sourced by both external input and internal lake metabolism with large variability between lakesSeasonally resolved estimates are needed in order to understand the role of high‐latitude lakes in landscape carbon budgets [ABSTRACT FROM AUTHOR]

Estuaries make an important contribution to the global greenhouse gas budget. Yet modeling predictions of carbon dioxide (CO2) and methane (CH4) emissions from estuaries remain highly uncertain due to both simplified assumptions about the underpinning hydrologic and biologic processes and inadequate data availability to uniquely define parameters related to CO2 and CH4 processes. This study presents a modeling framework to quantify the sensitivity and uncertainty of predicted CO2 and CH4 concentrations and emissions, which is demonstrated through application to a subtropical urban estuary (Brisbane River, Australia). A 3D hydrodynamic‐biogeochemical model was constructed, and calibrated using the model‐independent Parameter ESTimation software (PEST) with field data sets that captured strong gradients of CO2 and CH4 concentrations and emissions along the estuary. The approach refined uncertainty in the estimation of whole‐estuary annual emissions, and enabled us to assess the sensitivity and uncertainty of CO2 and CH4 dynamics. Estuarine CO2 concentrations were most sensitive to uncertainty in riverine inputs, whereas estuarine CH4 concentrations were most sensitive to sediment production and pelagic oxidation. Over the modeled year, variance in the daily fluctuations in carbon emissions from this case‐study spanned the full range of emission rates reported for estuaries around the world, highlighting that spatially or temporally limited sampling regimes could significantly bias estuarine greenhouse gas emission estimates. The combination of targeted field campaigns with the modeling approach presented in this study can help to improve carbon budgeting in estuaries, reduce uncertainty in emission estimates, and support management strategies to reduce or offset estuary greenhouse gas emissions. Plain Language Summary: Global estuaries are a major source of CO2 emission and play a disproportionate role in global carbon cycling relative to their area. Estuaries are also a source of CH4 that has approximately 34 times the global‐warming potential of CO2 over a 100‐year time period. However, large uncertainties remain in estimating the CO2 and CH4 emissions from estuaries due to limited observations to cover their large spatiotemporal variations, and lack of knowledge of their dynamics in response to the external inputs and internal biogeochemical reactions. In this study, we developed a modeling framework to address the sensitivity and uncertainty of CO2 and CH4 dynamics in the Brisbane River Estuary to both the riverine and wastewater inputs and biogeochemical reaction parameters. We showed that CO2 concentrations were most sensitive to catchment inputs and CH4 concentrations were most sensitive to the rate of sediment production and oxidation in the water. The estimated annual‐total CO2 equivalent emission from Brisbane River Estuary in 2016 was 3,514 ± 445 tonnes. The combination of targeted field campaigns with the modeling approach can help to improve carbon budgeting in estuaries, reduce uncertainty in emission estimates, and support management strategies to reduce or offset estuary greenhouse gas emissions. Key Points: Estuarine CO2 is most sensitive to external riverine CO2 inputsEstuarine CH4 is most sensitive to sediment production and oxidation in the waterThe modeling approach constrained the uncertainty in the estimation of CO2 and CH4 outgassing [ABSTRACT FROM AUTHOR]

Undisturbed peatlands are usually a sink of greenhouse gases (GHG), but turn to a GHG source when drained. While rewetting and restoration of drained peatlands play an important role in efforts to mitigate global warming, high uncertainties exist concerning the time period it takes after the start of rewetting until the point when the peatland becomes a GHG net sink. We investigated a site within the "Uchter Moor", NW Germany. It is an oligotrophic raised bog, which was never used for agriculture, but was drained in 1950 for peat mining. In 1999, restoration began; now the ecosystem is in transition back to a typical raised bog vegetation with Eriophorum vaginatum, Molinia caerulea and Eriophorum angustifolium being the dominant species. Sphagnum cuspidatum appears in patches. From June 2016 to January 2018, an eddy covariance (EC) tower measured the fluxes of carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O). The flux of ozone (O3) was also measured for 6 months by EC and modeled for the remaining experimental period. Using the sustained global warming potential for a 100‐year time horizon, the balance of CO2, CH4, and N2O in 2017 was +500 ± 120 g CO2‐equiv m−2 a−1. The plus sign indicates a net emission of GHG into the atmosphere. Methane contributed 78% to this flux. The inclusion of O3 into the balance yielded a somewhat smaller total flux of +430 ± 120 g CO2‐equiv m−2 a−1. Overall, the peatland is still a source of GHG, 18 years after re‐wetting. Plain Language Summary: Undisturbed peatlands usually have a long‐term cooling effect on global climate. If drained, peatlands turn from a sink to a source of greenhouse gases (GHG), which leads to a long‐term warming effect on climate. More recently, many drained peatlands have been re‐wetted. One of the goals is to bring back the peatland from a source to a sink of GHG, and thus mitigate the human‐made global warming. There is still uncertainty about how much time a peatland ecosystem requires in order to turn back to a GHG sink. We measured the exchange fluxes of GHG between peatland and the atmosphere in a restored raised bog in NW Germany, which was re‐wetted 18 years earlier. In the year 2017 the restored ecosystem was still a net source of long‐lived GHG (carbon dioxide, methane, nitrous oxide). When including the short‐lived GHG ozone, the total balance was still positive, but ozone adds a cooling effect. We hypothesize that it will take another few decades before the peatland is a net sink of these GHG. However, in direct comparison the GHG emissions are already lower than in comparable peatlands, which were still drained for the purpose of peat extraction. Key Points: First attempt to include ozone into the greenhouse gas balance of a rewetted raised bog. The deposition of ozone adds a cooling effectThe site is still a net source of greenhouse gasesIt is likely that the yearly GHG emissions are lower than they would have been if restoration had not started 18 years ago [ABSTRACT FROM AUTHOR]

Cities are taking the lead on climate change mitigation with ambitious goals to reduce carbon dioxide (CO2) emissions. The implementation of effective mitigation policies will require accurate measurements to guide policy decisions and monitor their efficacy. Here, we present a comprehensive CO2 inventory of an urban temperate forest and unmanaged grassland using field observations. We estimate the annual storage of CO2 by vegetation and soils and place our biogenic flux estimates in the context of local fossil fuel (FF) emissions to determine when, where, and by how much biogenic fluxes alter net CO2 flux dynamics. We compare our hourly estimates of biogenic fluxes in the forest site to modeled estimates using a modified version of Urban‐Vegetation Photosynthesis and Respiration Model (Urban‐VPRM) in Washington DC/Baltimore area presenting the first urban evaluation of this model. We estimate that vegetation results in a net biogenic uptake of −2.62 ± 1.9 Mg C ha−1 yr−1 in the forest site. FF emissions, however, drive patterns in the net flux resulting in the region being a net source of CO2 on daily and annual timescales. In the summer afternoons, however, the net flux is dominated by the uptake of CO2 by vegetation. The Urban‐VPRM closely approximates hourly forest inventory based estimates of gross ecosystem exchange but overestimates ecosystem respiration in the dormant season by 40%. Our study highlights the importance of including seasonal dynamics in biogenic CO2 fluxes when planning and testing the efficacy of CO2 emission reduction polices and development of monitoring programs. Plain Language Summary: The quantification of biogenic carbon dioxide (CO2) fluxes in cities is important to developing, reporting, and testing the efficacy of greenhouse gas emission reduction policies. Yet, to date there is a lack of detailed CO2 flux inventories conducted in urban ecosystems which is hindering our abilities to advance and evaluate urban carbon cycling models. Our study characterizes how unique features of urbanization influence carbon cycling dynamics in a temperate forest and grassland and provides a critical evaluation of a model for estimating biogenic fluxes of CO2. We identify key areas of needed model development and identify the seasonal and temporal significance of biogenic fluxes relative to total CO2 fluxes (biogenic + anthropogenic sources) in our study area. Key Points: In urban landscapes, the biogenic carbon dioxide (CO2) fluxes from forest patches and grassland areas vary at fine temporal and spatial scalesThe Urban‐Vegetation Photosynthesis and Respiration Model closely approximated temporal trends in gross ecosystem exchange but overestimated ecosystem respiration in the wintertimeFossil fuel emissions drive patterns in the net flux of CO2 except in summer afternoons when biogenic fluxes dominate [ABSTRACT FROM AUTHOR]

High global uncertainties remain in water‐air CO2, CH4, and N2O fluxes from estuaries due to spatial and temporal variability and the poor predictability of the gas transfer velocity (k600). This is the first study that directly compares k600 of CO2, CH4, and N2O in an estuary with the aim to evaluate the accuracy of using a uniform k600 value for estimating water‐air fluxes. We calculated 155 k600 values from CO2, CH4, and N2O fluxes over spatial (across, along) and temporal (tidal cycle) surveys in the subtropical Maroochy estuary using the floating chamber method. Combined k600 values showed a large range over the entire estuary (0.1–198.6 cm h−1) with slightly lower k600 in the lower compared to the upper estuary. Overall, temporal variability was greater than spatial variability of k600. We found the highest variability of k600 between gas species in the lower estuary, whereas the variability was less distinct in the upper estuary. In the Maroochy estuary, k600CO2 (mean 26.4 ± 37.3 cm h−1) was mostly higher than k600CH4 (mean 10.9 ± 10.6 cm h−1) and k600N2O (mean 9.9 ± 12.3 cm h−1), likely due to chemical and enzymatic enhancements and/or microbial activity in the surface microlayer. We demonstrate that empirical k600 models intended for CO2 may not accurately predict CH4 and N2O fluxes in estuaries. Our tested k600 models predicted the measured fluxes within an uncertainty range of 5%–40% (over or underestimation), but precise flux estimates should be based on in situk600 of all three gases. Plain Language Summary: Greenhouse gas emissions from estuaries are an important component of the coastal ocean carbon cycle. However, carbon dioxide, methane, and nitrous oxide fluxes from estuaries remain uncertain at the global scale, partially because of the gas transfer velocity, a relevant parameter of the flux computation. This study is the first study, to our knowledge, that directly compares gas transfer velocities of the three greenhouse gases with each other and over spatial and temporal gradients in a subtropical estuary in Australia. We find that gas transfer velocities were gas‐specific and differed in upper, mid, and lower estuary regions. The non‐uniformity of the gas transfer velocity in heterogeneous estuaries is likely due to different chemical, physical and biological processes that drive gas transfer of the three gases at the water‐air interface. This is important to consider when estimating estuary fluxes because models for one specific gas may not accurately predict the gas transfer of the other two greenhouse gases. Key Points: k600 of CO2, CH4, and N2O were non‐uniform over spatial and temporal estuary gradients and k600CO2 mostly greater than k600CH4 and k600N2OVariability of k600 between the gas species was greater at the estuary mouth compared to the mid and upper estuary sectionsEmpirical k600CO2 models may not accurately predict CH4 and N2O fluxes because they do not account for gas‐specific non‐diffusive processes [ABSTRACT FROM AUTHOR]

Serpentinization is a natural process that transforms ferromagnesian minerals such as olivine into serpentine and that produces waters at very high pH and gases enriched in methane (CH4) and hydrogen (H2). We report the composition of gases venting at two springs (Bain des Japonais and Rivière des Kaoris) of the serpentinizing environment of the Prony Bay (New Caledonia) collected eight times between 2011 and 2014, along with in situ measurements (temperature, pH, oxydo‐reduction potential, dissolved oxygen content) of on‐land alkaline springs of the Southern New Caledonia ophiolite. Venting gases are mainly composed of H2, CH4, and N2 and their composition has slightly varied during the 4‐year field survey. An elevated oxygen (O2) content in a high‐pH water sample is due to air uptake during surface flow. O2‐corrected gas compositions along with those published for gas data obtained at similar serpentinizing environments (Italy, Turkey, Philippines, and Oman) show that the H2 and CH4 concentrations display a linear correlation with a slope close to the value corresponding to the CH4 production from carbon dioxide rather from a less oxidized carbon such as carbon monoxide. Although these data are consistent with the stoichiometry of the Sabatier reaction, as such in the gas phase, it is also possible that microbial hydrogenotrophic methanogenesis takes place in the aqueous phase followed by degassing. A diagram is proposed that outlines the partitioning of H2 and CH4 between the gaseous and aqueous phases and the need to consider a two‐phase flow in the hydrology of these hyperalkaline environments. Plain Language Summary: Serpentinization is a natural process that transforms ferromagnesian minerals such as olivine into serpentine. This process is accompanied by the formation of high‐pH water, hydrogen, and methane. This forms a unique ecological niche for microorganisms adapted to extreme pH and/or involved in the hydrogen and/or methane cycles. There is an ongoing debate on the mechanisms at play and on the contribution of abiotic versus biogenic reactions. An analysis of the composition of free gases collected in hyperalkaline springs of New Caledonia combined with literature values for similar sites worldwide shows that the formation of methane follows the stoichiometry of a Sabatier‐type reaction involving fully oxidized carbon (CO2) and not carbon monoxide (CO) as required by a Fischer‐Tropsch‐Type process. Although it could be concluded that methane production occurs in the gas phase, and as such would be abiotic, it does not rule out the possibility that the relationship between gaseous hydrogen and methane is due to methane production and hydrogen consumption by microorganisms in the aqueous phase followed by degassing. Key Points: Hyperalkaline spring waters are anoxic. An elevated high O2 content in free gases results from air contaminationThe relationship between H2 and CH4 contents of free gases is consistent with the conversion of fully oxidized carbon (CO2)The H2‐CH4 relation is consistent with a Sabatier reaction in the gas or methane production by hydrogenotrophic methanogens in the water [ABSTRACT FROM AUTHOR]

Vegetated roofs or green roofs may supply a number of different urban ecosystem services such as thermal regulation of local air temperature by evaporative cooling or climate change mitigation by sequestration of CO2. However, knowledge about the annual and seasonal variations of green roof carbon uptake due to changes in meteorological conditions and water availability remains scarce. We studied the annual variation of net ecosystem exchange (NEE) by eddy‐covariance flux measurements during a 5‐year period from 2014 to 2019 on a large and unirrigated green roof at the Berlin‐Brandenburg airport (BER) in Berlin, Germany. The roof vegetation was dominated by Sedum species and herbs. The BER green roof showed net uptake of carbon in each of the 5 study years with an average uptake of −141.1 g C m−2 y−1 and an average measurement uncertainty of ±15.5 g C m−2 y−1. The NEE, however, was characterized by significant interannual variation in which the maximum annual uptake of −189 g C m−2 y−1 in a year with above average precipitation roughly doubles the minimum uptake of −95 g C m−2 y−1 in a dry year. The variation of NEE was a function of the annual amount of precipitation and the frequency of rainfall, that is, the number of dry days per year (DDY). The roof might lose its carbon uptake function in a warmer future climate with an increasing number of DDY. Sustainable irrigation of the roof (e.g., rainwater harvesting) could help to maintain the carbon uptake function of the green roof vegetation. Plain Language Summary: The popularity of and total surface area covered by urban green roofs is growing worldwide. Vegetated (green) roofs are able to take up carbon dioxide from the air via photosynthesis, but we know relatively little about the effects weather has on their annual rate of CO2 uptake. We studied the exchange of CO2 between green roof surface and air over a 5‐year period on a large, unirrigated green roof at the Berlin‐Brandenburg airport in Germany. There was a net uptake of CO2 by the green roof plants and soil in each year of the study. However, CO2 uptake was higher with increasing annual rainfall; drier conditions decrease the green roof's capacity to sequester CO2. Sustainable irrigation is likely to help maintain the rate of CO2 uptake of the green roof vegetation during periods of low rainfall. Key Points: Surface‐atmosphere exchange measurements document net annual carbon uptake for a green roof covered by Sedum speciesThe maximum carbon uptake in a wet year with above average precipitation is twice the amount as compared to a dry yearAn important driver of the annual variation in carbon uptake is the number of dry days per year [ABSTRACT FROM AUTHOR]

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Methane (CH4) exchange between trees and the atmosphere has recently emerged as an important, but poorly quantified process regulating global climate. The sources (soil and/or tree) and mechanisms driving the increase of CH4 in trees and degassing to the atmosphere are inadequately understood, particularly for coastal forests facing increased exposure to seawater. We investigated the eco‐physiological relationship between tree stem wood density, soil and stem oxygen saturation (an indicator of redox state), soil and stem CH4 concentrations, soil and stem carbon dioxide (CO2) concentrations, and soil salinity in five forests along the United States coastline. We aim to evaluate the mechanisms underlying greenhouse gas increase in trees and the influence of seawater exposure on stem CH4 accumulation. Seawater exposure corresponded with decreased tree survival and increased tree stem methane. Tree stem wood density was significantly correlated with increased stem CH4 in seawater exposed gymnosperms, indicating that dying gymnosperm trees may accumulate higher levels of CH4 in association with seawater flooding. Further, we found that significant differences in seawater exposed and unexposed gymnosperm tree populations are associated with increased soil and stem CH4 and CO2, indicating that seawater exposure significantly impacts soil and stem greenhouse gas abundance. Our results provide new insight into the potential mechanisms driving tree CH4 accumulation within gymnosperm coastal forests. Plain Language Summary: Trees emit greenhouse gases such as methane and carbon dioxide to the atmosphere. The origin of these gases includes production in the tree or in the surrounding soils. Disturbances to these systems, such as seawater exposure that increases soil salinity, have an unknown impact on gas production and connectivity between soil and trees. We found that higher soil salinities corresponded to higher soil methane content and increased stem methane. The accumulation of soil and tree methane was lower in sites with no salinity exposure and higher in sites with high salinity. As coastal systems become more vulnerable to changes in seawater exposure, this may have consequences on methane emitted from trees to the atmosphere. Key Points: Seawater exposure significantly increased internal stem CH4 concentrationThere is a significant correlation between stem wood density and stem CH4 concentrationLow stem and soil O2 were significantly linked to seawater exposure and decreased tree survival [ABSTRACT FROM AUTHOR]

Experimental and ambient warming of Arctic tundra results in emissions of greenhouse gases to the atmosphere, contributing to a positive feedback to climate warming. Estimates of gas emissions from lakes and terrestrial tundra confirm the significance of aquatic fluxes in greenhouse gas budgets, whereas few estimates describe emissions from fluvial networks. We measured dissolved gas concentrations and estimated emissions of carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) from water tracks, vegetated depressions that hydrologically connect hillslope soils to lakes and streams. Concentrations of trace gases generally increased as ground thaw deepened through the growing season, indicating active production of greenhouse gases in thawed soils. Wet antecedent conditions were correlated with a decline in CO2 and CH4 concentrations. Dissolved N2O in excess of atmospheric equilibrium occurred in drier water tracks, but on average water tracks took up N2O from the atmosphere at low rates. Estimated CO2 emission rates for water tracks were among the highest observed for Arctic aquatic ecosystems, whereas CH4 emissions were of similar magnitude to streams. Despite occupying less than 1% of total catchment area, surface waters within water tracks were an estimated source of up to 53–85% of total CH4 emissions from their catchments and offset the terrestrial C sink by 5–9% during the growing season. Water tracks are abundant features of tundra landscapes that contain warmer soils and incur deeper thaw than adjacent terrestrial ecosystems and as such might contribute to ongoing and accelerating release of greenhouse gases from permafrost soils to the atmosphere. Plain Language Summary: The Arctic is warming at least twice as rapidly as the rest of the Earth. Warming thaws permafrost, defined as ground that remains frozen for more than 2 years, and results in increased production and release of greenhouse gases by microbes. These gases, including carbon dioxide, methane, and nitrous oxide, trap heat in the atmosphere and contribute to further warming. Emissions of these gases have been extensively observed from soils and lakes in the Arctic, but fewer observations describe greenhouse gas emissions from stream networks. We estimated emissions of greenhouse gases from water flowing in vegetated depressions on Arctic hillslopes, called water tracks, which are tributaries of lakes and rivers. Water tracks may be conduits for gases from soils to the atmosphere. Compared to terrestrial tundra, water tracks emitted more carbon dioxide and methane than expected from their small areal coverage. Water tracks typically withdrew nitrous oxide from the atmosphere, though smaller water tracks supported higher concentrations and net emission of nitrous oxide. The abundance of water tracks combined with high rates of carbon dioxide and methane emission relative to adjacent ecosystems suggests that these features might contribute to increased release of greenhouse gases from Arctic landscapes as permafrost thaws. Key Points: Water tracks draining Arctic hillslopes are estimated to emit CO2 and CH4 to the atmosphere at greater rates than terrestrial tundraWater tracks produced and consumed N2O, resulting in low net emissions to the atmosphereDissolved gas concentrations varied among water tracks and over time and were related to precipitation, redox, and primary productivity [ABSTRACT FROM AUTHOR]

Measuring the soil‐to‐atmosphere carbon dioxide (CO2) flux (soil respiration, RS) is important to understanding terrestrial carbon balance and to forecasting climate change. Such measurements are frequently made using measurement collars permanently inserted into the soil surface. However, differences in measurement duration and frequency, as well as collar properties, may lead to biases in the estimation of annual RS. Using a newly updated global RS database (SRDB‐V5), we investigated the annual RS bias associated with five methodological factors: collar height, collar coverage area, collar insertion depth, measurement duration, and measurement frequency. We found that annual RS was negatively correlated with collar insertion depth, consistent with the idea that collar insertion cuts roots and thus reduces RS. Annual RS was also negatively related with collar height and collar coverage area, perhaps because uniform head‐space mixing is difficult to achieve in larger volume chambers; however, these effects were quantitatively small (bias of ~2% to 10% of mean RS). We found no correlation of measurement duration or measurement frequency with annual RS. These findings suggest that variation in RS methodology generally introduces minimal bias overall. Therefore, compilations of minimally adjusted annual RS measurements provide a reliable resource for synthesis studies, global annual RS modeling, and investigation of how soil carbon responds to climate change. Plain Language Summary: Soil‐to‐atmosphere carbon dioxide (CO2) is the second largest component in the terrestrial carbon cycle; thus, our ability to balance the terrestrial carbon budget and forecast climate change relies upon accurate measurements of this process. Collars permanently installed in the soil are commonly used to measure soil‐to‐atmosphere CO2. However, differences in collar properties, measurement duration, and measurement frequency may lead to biases in the estimation of annual soil‐to‐atmosphere CO2 amount. While many studies on the methodology for measuring soil‐to‐atmosphere CO2 have been conducted, a comprehensive evaluation of the influence of collar properties and measurement duration on annual soil‐to‐atmosphere CO2 variability has not been investigated before. In this study, we use a global data set to analyze soil‐to‐atmosphere CO2 measurement bias related to collar properties and measurement duration. We found that annual soil‐to‐atmosphere CO2 amount negatively correlated with collar height and insertion depth but showed no significant relationship to measurement duration and measurement frequency. Overall, collar properties and measurement duration contributed minimal bias. The results provide strong support for compiling site‐scale soil‐to‐atmosphere CO2 measurements to support synthesis analysis, as well as global soil‐to‐atmosphere CO2 modeling. Key Points: Annual soil respiration (RS) was negatively correlated with collar insertion depth, collar height, and collar coverage areaWe found no correlation of measurement duration or measurement frequency with annual RSOverall, collar properties, measurement duration, and measurement frequency confer minimal bias to annual RSThe results provide strong support for compiling site‐scale RS measurements to support synthesis analysis, as well as global RS modeling [ABSTRACT FROM AUTHOR]

Estimating carbon dioxide (CO2) and methane (CH4) emission rates from reservoirs is important for regional and national greenhouse gas inventories. A lack of methodologically consistent data sets for many parts of the world, including agriculturally intensive areas of the United States, poses a major challenge to the development of models for predicting emission rates. In this study, we used a systematic approach to measure CO2 and CH4 diffusive and ebullitive emission rates from 32 reservoirs distributed across an agricultural to forested land use gradient in the United States. We found that all reservoirs were a source of CH4 to the atmosphere, with ebullition being the dominant emission pathway in 75% of the systems. Ebullition was a negligible emission pathway for CO2, and 65% of sampled reservoirs were a net CO2 sink. Boosted regression trees (BRTs), a type of machine learning algorithm, identified reservoir morphology and watershed agricultural land use as important predictors of emission rates. We used the BRT to predict CH4 emission rates for reservoirs in the U.S. state of Ohio and estimate they are the fourth largest anthropogenic CH4 source in the state. Our work demonstrates that CH4 emission rates for reservoirs in our study region can be predicted from information in readily available national geodatabases. Expanded sampling campaigns could generate the data needed to train models for upscaling in other U.S. regions or nationally. Key Points: Reservoirs are the fourth largest anthropogenic methane source in the U.S. state of OhioVariables in national databases (reservoir size) predict methane emission rates nearly as well as variables measured on‐site (nutrients)The global warming potential of methane emissions from reservoirs exceeded that of carbon dioxide emissions [ABSTRACT FROM AUTHOR]

CO2 efflux from the litter layer, litter heterotrophic respiration, is an important component of the forest carbon cycle. Litter heterotrophic respiration mediated by microorganisms varies in response to seasonal environmental changes, such as temperature and moisture. Here, we aimed to quantify seasonal variation in litter heterotrophic respiration and determine how the microbial biomass influences microbial activity and hence litter heterotrophic respiration in a warm temperate forest. We performed in situ high‐frequency measurements of litter heterotrophic respiration per unit area (R_area), which are able to capture CO2 pulses during rainfall, for over 2 years. Microbial activity, which is the CO2 efflux per unit weight (R_mass) considering the change in the amount of substrate, was calculated based on R_area. In parallel, we measured substrate‐induced respiration (SIR) each month, as an index of microbial biomass. We identified seasonal hysteresis in R_area, which was higher in spring (January to July) than in fall (August to December), despite the temperature being similar in both periods. Of interest, R_mass and SIR also showed similar seasonal hysteresis in relation to temperature. Additionally, potential microbial activity without the effect of temperature and moisture was positively related to SIR. This result indicates that seasonal hysteresis with temperature in microbial activity was driven by microbial biomass seasonality, and thus, it leads to seasonal hysteresis in litter heterotrophic respiration in relation to temperature. Our findings highlight the importance of not only the environmental factors and substrate depletion but also biotic factors for estimating heterotrophic respiration. Key Points: Litter heterotrophic respiration measured by an automated chamber system showed seasonal hysteresis in relation to temperatureMicrobial activity also showed similar seasonal hysteresis with temperature, which was driven by microbial biomass seasonalityIt highlights the important role of microbial biomass seasonality in determining litter heterotrophic respiration [ABSTRACT FROM AUTHOR]

Our study of a St. Lawrence Estuary marsh reveals that, compared to native Spartina patens‐dominated vegetation, invasive Phragmites australis makes a greater contribution to soil volume and carbon stock (referred to as blue carbon). Phragmites' contributions to soil volume enhance marsh sustainability in face of sea level rise, and its greater contribution to soil carbon helps to reduce the atmospheric concentration of CO2. Phragmites australis (common reed) is a cosmopolitan species growing in fresh to brackish wetlands. An invasive genetic strain, introduced from Europe or Asia, has expanded extensively along the St. Lawrence River in the last few decades but has been little studied on the estuarine portion. We collected soil cores from three sites within an invasive Phragmites stand and one site within S. patens‐dominated stand in a St. Lawrence Estuary salt marsh near la Pocatiere, Quebec. We measured the bulk density, carbon content, volume, and mass of belowground organic matter in 2‐cm‐thick soil layers of three cores at each Phragmites site. Bulk density and carbon content were measured in 5‐cm‐thick soil layers of three cores at S. patens site. Results showed that soil in the Phragmites stands held 37–77% more blue carbon than in the S. patens‐dominated marsh. Based upon their diameter size, Phragmites rhizomes could be contributing 7.4–10.2 cm to the thickness in the upper 20 cm of soil. We suggest that any management of invasive Phragmites include consideration of its role in increasing blue carbon stocks and marsh resilience along with other ecosystem services. Plain Language Summary: An aggressive invasive strain of the common reed (Phragmites australis) is widespread on the eastern coast of North America. It is rapidly spreading through salt marshes, the grassy meadows, which straddle mean sea level on the coast of Quebec's St. Lawrence Estuary. This reed is considered to degrade the value of the native salt marsh and its biodiversity by displacing native vegetation and the habitat it provides for birds and wildlife such as the Nelson's Sparrow, designated as a Species of Special Concern in nearby Maine. Our study of a St. Lawrence salt marsh, however, shows that the invasive reed can have positive effects on soil properties. We compared soils below the reed and the most common native grass, salt meadow hay (Spartina patens) and found 37–77% more carbon stored in the reed soil. Thus, growth of the reed increases the ability of salt marshes to reduce concentrations of atmospheric carbon dioxide and mitigate climate change. We measured how much the roots and rhizomes (underground stems) of the two species contributed to the soil volume and found that the reed contributed considerably more. Greater contributions to soil volume means that the elevation of the soil surface will increase faster in the reed marsh and help this tidal ecosystem to keep up with increased rates of sea level rise that will accompany climate warming. Thus, any management of the invasive reed should include consideration of its role in mitigation of climate change and marsh adaptation to sea level rise, along with other ecosystem values provided by the native vegetation. Key Points: Phragmites australis has invaded Spartina patens‐dominated marshes of the St. Lawrence EstuarySoils of invasive Phragmites had 37–77% greater C stocks than S. patens soilsPhragmites contributed 7.4–10.2 cm to the thickness of the top 20 cm of soil [ABSTRACT FROM AUTHOR]

High‐altitude tropical grasslands, known as "páramos," are characterized by high solar radiation, high precipitation, and low temperature. They also exhibit some of the highest ecosystem carbon stocks per unit area on Earth. Recent observations have shown that páramos may be a net source of CO2 to the atmosphere as a result of climate change; however, little is known about the source of this excess CO2 in these mountainous environments or which landscape components contribute the most CO2. We evaluated the spatial and temporal variability of surface CO2 fluxes to the atmosphere from adjacent terrestrial and aquatic environments in a high‐altitude catchment of Ecuador, based on a suite of field measurements performed during the wet season. Our findings revealed the importance of hydrologic dynamics in regulating the magnitude and likely fate of dissolved carbon in the stream. While headwater catchments are known to contribute disproportionately larger amounts of carbon to the atmosphere than their downstream counterparts, our study highlights the spatial heterogeneity of CO2 fluxes within and between aquatic and terrestrial landscape elements in headwater catchments of complex topography. Our findings revealed that CO2 evasion from stream surfaces was up to an order of magnitude greater than soil CO2 efflux from the adjacent terrestrial environment. Stream carbon flux to the atmosphere appeared to be transport limited (i.e., controlled by flow characteristics, turbulent flow, and water velocity) in the upper reaches of the stream, and source limited (i.e., controlled by CO2 and carbon availability) in the lower reaches of the stream. A 4‐m waterfall along the channel accounted for up to 35% of the total evasion observed along a 250‐m stream reach. These findings represent a first step in understanding ecosystem carbon cycling at the interface of terrestrial and aquatic ecosystems in high‐altitude, tropical, headwater catchments. Plain Language Summary: Mountain streams are a critical part of the global carbon cycle. Multiple studies have highlighted this importance in different regions around the globe, and some have shown that mountain streams are a significant source of carbon dioxide emissions. The Tropical Andes are unique because of proximity to the equator, high elevation, and high rainfall. We studied a high‐altitude, tropical grassland ecosystem known as a "páramo," which contains some of the highest carbon stocks on Earth. Our results demonstrate that hydrologic connectivity between wetlands and streams is critical for the movement of carbon downstream and also for the emissions of carbon to the atmosphere. Our study highlights that hydrologic dynamics play an important role in determining the amount and the fate of dissolved carbon within the stream as well as the spatial differences in carbon emissions across the aquatic‐terrestrial interface. These results represent a first step in understanding ecosystem‐level carbon cycling between wetland and stream ecosystems in high‐altitude, tropical, headwater catchments. Key Points: Dissolved CO2 from a high‐altitude peatland was carried into the stream and evaded to the atmosphere within the first ~150 m of the channelStream CO2 evasion was controlled mainly by geomorphic and hydrologic variablesAquatic CO2 fluxes were up to an order of magnitude greater than terrestrial CO2 fluxes in adjacent landscape positions [ABSTRACT FROM AUTHOR]

The article presents a study which examined the diel patterns of soil respiration in a tropical moist forest in Puerto Rico to determine their effects on the production of carbon dioxide (CO2) affecting the atmosphere. Also cited are the effects of climate change and the role of tropical forests in the carbon cycle, and the data collected in the study like soil temperature/moisture, air temperature, photosynthetically active radiation, and relative humidity.

The article presents a study which examined the effects of anthropogenic activity on global terrestrial gross primary production (GPP). Also cited are the use of factorial simulations and biosphere models to calculate the impacts of climate change on GPP, and some factors affecting GPP like carbon dioxide (CO2) fertilization, volcanic eruptions, and El Niñno-Southern Oscillation conditions.

A carbon cycle model is built to describe the behavior of carbon materials in the Pearl River Estuary (PRE), China. The distributions and transformations of dissolved inorganic carbon (DIC), dissolved organic carbon (DOC), and particulate organic carbon (POC) are simulated in the water column and sediment for the year 2006. The terrestrial carbon input is the dominant factor that determines the seasonal variation of carbon, while physical and biochemical processes contribute to the spatial‐temporal circulation of carbon. The simulation results reveal that the PRE acts as a net source for atmospheric CO2 throughout the year, and it buffers the export of DIC from the river and sediment to the adjacent system via the DIC consumption by primary production. As a consequence of biochemical processes, the PRE exports more organic carbon to the sediment and adjacent marine ecosystems than the amount that it receives from upstream river reaches. POC burial in sediment and refractory DOC export to the adjacent marine ecosystems are the main carbon fixation pathways in the PRE. The total amount of carbon fixation in PRE is estimated 6.92 × 1010 mol C year−1. Carbon fixation analysis shows that (1) 20.4% of the POC deposited into sediment is fixed through burial, while the remaining continues to participate in the circulation of carbon materials, and (2) the combined effects of river discharge and monsoon dominate the amount and direction of refractory DOC exports. Plain Language Summary: Carbon cycle in estuaries is an important part of the global carbon cycle. Due to the intensive human impact and complicated biogeochemical processes, there exist great uncertainties in estimating carbon fixation and difficulties in illustrating the entire carbon cycle in estuaries. In this study, we chose the Pearl River Estuary as the study area and used a carbon cycle model to simulate the distribution and transformation of carbon materials. Our study found that the carbon cycle in the PRE is very active. Even though CO2 undergoes continuous evasion from the sea surface to the atmosphere, the PRE fixes 6.92 × 1010 mol C year−1 via particulate organic carbon burial into deep sediment and refractory dissolved organic carbon export to adjacent marine systems. In different seasons, with river discharge and monsoon changes, carbon fixation ability and the direction of carbon export flux vary as well. Key Points: The entire carbon system of Pearl River Estuary is simulated in this studyCarbon fixation pathways are estimated, and carbon fixation ability is simulated in this study [ABSTRACT FROM AUTHOR]

The rapid development of novel technologies to obtain high‐frequency observations has provided new possibilities to observe and understand carbon cycling in inland waters. This study investigates carbon dioxide (CO2) dynamics along a boreal soil‐stream transect using a state‐of‐the‐art data set in combination with a spectral methodology to identify controls on stream CO2. The spectral decomposition of hourly observations revealed intermediate (multiday) to long‐term (monthly) patterns across the upslope‐riparian‐stream continuum, with similar power law increases in CO2 concentration fluctuations with increasing period. High‐frequency CO2 variabilities, specifically diel CO2 concentration fluctuations, were also identified at all locations but were substantially amplified in the stream compared to in the riparian groundwater. Moreover, the spectral coherence between soil and stream CO2 fluctuations was inconsistent and restricted to episodic events. In contrast, we found a strong and consistent spectral coherence between the riparian groundwater level and stream CO2 concentration, indicating a hydrological control on stream CO2 dynamics. However, during some time spans even these patterns were obscured, suggesting that additional processes, such as CO2 evasion and in‐stream metabolism, modulated the influence from riparian sources. The scales and patterns of temporal coherences (or lack thereof) between CO2 at different points in the catchment, as well as with other factors, for example, groundwater levels and Photosynthetically Active Radiation, provide new perspectives on the range of processes governing stream CO2 dynamics. Thus, this study highlights the potential of using spectral decomposition of high‐resolution, spatially distributed data of different types to investigate biogeochemical transformations and pathways linking terrestrial and aquatic systems. Plain Language Summary: Boreal streams are typically rich in carbon dioxide (CO2), a major greenhouse gas, which causes considerable CO2 emissions to the atmosphere. Multiple studies have attempted to explore the sources and drivers of aquatic CO2, but these attempts are often based on sampling methods restricted to low time resolutions. Recently, the development of new cost‐efficient sensor technologies has allowed high temporal resolution exploration of CO2 concentration patterns in streams and the catchment ground waters that feed into the streams. Here we demonstrate how such information can be used to study how the carbon dissolved in water is transported and processed as it moves between terrestrial and aquatic systems. We analyzed hourly sensor data along a soil‐stream transect using a state‐of‐the‐art data set for CO2 and cross‐compared the fluctuations in CO2 concentration with water level fluctuations and Photosynthetically Active Radiation. The analyses revealed important correlations of the hydrochemical connectivity that were further used to explore soil‐stream linkages and subsequent in‐stream processes that affect the stream CO2 concentration. We provide new perspectives on the terrestrial sources of stream CO2 and the multiple ways hydrological drivers affect the stream water CO2 across hourly, daily, seasonal, and interannual time scales. Key Points: Spectral analyses provide a means to exploit spatially distributed, high‐frequency hydrochemical data to study spatiotemporal connectivityWavelet analyses demonstrated nonstationary CO2 patterns, indicative of temporal variabilities in the processes controlling stream CO2A wide spectrum of coherent time scales and consistent phase shifts revealed a strong control of riparian hydrology on stream CO2 dynamics [ABSTRACT FROM AUTHOR]

Lakes are conduits of greenhouse gases to the atmosphere; however, most efflux estimates for individual lakes are based on extrapolations from a limited number of locations. Within‐lake variability in carbon dioxide (CO2) and methane (CH4) arises from differences in water sources, mixing, atmospheric exchange, and biogeochemical transformations, all of which vary across multiple temporal and spatial scales. We asked, how variable are CO2 and CH4 across the surface of a single lake, how do spatial patterns change seasonally, and how well does the typical sampling location represent the entire lake surface? During the 2016 ice‐free period, we mapped surface water concentrations of CO2 and CH4 approximately weekly in Lake Mendota (USA) and modeled diffusive gas exchange. During stratification, CO2 was generally lower than atmospheric saturation (mean 19.81 μM) and relatively homogenous (mean coefficient of variation 0.12), whereas CH4 was routinely extremely supersaturated (mean 0.29 μM) with greater spatial heterogeneity (mean coefficient of variation 0.65). During fall mixis, concentrations of both gases increased and became more spatially variable, but their spatial arrangements differed. In this system, samples collected from the lake center reasonably well represented the spatially weighted mean CO2 concentration but overestimated annual CO2 efflux by 21%. For CH4, the lake center underestimated annual diffusive efflux by only 8.6% but poorly represented lakewide concentrations and fluxes on any given day. Upscaling from a single site to the whole lake requires consideration of spatial variation to assess lakewide carbon dynamics due to heterogeneity in within‐lake processing, transport to the lake surface, and exchange with the atmosphere. Plain language summary: Numerous physical, chemical, and biological properties vary across the surface of individual lakes. However, researchers frequently use the lake center to represent the entire lake and ignore spatial heterogeneity. Using a boat‐mounted sampling system, we mapped carbon dioxide and methane across the surface of Lake Mendota 26 times spanning the entire ice‐free season. We described the progression of each gas's spatial pattern and evaluated the consequences of using only the lake center to calculate emissions to the atmosphere. The lake surface alternated between periods of relative uniformity to periods or remarkable spatial heterogeneity, and the spatial patterns of the two gases did not always align. At the annual scale, samples at the lake center overestimated lakewide carbon dioxide efflux and underestimated methane efflux. These results have consequences for our understanding of lake carbon cycling and the contribution of lakes to atmospheric greenhouse gas budgets. Key Points: Concentrations of carbon dioxide and methane in Lake Mendota varied through time and across the lake surfaceEfflux estimates at the lake center overestimated lakewide carbon dioxide emissions and underestimated methane emissions to the atmosphereUpscaling concentrations, fluxes, and gas transfer velocities from point samples to whole lakes requires consideration of spatial variability [ABSTRACT FROM AUTHOR]

Coccolithophores, originating in the Late Triassic, are one of the most successful marine calcifying algae living in modern oceans. Coccoliths are the calcareous fossil remnants left in marine sediments after coccolithophores die. These calcite scales record the conditions of the surface ocean (e.g., primary productivity, seawater temperature, and carbonate chemistry) and are expected to be a promising subject in paleoceanographic and paleoclimatic studies. Hence, a comprehensive understanding of the ecology and phenology of living coccolithophores, as well as their interactions with other plankton groups, is needed to develop better constraints on their uses in paleoenvironmental studies. Here we show modern coccolithophore production through the coccolith fluxes from sediment traps at a ~500‐m water depth from 2013 to 2015 in the northern South China Sea. In addition to the expected seasonality of the coccolithophore production that occurs during winter seasons due to the strong water mixing induced by monsoon winds, a two‐stage mode for the coccolithophore production is also recognized in relatively "coastal" waters. The first stage includes the production of Gephyrocapsa oceanica in December, when the macronutrient inventory is built, and the second stage describes the growth of Emiliania huxleyi in late February with the depletion of silicate nutrients. This two‐stage mode originates from subtle differences in the nutrient assimilation ability between the two species. In addition, coccolithophore production is significantly influenced by the decadal oceanic events (i.e., El Niño) in the northern South China Sea. Plain Language Summary: Coccolithophores are a kind of marine unicellular algae. They can secrete calcareous scales named coccoliths around their cells. These scales can record the environmental conditions, for example, temperature and CO2 of surface ocean, when coccolithophores are alive. After death, these scales settle to the seafloor and are stored in sediments for up to millions of years. The scales in old sediments can reflect ancient climates and environments. Thus, understanding coccolithophores' life in modern oceans is important. In this study, we deployed a sediment trap at a 500‐m water depth in the northern South China Sea. These traps collected the scales of coccolithophores and record the annual variations in modern coccolithophore population. Higher numbers of scales are found in winter, thereby indicating that more coccolithophores live during this period because of a higher nutrient availability. The coccolithophore scale records in the traps differ spatially and temporally. In coastal waters, one extreme nutrient‐preference species can live with diatoms (large and completive algae) in early winter, and in later winter, another relatively cosmopolitan species appears. The different phenology of these two species is related to seawater nitrogen and silicon. In addition, we also find that the coccolithophore population declines during El Niño periods. Key Points: Coccolith fluxes increased during winter seasons due to the enhanced water mixing and nutrient supply in the northern South China SeaTwo stages of coccolithophore production of Emiliania huxleyi and Gephyrocapsa oceanica were found in relatively "coastal" watersLower winter coccolithophore production was found during an El Niño event in the northern South China Sea [ABSTRACT FROM AUTHOR]

Soil respiration (Rs), the largest carbon emission flux in ecosystems, is usually modeled as an empirically parameterized function of temperature, and sometimes water availability. The likely contribution by other factors, such as carbohydrate substrate supply from photosynthesis, has been recognized, but modeling capacity to use this information is limited. Wavelet transformations of the residuals of a seasonal Q10 temperature response model indicated structure at different temporal scales. We hypothesize that this indicates the lack of explicit representation of relevant processes in the models. Using cross‐spectral analysis, we found that time series of photosynthetically active radiation explained most of the diurnal variation, temperature, explained variability at multiple time scales (diurnal‐synoptic), whereas volumetric soil water content correlated with variability in Rs at scales 15‐30 days. The results suggest that the time domains of influence for different driving variables of Rs are discrete, and largely nonoverlapping, and represent functional relationships between soil biological activity and its constraints. Analysis of phase angles showed that Rs lagged photosynthetically active radiation by 1.5‐3.0 hr. As this time lag was the same in both young and mature trees, with more than fivefold difference in transport distances, we hypothesize that this finding adds to the body of literature that support the pressure‐concentration‐wave model of carbohydrate availability in plants. Plain Language Summary: "Soil respiration" (RS) is the net emission of carbon dioxide (CO2) from the soil to the atmosphere. Rs has traditionally been quantified as a function of soil temperature, even though it originates from a suite of biological processes in the soil. Recent experimental studies have shown strong dependence of both plant root and microbial activity on plant‐internal carbohydrate status. Here we report how different physical (soil temperature and soil moisture) and biological (plant carbohydrate supply, approximated with photosynthetically active radiation [PAR]) process covary with soil CO2 emissions in two forest stands of different age. The individual and pairwise variation in Rs, soil temperature, soil moisture, and PAR time series at different frequencies was used to infer causal relationships between them. Environmental factor temperature and soil moisture significantly covaried with Rs at weekly to seasonal time scales, whereas diurnal variability of Rs was attributed to carbohydrate substrate availability. The consistent 1.5‐ to 3‐hr time lag between Rs and PAR (as a proxy for carbohydrate availability) did not differ between forests of different stature. We hypothesize that this suggests that carbohydrate availability throughout the plant was regulated by pressure‐concentration waves from phloem loading rather than actual mass transport of the loaded carbohydrates. Key Points: The diurnal variation in soil respiration is driven primarily by PAR (surrogate for photosynthesis), whereas temperature and soil moisture affect primarily synoptic (8‐15 days) and seasonal (15‐30 days) variability, respectivelySoil respiration lagged PAR by 1.5 to 3 hr and was independent of vegetation heightThe discrete and nonoverlapping influences from different factors call for a revision of current respiration models so as to realistically capture their temporal domain of influence [ABSTRACT FROM AUTHOR]

Soils exert considerable influence on the global biogeochemical cycles, but their role in the abiotic production of volatile compounds remains poorly understood. In this study, we demonstrated evidence for the abiotic production of three major greenhouse gases (GHGs), namely carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O), simultaneously in six different subtropical surface soils under thermal treatment at 30 to 90 °C, wetting, and the addition of hydrogen peroxide. This study identified soils as a novel source of N2O through abiotic processes such as thermal treatment and the oxidation of reactive oxygen species. Our results showed that the abiotic production rate of all three GHGs increased exponentially with temperature, while that of CO2 and CH4 increased linearly with hydrogen peroxide concentrations. Wetting of soil samples could further enhance the rate of abiotic CO2 and N2O production in soils. These findings highlight the potential contribution of abiotic processes in the production of GHGs in subtropical soils and help constrain the uncertainties of the global GHG budgets. Plain Language Summary: While the production of greenhouse gases (GHGs) in soils by microbial activities has been widely documented and investigated, the role of nonmicrobial processes in soil GHG production is still poorly understood. In this study, we found that sterilized soil samples, in the absence of microbial activities, could produce three major GHGs, including carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O), simultaneously under heating, wetting, and the addition of hydrogen peroxide (a powerful oxidizing agent). Also, this study discovered a new source of atmospheric N2O arising from the abiotic production in soils under a high temperature and the presence of hydrogen peroxide. The abiotic production rates of all three GHGs increased exponentially with temperature, implying that global warming could enhance nonmicrobial GHG production rates and induce a positive climate feedback. Key Points: Abiotic production of three greenhouse gases (CO2, CH4, and N2O) occurred simultaneously in subtropical soilsSoil CO2 and CH4 production rates increased exponentially with temperature and increased linearly with H2O2 concentrationWetting enhanced the abiotic production rates of CO2 and N2O in soils [ABSTRACT FROM AUTHOR]

Thermokarst can rapidly mobilize vast amounts of sediment, solutes, and organic carbon previously maintained in frozen soils to inland waters. Streams provide a critical pathway for transforming these materials into carbon dioxide (CO2) and methane (CH4), yet the direct effects of thermokarst on fluvial C gas efflux from streams to the atmosphere are largely unknown. Working on the Peel Plateau in the western Canadian Arctic, we show that CO2 efflux in rill runoff thaw streams (runoff) within retrogressive thaw slumps (RTSs) was four times greater than in adjacent streams and contributed modestly but disproportionately to efflux at the landscape scale. In contrast, CH4 efflux was generally greater in adjacent streams than in RTS runoff and, overall, was within the range of values reported for other northern streams. While RTS occurrence was a primary driver of CO2 efflux, CH4 efflux was more strongly associated with conditions reflective of biological activity. Transects downstream of two RTSs revealed that CH4 consistently and rapidly degassed to the atmosphere, while elevated CO2 was sustained downstream of one RTS feature. At the watershed scale, streams adjacent to RTSs rather than runoff streams within RTSs dominated fluvial CO2 and CH4 efflux. Intensifying thermokarst activity in the western Canadian Arctic will likely amplify contributions from runoff streams in RTSs to watershed‐scale fluvial C gas efflux. Plain Language Summary: Rapid Arctic warming is thawing perennially frozen ground (permafrost) and driving terrain subsidence in ice‐rich areas (thermokarst). In glaciated permafrost landscapes, thermokarst features called retrogressive thaw slumps, which are caused by thawing of ice‐rich permafrost, release sediment, solutes, and organic materials that have been preserved in permafrost for millennia. Following thaw, permafrost carbon can rapidly transform into the greenhouse gas carbon dioxide and methane in streams. Yet the effects of thermokarst on fluvial gas flux are largely unknown. Working on the Peel Plateau in northwest Canada, we found that carbon dioxide efflux to the atmosphere from runoff draining from thaw slumps was four times greater than in streams adjacent to thaw slumps. In contrast, methane efflux in adjacent streams was twice that in runoff streams and both were similar to streams in other northern regions. While thaw slump occurrence was a primary driver of carbon dioxide efflux, methane efflux was more strongly associated with conditions reflective of biological activity. Generally, both carbon dioxide and methane were rapidly degassed downstream. While efflux of carbon dioxide and methane in thaw slumps were a small component of watershed‐scale fluvial C gas efflux, these fluxes will likely increase as regional thermokarst activity accelerates. Key Points: CO2 efflux in thaw slump runoff streams was four times greater than in adjacent streams, but a small part of watershed‐scale effluxCH4 efflux was generally small in runoff streams, variable in adjacent streams, and associated with conditions reflecting biological activityElevated CO2 partial pressures persisted downstream of a large thaw slump but dissipated downstream of a smaller thaw slump [ABSTRACT FROM AUTHOR]

Inland waters are major sources of CO2 to the atmosphere. The origin of this CO2 is often elusive, especially in high‐altitude streams that remain poorly studied at present. Here we study the spatial and seasonal variations in streamwater CO2, its potential sources and drivers in an Alpine stream network (Switzerland). High‐resolution sampling combined with stable isotope analysis and mixing models enabled us to capture the fine‐scale spatial heterogeneity in streamwater pCO2 as the stream network expanded and contracted during seasons. We identified soil respiration as a major source of CO2 to the stream. We also identified a major groundwater upwelling zone as an ecosystem "control point" that disproportionately influenced stream biogeochemistry. This was particularly pronounced when the stream network expanded during snowmelt, when it covered a five times larger area compared to winter (35,300 m2 compared to 7,100 m2). Downstream from this control point, CO2 evaded rapidly owing to high gas transfer velocity. The stream network was a net source of CO2 to the atmosphere with an average areal evasion flux of 30.1 (18.0–43.1) μmol · m‐2 · s‐1 and a total flux at network scale ranging from 237 (141–339) kg C/day in winter to 1793 (1069–2565) kg C/day during spring snowmelt. Our study highlights the role of stream network dynamics and control points for the CO2 dynamics in high‐altitude streams. Plain Language Summary: Streams are significant sources of CO2 to the atmosphere. However, the origin of this CO2 and its spatial and temporal dynamics are often not clear. Here we study the small‐scale spatial variation of streamwater CO2 and its evasion from an Alpine stream network in Switzerland. We found that groundwater upwelling, as constrained by local geomorphology, delivered large quantities of respiratory CO2 from adjacent soils to the stream. The relevance of these groundwater deliveries changes seasonally as the stream network expanded and contracted. Our spatially resolved sampling design allowed us to identify hot spots of CO2 evasion, based on which we were able to assess CO2 evasion at the scale of the entire stream network. Our findings emphasize the relevance of such approaches to properly constrain CO2 evasion fluxes beyond the reach scale. Key Points: We identified a geomorphological "control point" to deliver respiratory CO2 from soils to an Alpine stream via groundwaterWe show how streamwater CO2 dynamics and evasion varies with stream network expansion and contractionCO2 evasion from mountain streams might be underestimated due to high spatiotemporal variability in pCO2 [ABSTRACT FROM AUTHOR]

Ocean acidification (OA), a consequence of increased global carbon dioxide (CO2) emissions, is considered a major threat to marine ecosystems. Its effects on bacterioplankton activity, diversity, and community composition have received considerable attention. However, the direct impact of OA on heterotrophic bacterioplankton is often masked by the significant response of phytoplankton due to the close coupling of heterotrophic bacterioplankton and autotrophs. Here we investigated the responses of a heterotrophic bacterioplankton assemblage to high pCO2 (790‐ppm) treatment in warm tropical western Pacific waters by conducting a microcosm experiment in dark for 12 days. Heterotrophic bacterioplankton abundance and production were enhanced by OA over the first 6 days of incubation, while the diversity and species richness were negatively affected. Bacterioplankton community composition in the high pCO2 treatment changed faster than that in the control. The molecular ecological network analysis showed that the elevated CO2 changed the overall connections among the bacterial community and resulted in a simple network under high CO2 condition. Species‐specific responses to OA were observed and could be attributed to the different life strategies and to the ability of a given species to adapt to environmental conditions. In addition, high‐throughput functional gene array analysis revealed that genes related to carbon and nitrogen cycling were positively affected by acidification. Together, our findings suggest that OA has direct effects on heterotrophic bacterioplankton in a low‐latitude warm ocean and may therefore affect global biogeochemical cycles. Key Points: Heterotrophic bacterioplankton abundance, production, and activity were stimulated by ocean acidification, while diversity decreasedOcean acidification changed the overall connections among the bacterial community and resulted in a simple networkGeoChip analysis showed that bacterial functional genes related to carbon and nitrogen cycling could be enhanced by ocean acidification [ABSTRACT FROM AUTHOR]

Greenhouse gas evasion from inland waters is a globally significant yet highly uncertain flux, especially in regard to effects of wetlands and hydrologic variability. We sampled five first‐order and two second‐order streams with variable wetland influence during storm events for dissolved CO2, CH4, and N2O. We also calculated gas evasion rates. In first‐order streams, pCO2 and pN2O were significantly higher in the stream with the most wetland influence (mean ± 1 std: 3,965 ± 1,504 and 1.18 ± 0.37 μatm, respectively) than the forested stream (2,927 ± 439 and 0.47 ± 0.08 μatm, respectively). In second‐order streams, pCO2, pCH4, and pN2O were higher in the 14% wetland stream (3,274 ± 825, 501 ± 207, and 1.37 ± 0.43 μatm, respectively) than in the 2% wetland stream (1,858 ± 423, 137 ± 53, and 0.37 ± 0.08 μatm, respectively). In first‐order streams, pCO2 in streams with wetland influence increased during rain events, while pCO2 in streams with little to no wetland influence decreased or remained constant. Generally, pCH4 and pN2O followed the same trend, except in one stream with intermediate wetland influence. Gas transfer velocity increased in all streams during storm events. However, the forested streams had higher gas transfer velocities than the wetland streams due to steeper topography. CO2, CH4, and N2O evasion peaked in one of the intermediate wetland streams at high flow (maximum: 66 g C·m−2·day−1, 177 mg C·m−2·day−1, and 9.7 mg N·m−2·day−1, respectively). These findings suggest that gases are shunted downstream in flatter, wetland streams, while gases are evaded closer to their source in steeper, forested streams. Plain Language Summary: Greenhouse gas evasion from inland waters is an important component of the global carbon cycle. However, this flux has only just been included in the most recent IPCC carbon cycle (AR5). Headwater streams are areas where gases, such as carbon dioxide, methane, and nitrous oxide, tend to collect and are released to the atmosphere. Gas dynamics in headwater streams must be understood to improve global evasion estimates. We sampled streams covering a gradient of wetland to forest land covers during rainstorms to examine the effects on gas evasion. We found that gas concentrations generally increased during storms in wetland streams, while concentrations tended to decrease or remain constant in forested streams. However, we found that gases were more easily evaded from forested streams than from wetland streams. Forested streams tend to be found in steeper areas and have more turbulent flows than wetland streams, which promote evasion. Conversely, gas inputs to wetland streams are carried downstream and released farther from the source. The significance of this work is that greenhouse gas evasion from headwater streams is a spatially and temporally variable flux from the landscape. Inclusion of this variability in global models may support higher global evasion estimates. Key Points: Carbon dioxide, methane, and nitrous oxide fluxes are spatially and temporally variable in part due to land cover and storm dynamicsAqueous pCO2 increases with rain in wetland streams and decreases or remains constant in forest streamsWetland streams have lower gas transfer velocity than forest streams due to flatter topography, which favors downstream transport [ABSTRACT FROM AUTHOR]

Fires play an important role in the terrestrial biosphere carbon cycle, not only through direct carbon release but also contributing to a potential long‐term storage as pyrogenic carbon (PyC). PyC is formed through fires, and, because it may resist further biological and chemical degradation, is more stable in soil and sediment than original biomass. At the global scale, contributions of fires to both atmospheric CO2 emissions and PyC accumulation are potentially large but difficult to estimate. Our analysis was based on existing simulation results from two different modeling approaches (Global Fire Emissions Database version 4 [GFED4s] and Terrestrial Ecosystem Model version 6 [TEM6]) that used global area burned data to provide recent, retrospective estimates of CO2 emissions from vegetation combustion, together with published, biome‐ and continental‐scale conversion ratios that relate CO2 emissions to PyC production (PyC/CO2) during combustion. The estimates of global CO2 emissions from fires differed substantially between the two models' results. GFED4s estimated 2,041 Tg C/year during the 2000–2016 time period, whereas the TEM6 estimate was considerably lower at 643 Tg C/year from 2000 to 2010. Global PyC production estimates from fires were 153.4 ± 18.7 and 49.5 ± 4.9 Tg C/year based on the emission estimates from GFED4s and TEM6, respectively. Our results suggest that African tropical savanna fires produced the largest amount of CO2 emissions and PyC among global biomes, the most significant interannual variations in CO2 emissions and PyC production were found in tropical forests, and the magnitude of PyC produced by fires each year represented a potentially significant long‐term sink of atmospheric CO2. Key Points: We present two modeling approaches to estimate the spatiotemporal dynamics of global pyrogenic carbon (PyC) produced from firesWe calculate global PyC production from fires as 153.4 ± 18.7 and 49.5 ± 4.9 Tg C/year based on the estimates from GFED4s and TEM6The magnitude of PyC produced by fires represents a potentially significant long‐term sink of atmospheric CO2 [ABSTRACT FROM AUTHOR]

As the world's population increases, so will the demand for food and timber resources. Though carbon dioxide (CO2) fertilization and, to a lesser extent, nitrogen (N) deposition are expected to increase future resource production, changes in ozone (O3) and climate have the potential to decrease production due to increased phytotoxic damage, drought, and heat stress. To determine how crop and timber production may change in the future, we use the Community Land Model version 4.5 with prognostic crops to simulate responses of wood biomass and crop yields to CO2, O3, N deposition, and climate under Representative Concentration Pathway 8.5 forcings. Generally, rising CO2 increases wood biomass and crop yields, while projected climate change causes decreases. Small projected changes in O3 and N deposition do not strongly affect yields, though additional research is needed on future O3 and N deposition trends and impacts. By the end of the 21st century, global wood biomass increases by ~16% due to the dominating impact of CO2. The positive effect of CO2 on future crop yields is muted by the negative impacts of climate, with a ~5% net global increase. Future projections suggest that rice and wheat yields typically increase under the combination of future forcings, whereas soy and corn yields are regionally variable. While short‐term resource management strategies can benefit from planting heat‐tolerant species and cultivars, technological advances and intensification, among other management strategies not included here, must be employed to meet the future demand for these resources. Plain Language Summary: As the world's population increases, so will the demand for food and wood resources. Though these resources might increase due to the fertilizing effects of carbon dioxide (CO2) and additional nitrogen (N) created by human sources, food and wood resources might decrease due to other environmental changes, like ground‐level ozone (O3), a toxic air pollutant, and climate, which can cause drought and heat stress. This study uses a global model to understand how food and wood resources may change in the future if there is no climate mitigation. Our results show that global wood biomass will increase by 16% and crop yields will increase by 5% at the end of the 21st century. While CO2 increases wood biomass and crop yields, it does not always outweigh the negative effects of climate, especially for crops like soybean and corn. The future changes in O3 and additional N were small and therefore did not change resource production very much. With the global food demand expected to increase by 65% and timber by 100%, these results suggest that resources may not increase enough to meet future demand unless additional, intensive management techniques are used, like expansion of managed area and irrigation, development of heat‐ and drought‐tolerant crop and timber varieties, and other technological advances that can increase production in light of future environmental changes. Key Points: Community Land Model simulations suggest that future global wood biomass and crop yields increase by 16% and 5%, respectively under RCP8.5Under RCP8.5, future carbon dioxide increases wood biomass and crop yields, while future climate change typically decreases yieldsSmall future changes in ozone and nitrogen deposition under RCP8.5 have little effect on wood biomass and crop yields [ABSTRACT FROM AUTHOR]

Accurate determination of air‐water gas exchange fluxes is critically important for calculating ecosystem metabolism rates from dissolved oxygen in shallow aquatic environments. We present a unique data set of the noble gases neon, argon, krypton, and xenon in a salt marsh pond to demonstrate how the dissolved noble gases can be used to quantify gas transfer processes and evaluate gas exchange parameterizations in shallow, near‐shore environments. These noble gases are sensitive to a variety of physical processes, including bubbling. We thus additionally use this data set to demonstrate how dissolved noble gases can be used to assess the contribution of bubbling from the sediments (ebullition) to gas fluxes. We find that while literature gas exchange parameterizations do well in modeling more soluble gases, ebullition must be accounted for in order to correctly calculate fluxes of the lighter noble gases. In particular, for neon and argon, the ebullition flux is larger than the differences in the diffusive gas exchange flux estimated by four different wind speed‐based parameterizations for gas exchange. We present an application of noble gas derived ebullition rates to improve estimates of oxygen metabolic fluxes in this shallow pond environment. Up to 21% of daily net oxygen production by photosynthesis may be lost from the pond via ebullition during some periods of biologically and physically produced supersaturation. Ebullition could be an important flux of oxygen and other gases that is measurable with noble gases in other shallow aquatic environments. Key Points: Concentrations of high solubility noble gases in a salt marsh pond are predicted well by wind speed‐based gas exchange parameterizationsHowever, ebullition must occur to explain concentrations of less soluble noble gases, and we infer ebullition rates from neon observationsIncluding ebullition fluxes increased daily net oxygen production by 1–21%, making it an important oxygen flux in this setting [ABSTRACT FROM AUTHOR]

Abstract: The capacity of forest soils to store organic carbon is influenced by changing hydrologic connectivity. We hypothesized that hydrologic connectivity, the water‐mediated transfer of matter and energy between different landscape positions, controls the partitioning between aquatic and atmospheric soil carbon fates. Results from a 5‐year study of a northern hardwood forested catchment indicated that hydrologic connectivity affected both the magnitude and fate of carbon export. Atmospheric carbon export was the major export pathway from the catchment; its rate was regulated by topographic position (i.e., uplands, ecotones, and wetlands) but enhanced or supressed through changes in soil moisture and hydrologic connectivity. Wetter soil conditions reduced CO2 flux from the ecotones and wetlands where microbial respiration was oxygen‐limited, whereas drier soil conditions that decreased hydrologic connectivity increased CO2 flux by relieving the oxygen limitation. In contrast, aquatic carbon export was a minor export pathway from the catchment and was driven by hydrologic connectivity, with less carbon export during relatively low discharge years. Past trends suggest a shift to a warmer climate and changes in the timing, duration, and intensity of hydrologic connectivity that are leading to an increase in annual atmospheric carbon export but a decrease in annual aquatic carbon export, despite the intensification of autumn storms. The increase in atmospheric carbon export creates a positive feedback for climate warming that will further disrupt hydrologic connectivity and aquatic carbon export, with consequences for downstream streams and lakes. [ABSTRACT FROM AUTHOR]

Abstract: Climate change may have notable impacts on carbon cycling in freshwater ecosystems, especially in the boreal zone. Higher atmospheric temperature and changes in annual discharge patterns and carbon loading from the catchment affect the thermal and biogeochemical conditions in a lake. We developed an extension of a one‐dimensional process‐based lake model MyLake for simulating carbon dioxide (CO2) dynamics of a boreal lake. We calibrated the model for Lake Kuivajärvi, a small humic boreal lake, for the years 2013–2014, using the extensive data available on carbon inflow and concentrations of water column CO2 and dissolved organic carbon. The lake is a constant source of CO2 to the atmosphere in the present climate. We studied the potential effects of climate change‐induced warming on lake CO2 concentration and air‐water flux using downscaled air temperature data from three recent‐generation global climate models with two alternative representative concentration pathway forcing scenarios. Literature estimates were used for climate change impacts on the lake inflow. The scenario simulations showed a 20–35% increase in the CO2 flux from the lake to the atmosphere in the scenario period 2070–2099 compared to the control period 1980–2009. In addition, we estimated possible implications of different changes in terrestrial inorganic and organic carbon loadings to the lake. The scenarios with plausible increases of 10% and 20% in CO2 and dissolved organic carbon loadings, respectively, produced increases of 2.1–2.5% and 2.2–2.3% in the annual CO2 flux. [ABSTRACT FROM AUTHOR]

Abstract: Spatiotemporal variation in climate and weather, allochthonous carbon loads, and autochthonous factors such as lake metabolism (photosynthesis and respiration) interacts to regulate atmospheric CO2 exchange of lakes. Understanding this interplay in diverse basin types at different timescales is required to adequately place lakes into the global carbon cycle and predict CO2 flux through space and time. We analyzed 18 years of data from seven moderately hard lakes in an agricultural prairie landscape in central Canada. We applied generalized additive models and sensitivity analyses to evaluate the roles of metabolic and climatic drivers in regulating CO2 flux at the intra‐annual scale. At mean conditions with respect to other predictors, metabolic controls resulted in uptake of atmospheric CO2 when surface waters exhibited moderate primary production but released CO2 only when primary production was very low (<8 μg/L or when dissolved nitrogen was elevated (>2,000 μg/L), implying that respiratory controls offset photosynthetic CO2 uptake under these conditions. Climatically, dry conditions increased the likelihood of in‐gassing, likely due to evaporative concentration of base cations and/or reduced allochthonous carbon loads. While more research is required to establish the relative importance of climate and metabolism at other timescales (diel, autumn/winter), we conclude that these hard fresh waters characteristic of continental interiors are mainly affected by metabolic drivers of pCO2 at daily‐monthly timescales, are climatically controlled at interannual intervals, and are more likely to in‐gas CO2 for a given level of algal abundance than are soft water, boreal ecosystems. [ABSTRACT FROM AUTHOR]

Abstract: We investigated the direct and indirect influence of tides on net ecosystem exchange (NEE) of carbon dioxide (CO2) in a temperate brackish tidal marsh. NEE displayed a tidally driven pattern with obvious characteristics at the multiday scale, with greater net CO2 uptake during spring tides than neap tides. Based on the relative mutual information between NEE and biophysical variables, this was driven by a combination of higher water table depth (WTD), cooler air temperature, and lower vapor pressure deficit (VPD) during spring tides relative to neap tides, as the fortnightly tidal cycle not only influenced water levels but also strongly modulated water and air temperature and VPD. Tides also influenced NEE at shorter timescales, with a reduction in nighttime fluxes during growing season spring tides when the higher of the two semidiurnal tides caused inundation at the site. WTD significantly influenced ecosystem respiration (Reco), with lower Reco during spring tides than neap tides. While WTD did not appear to affect ecosystem photosynthesis (gross ecosystem production, GPP) directly, the impact of tides on temperature and VPD influenced GPP, with higher daily light‐use efficiency and photosynthetic activity during spring tides than neap tides when temperature and VPD were lower. The strong direct and indirect influence of tides on NEE across the diel and multiday timescales has important implications for modeling NEE in tidal wetlands and can help inform the timing and frequency of chamber measurements as annual or seasonal net CO2 uptake may be underestimated if measurements are only taken during nonflooded periods. [ABSTRACT FROM AUTHOR]

Abstract: Decomposition of soil organic matter (SOM) in permafrost terrain and the production of greenhouse gases is a key factor for understanding climate change‐carbon feedbacks. Previous studies have shown that SOM decomposition is mostly controlled by soil temperature, soil moisture, and carbon‐nitrogen ratio (C:N). However, focus has generally been on site‐specific processes and little is known about variations in the controls on SOM decomposition across Arctic sites. For assessing SOM decomposition, we retrieved 241 samples from 101 soil profiles across three contrasting Arctic regions and incubated them in the laboratory under aerobic conditions. We assessed soil carbon losses (Closs) five times during a 1 year incubation. The incubated material consisted of near‐surface active layer (ALNS), subsurface active layer (ALSS), peat, and permafrost samples. Samples were analyzed for carbon, nitrogen, water content, δ13C, δ15N, and dry bulk density (DBD). While no significant differences were observed between total ALSS and permafrost Closs over 1 year incubation (2.3 ± 2.4% and 2.5 ± 1.5% Closs, respectively), ALNS samples showed higher Closs (7.9 ± 4.2%). DBD was the best explanatory parameter for active layer Closs across sites. Additionally, results of permafrost samples show that C:N ratio can be used to characterize initial Closs between sites. This data set on the influence of abiotic parameter on microbial SOM decomposition can improve model simulations of Arctic soil CO2 production by providing representative mean values of CO2 production rates and identifying standard parameters or proxies for upscaling potential CO2 production from site to regional scales. [ABSTRACT FROM AUTHOR]

Abstract: Large CO2 evasion to the atmosphere occurs as dissolved inorganic carbon (DIC) is transported from soils to streams. While this physical process has been the focus of multiple studies, less is known about the underlying biogeochemical transformations that accompany this transfer of C from soils to streams. Here we used patterns in stream water and groundwater δ13C‐DIC values within three headwater catchments with contrasting land cover to identify the sources and processes regulating DIC during its transport. We found that although considerable CO2 evasion occurs as DIC is transported from soils to streams, there were also other processes affecting the DIC pool. Methane production and mixing of C sources, associated with different types and spatial distribution of peat‐rich areas within each catchment, had a significant influence on the δ13C‐DIC values in both soils and streams. These processes represent an additional control on δ13C‐DIC values and the catchment‐scale cycling of DIC across different northern landscape types. The results from this study demonstrate that the transport of DIC from soils to streams results in more than just rapid CO2 evasion to the atmosphere but also represents a channel of C transformation, which questions some of our current conceptualizations of C cycling at the landscape scale. [ABSTRACT FROM AUTHOR]

Quantification of small stream contributions to global carbon cycling is key to understanding how freshwater systems transmit and transform carbon between terrestrial and atmospheric pools. To date, greenhouse gas emissions of carbon dioxide and methane from freshwaters, particularly in mountainous regions, remain poorly characterized due to a lack of direct field observations. Using a unique longitudinal approach, we conducted field surveys across two ecoregions (Middle Rockies and Great Plains) in the Clear Creek watershed, a subwatershed of Wyoming's Powder River Basin. We took direct measurements of stream gases using headspace sampling at 30 sites (8 June to 23 October). We observed the lowest and most variable concentrations in headwaters, which flow through a federally designated alpine wilderness area. By contrast, the Great Plains exhibited 1.45 and 4 times higher pCO2 and pCH4 concentrations and the relative contributions of methane increased downstream. Fluxes during snowmelt were 45% and 58% higher for CO2 and CH4 than during base flow but overall were lower than estimates for other systems. Variability for pCO2 was highest during late summer and in the uppermost sections of the headwaters. The high heterogeneity and common undersaturation observed through space and time, especially in the mountains, suggest that downscaled regional estimates may fail to capture variability in fluxes observed at these smaller scales. Based on these results, we strongly recommend higher resolution time series studies and increased scrutiny of systems at near equilibrium, inclusive of winter storage and ice-off events, to improve our understanding of the effects of seasonal dynamics on these processes. [ABSTRACT FROM AUTHOR]