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Turfgrass is an important component of the urban landscape frequently considered as an alternative land cover to offset anthropogenic CO 2 emissions. However, quantitative information of the potential to directly remove CO 2 from the atmosphere by turfgrass systems is lacking, especially in the tropics. Most assessments have considered the carbon accumulated by grass shoots and soil, but not the release of CO 2 to the atmosphere by soil respiration (i.e., soil CO 2 efflux). Here, we measured at high-temporal resolution (30-min) soil CO 2 efflux, production, and storage rate for nearly three years in a residential lawn of Singapore. Furthermore, we quantified the carbon capture related to biomass production and CO 2 emissions from fossil fuel consumption associated with maintenance activities (e.g., mowing equipment). Warm and humid conditions resulted in relatively constant rates of soil CO 2 efflux, CO 2 storage in soil, and aboveground biomass production (3370, 652, 1671 Mg CO 2 km -2 yr -1 ; respectively), while the systematic use of mowing machinery emitted 27 Mg CO 2 km -2 yr -1 . Soil CO 2 efflux and CO 2 mowing emissions represent carbon losses to the atmosphere, while CO 2 storage in soil and biomass productivity represent gains of carbon into the ecosystem. Under a steady state in which soil CO 2 losses are only compensated by atmospheric CO 2 uptake by photosynthesis, an ideal clipping waste disposal management, in which no CO 2 molecule returns to the atmosphere (i.e., clippings are not burnt), and a 3-week mowing regime, this site can act as a sink of 2296 Mg CO 2 km -2 yr -1 . In the scenario of incinerating all clippings, the lawn acts as an emission source of 1046 Mg CO 2 km -2 yr -1 . Thus, management practices that reduce mowing frequency together with clipping disposal practices that minimize greenhouse gas emissions are needed to make urban lawns a potential natural solution to mitigate global environmental change., (Copyright © 2020 Elsevier Ltd. All rights reserved.)

Storm surges can substantially alter the water level and salinity in tidal salt marshes. Little is known about how changes experienced during storm surges affect greenhouse gas emissions (GHG; CO 2 , CH 4 , N 2 O) from tidal salt marsh soils. Understanding how storm surges influence ecosystem processes is critical for evaluating the ecosystem's sensitivity to sea level rise. To explore how hurricane-induced changes in salinity affect GHG emissions, we exposed intact soil mesocosms (0-9 cm depth) from a Mid-Atlantic temperate salt marsh to pulse changes in salinity experienced at the site before, during, and after Hurricane Joaquin in 2015. Soil temperature, oxygen, and water level were kept constant to avoid confounding effects throughout the experiment. Automated measurements (hourly resolution) of soil GHG emissions were recorded in control (i.e., no salinity changes) and treatment mesocosms, and combined with soil pore water chemistry (i.e., SO 4 2- , S 2- , Fe 2+ , TN b , redox potential, pH) to characterize the biogeochemical responses. Using mixed effects models, we found that the role of different biogeochemical processes, such as sulfur cycling, changed throughout the experiment, underscoring the complex nature of GHG emissions in tidal salt marsh soils. Overall, soils subjected to a salinity decrease had greater GHG emissions than control soils, which were maintained at 17 ppt. The treatment soils had a 24% and 23% increase in global warming potential (20- and 100-year scenarios, respectively) indicating that storm surges can produce pulses of GHG emissions. However, both CH 4 and N 2 O emissions returned to baseline values (following hysteresis responses) when initial conditions were reestablished. The results support the fact that tidal salt marshes are resilient ecosystems, as soil GHG emissions recovered relatively quickly from the pulse event., (Copyright © 2019 Elsevier B.V. All rights reserved.)

Although there is increasing evidence of the temporal correlation between photosynthesis and soil CO(2) efflux, no study has so far tested its generality across the growing season at multiple study sites and across several time scales. Here, we used continuous (hourly) data and applied time series analysis (wavelet coherence analysis) to identify temporal correlations and time lags between photosynthesis and soil CO(2) efflux for three forests from different climates and a grassland. Results showed the existence of multi-temporal correlations at time periods that varied between 1 and 16 d during the growing seasons at all study sites. Temporal correlations were strongest at the 1 d time period, with longer time lags for forests relative to the grassland. The multi-temporal correlations were not continuous throughout the growing season, and were weakened when the effect of variations in soil temperature and CO(2) diffusivity on soil CO(2) efflux was taken into account. Multi-temporal correlations between photosynthesis and soil CO(2) efflux exist, and suggest that multiple biophysical drivers (i.e. photosynthesis, soil CO(2) diffusion, temperature) are likely to coexist for the regulation of allocation and transport speed of carbon during a growing season. Future studies should consider the multi-temporal influence of these biophysical drivers to investigate their effect on the transport of carbon through the soil-plant-atmosphere continuum., (No claim to original US government works. New Phytologist © 2011 New Phytologist Trust.)

We seek to understand how biophysical factors such as soil temperature (Ts), soil moisture (theta), and gross primary production (GPP) influence CO2 fluxes across terrestrial ecosystems. Recent advancements in automated measurements and remote-sensing approaches have provided time series in which lags and relationships among variables can be explored. The purpose of this study is to present new applications of continuous measurements of soil CO2 efflux (F0) and soil CO2 concentrations measurements. Here we explore how variation in Ts, theta, and GPP (derived from NASA's moderate-resolution imaging spectroradiometer [MODIS]) influence F0 and soil CO2 production (Ps). We focused on seasonal variation and used continuous measurements at a daily timescale across four vegetation types at 13 study sites to quantify: (1) differences in seasonal lags between soil CO2 fluxes and Ts, theta, and GPP and (2) interactions and relationships between CO2 fluxes with Ts, theta, and GPP. Mean annual Ts did not explain annual F0 and Ps among vegetation types, but GPP explained 73% and 30% of the variation, respectively. We found evidence that lags between soil CO2 fluxes and Ts or GPP provide insights into the role of plant phenology and information relevant about possible timing of controls of autotrophic and heterotrophic processes. The influences of biophysical factors that regulate daily F0 and Ps are different among vegetation types, but GPP is a dominant variable for explaining soil CO2 fluxes. The emergence of long-term automated soil CO2 flux measurement networks provides a unique opportunity for extended investigations into F0 and Ps processes in the near future.

The respiratory release of carbon dioxide (CO(2)) from the land surface is a major flux in the global carbon cycle, antipodal to photosynthetic CO(2) uptake. Understanding the sensitivity of respiratory processes to temperature is central for quantifying the climate-carbon cycle feedback. We approximated the sensitivity of terrestrial ecosystem respiration to air temperature (Q(10)) across 60 FLUXNET sites with the use of a methodology that circumvents confounding effects. Contrary to previous findings, our results suggest that Q(10) is independent of mean annual temperature, does not differ among biomes, and is confined to values around 1.4 +/- 0.1. The strong relation between photosynthesis and respiration, by contrast, is highly variable among sites. The results may partly explain a less pronounced climate-carbon cycle feedback than suggested by current carbon cycle climate models.

Here, we explore how interannual variations in environmental factors (i.e. temperature, precipitation and light) influence CO(2) fluxes (gross primary production and ecosystem respiration) in terrestrial ecosystems classified by vegetation type and the mycorrhizal type of dominant plants (arbuscular mycorrhizal (AM) or ectomycorrhizal (EM)). We combined 236 site-year measurements of terrestrial ecosystem CO(2) fluxes and environmental factors from 50 eddy-covariance flux tower sites with information about climate, vegetation type and dominant plant species. Across large geographical distances, interannual variations in ecosystem CO(2) fluxes for EM-dominated sites were primarily controlled by interannual variations in mean annual temperature. By contrast, interannual variations in ecosystem CO(2) fluxes at AM-dominated sites were primarily controlled by interannual variations in precipitation. This study represents the first large-scale assessment of terrestrial CO(2) fluxes in multiple vegetation types classified according to dominant mycorrhizal association. Our results support and complement the hypothesis that bioclimatic conditions influence the distribution of AM and EM systems across large geographical distances, which leads to important differences in the major climatic factors controlling ecosystem CO(2) fluxes.

While wetlands are the largest natural source of methane (CH4 ) to the atmosphere, they represent a large source of uncertainty in the global CH4 budget due to the complex biogeochemical controls on CH4 dynamics. Here we present, to our knowledge, the first multi-site synthesis of how predictors of CH4 fluxes (FCH4) in freshwater wetlands vary across wetland types at diel, multiday (synoptic), and seasonal time scales. We used several statistical approaches (correlation analysis, generalized additive modeling, mutual information, and random forests) in a wavelet-based multi-resolution framework to assess the importance of environmental predictors, nonlinearities and lags on FCH4 across 23 eddy covariance sites. Seasonally, soil and air temperature were dominant predictors of FCH4 at sites with smaller seasonal variation in water table depth (WTD). In contrast, WTD was the dominant predictor for wetlands with smaller variations in temperature (e.g., seasonal tropical/subtropical wetlands). Changes in seasonal FCH4 lagged fluctuations in WTD by ~17 ± 11 days, and lagged air and soil temperature by median values of 8 ± 16 and 5 ± 15 days, respectively. Temperature and WTD were also dominant predictors at the multiday scale. Atmospheric pressure (PA) was another important multiday scale predictor for peat-dominated sites, with drops in PA coinciding with synchronous releases of CH4 . At the diel scale, synchronous relationships with latent heat flux and vapor pressure deficit suggest that physical processes controlling evaporation and boundary layer mixing exert similar controls on CH4 volatilization, and suggest the influence of pressurized ventilation in aerenchymatous vegetation. In addition, 1- to 4-h lagged relationships with ecosystem photosynthesis indicate recent carbon substrates, such as root exudates, may also control FCH4. By addressing issues of scale, asynchrony, and nonlinearity, this work improves understanding of the predictors and timing of wetland FCH4 that can inform future studies and models, and help constrain wetland CH4 emissions.

Globally, soils store two to three times as much carbon as currently resides in the atmosphere, and it is critical to understand how soil greenhouse gas (GHG) emissions and uptake will respond to ongoing climate change. In particular, the soil-to-atmosphere CO2 flux, commonly though imprecisely termed soil respiration (RS ), is one of the largest carbon fluxes in the Earth system. An increasing number of high-frequency RS measurements (typically, from an automated system with hourly sampling) have been made over the last two decades; an increasing number of methane measurements are being made with such systems as well. Such high frequency data are an invaluable resource for understanding GHG fluxes, but lack a central database or repository. Here we describe the lightweight, open-source COSORE (COntinuous SOil REspiration) database and software, that focuses on automated, continuous and long-term GHG flux datasets, and is intended to serve as a community resource for earth sciences, climate change syntheses and model evaluation. Contributed datasets are mapped to a single, consistent standard, with metadata on contributors, geographic location, measurement conditions and ancillary data. The design emphasizes the importance of reproducibility, scientific transparency and open access to data. While being oriented towards continuously measured RS , the database design accommodates other soil-atmosphere measurements (e.g. ecosystem respiration, chamber-measured net ecosystem exchange, methane fluxes) as well as experimental treatments (heterotrophic only, etc.). We give brief examples of the types of analyses possible using this new community resource and describe its accompanying R software package.

Global modeling efforts indicate semiarid regions dominate the increasing trend and interannual variation of net CO2 exchange with the atmosphere, mainly driven by water availability. Many semiarid regions are expected to undergo climatic drying, but the impacts on net CO2 exchange are poorly understood due to limited semiarid flux observations. Here we evaluated 121 site-years of annual eddy covariance measurements of net and gross CO2 exchange (photosynthesis and respiration), precipitation, and evapotranspiration (ET) in 21 semiarid North American ecosystems with an observed range of 100 - 1000 mm in annual precipitation and records of 4-9 years each. In addition to evaluating spatial relationships among CO2 and water fluxes across sites, we separately quantified site-level temporal relationships, representing sensitivity to interannual variation. Across the climatic and ecological gradient, photosynthesis showed a saturating spatial relationship to precipitation, whereas the photosynthesis-ET relationship was linear, suggesting ET was a better proxy for water available to drive CO2 exchanges after hydrologic losses. Both photosynthesis and respiration showed similar site-level sensitivity to interannual changes in ET among the 21 ecosystems. Furthermore, these temporal relationships were not different from the spatial relationships of long-term mean CO2 exchanges with climatic ET. Consequently, a hypothetical 100-mm change in ET, whether short term or long term, was predicted to alter net ecosystem production (NEP) by 64 gCm(-2) yr(-1). Most of the unexplained NEP variability was related to persistent, site-specific function, suggesting prioritization of research on slow-changing controls. Common temporal and spatial sensitivity to water availability increases our confidence that site-level responses to interannual weather can be extrapolated for prediction of CO2 exchanges over decadal and longer timescales relevant to societal response to climate change.

Forest ecosystems can act as sinks of carbon and thus mitigate anthropogenic carbon emissions. When forests are actively managed, treatments can alter forests carbon dynamics, reducing their sink strength and switching them from sinks to sources of carbon. These effects are generally characterized by fast temporal dynamics. Hence this study monitored for over a decade the impacts of management practices commonly used to reduce fire hazards on the carbon dynamics of mixed-conifer forests in the Sierra Nevada, California, USA. Soil CO2 efflux, carbon pools (i.e. soil carbon, litter, fine roots, tree biomass), and radial tree growth were compared among un-manipulated controls, prescribed fire, thinning, thinning followed by fire, and two clear-cut harvested sites. Soil CO2 efflux was reduced by both fire and harvesting (ca. 15%). Soil carbon content (upper 15 cm) was not significantly changed by harvest or fire treatments. Fine root biomass was reduced by clear-cut harvest (60-70%) but not by fire, and the litter layer was reduced 80% by clear-cut harvest and 40% by fire. Thinning effects on tree growth and biomass were concentrated in the first year after treatments, whereas fire effects persisted over the seven-year post-treatment period. Over this period, tree radial growth was increased (25%) by thinning and reduced (12%) by fire. After seven years, tree biomass returned to pre-treatment levels in both fire and thinning treatments; however, biomass and productivity decreased 30%-40% compared to controls when thinning was combined with fire. The clear-cut treatment had the strongest impact, reducing ecosystem carbon stocks and delaying the capacity for carbon uptake. We conclude that post-treatment carbon dynamics and ecosystem recovery time varied with intensity and type of treatments. Consequently, management practices can be selected to minimize ecosystem carbon losses while increasing future carbon uptake, resilience to high severity fire, and climate related stresses.

Application of the best available science to improve quantification of greenhouse gas (GHG) emissions at regional and national scales is key to climate action. Here, we present a two‐decade (2000–2019) GHG (CO2, CH4, and N2O) budget for Mexico derived from multiple products. Data from the National GHG Inventory, global observations, and the scientific literature were compared to identify knowledge gaps on GHG flux dynamics and discrepancies among estimates. Total mean annual GHG emissions were estimated at 695–910 TgCO2‐eq year−1 over these two decades, with 70% of the emissions attributable to CO2, 23% to CH4, and 5% to N2O (2% to other gases). When divided by sectors, we found agreement across emission estimates from various sources for fossil fuels, cattle, agriculture, and waste for all GHGs. However, considerable discrepancies were identified in the fluxes from terrestrial ecosystems. The disagreement was particularly large for the land CO2 sink, where net biome production estimations from the national inventory were double those from any other observational product. Extensive knowledge gaps exist, mainly related to aquatic systems (e.g., outgassing in rivers) and the lateral fluxes (e.g., wood trade). In addition, limited information is available on CH4 emissions from wetlands and soil CH4 consumption. We expect these results to guide future research to reduce estimation uncertainties and fill the information gaps across Mexico. Plain Language Summary: Mexico represents the 13th global emitter of greenhouse gases (GHGs) and the second among Latin American countries, releasing roughly 485 million tons of CO2, six million tons of CH4 (equivalent to 160 million tons of CO2), and 0.1 million tons of N2O (equivalent to 35 million tons of CO2). However, large uncertainties prevail in the estimation of several components of each flux; thus, the application of the best available science to account for and improve these estimations is essential for sound mitigation policies. Here, we used multiple available products (national inventories, satellites, flux towers, dynamic global vegetation models, inversion models, and other data sources) to advance our understanding of the GHG budget of Mexico over the last 20 years. We found that fluxes from fossil fuel combustion (including energy production, transportation, industry, etc.), agriculture, livestock, and waste management agree remarkably well across products. In contrast, we documented large differences among products for land‐based fluxes (e.g., CO2 capture) and scant available information on aquatic ecosystems (e.g., CH4 emission from wetlands) and on lateral fluxes (e.g., emissions from trade). We expect these results to help in guiding future measuring efforts and policies to mitigate GHG emissions from Mexico. Key Points: A greenhouse gas (GHG) budget for Mexico was calculated based on multiple products. We estimated fluxes for CO2, CH4, and N2O over the 2000–2019 periodTotal GHG emissions were 695 TgCO2‐eq year−1 in the national inventory and as much as a mean 910 TgCO2‐eq year−1 across productsGHG fluxes agreed well across products for emissions, but large discrepancies exist for the land CO2 and CH4 sinks [ABSTRACT FROM AUTHOR]

Global soils store at least twice as much carbon as Earth's atmosphere.sup.1,2. The global soil-to-atmosphere (or total soil respiration, R.sub.S) carbon dioxide (CO.sub.2) flux is increasing.sup.3,4, but the degree to which climate change will stimulate carbon losses from soils as a result of heterotrophic respiration (R.sub.H) remains highly uncertain.sup.5-8. Here we use an updated global soil respiration database.sup.9 to show that the observed soil surface R.sub.H:R.sub.S ratio increased significantly, from 0.54 to 0.63, between 1990 and 2014 (P = 0.009). Three additional lines of evidence provide support for this finding. By analysing two separate global gross primary production datasets.sup.10,11, we find that the ratios of both R.sub.H and R.sub.S to gross primary production have increased over time. Similarly, significant increases in R.sub.H are observed against the longest available solar-induced chlorophyll fluorescence global dataset, as well as gross primary production computed by an ensemble of global land models. We also show that the ratio of night-time net ecosystem exchange to gross primary production is rising across the FLUXNET2015.sup.12 dataset. All trends are robust to sampling variability in ecosystem type, disturbance, methodology, CO.sub.2 fertilization effects and mean climate. Taken together, our findings provide observational evidence that global R.sub.H is rising, probably in response to environmental changes, consistent with meta-analyses.sup.13-16 and long-term experiments.sup.17. This suggests that climate-driven losses of soil carbon are currently occurring across many ecosystems, with a detectable and sustained trend emerging at the global scale.Global soil respiration is rising, probably in response to environmental changes, suggesting that climate-driven losses of soil carbon are occurring worldwide., Author(s): Ben Bond-Lamberty [sup.1] , Vanessa L. Bailey [sup.2] , Min Chen [sup.1] , Christopher M. Gough [sup.3] , Rodrigo Vargas [sup.4] Author Affiliations:(1) Pacific Northwest National Laboratory, Joint Global [...]

Quantifying the role of soils in nature-based solutions require accurate estimates of soil greenhouse gas (GHG) fluxes. Technological advances allow to simultaneously measure multiple GHGs and now is possible to provide complete GHG budgets from soils (i.e., CO2, CH4 and N2O fluxes). We propose that there is a conflict between the convenience of simultaneously measuring multiple soil GHG fluxes at fixed time intervals (e.g., once, or twice per month) and the intrinsic temporal variability and patterns of different GHG fluxes. Information derived from fixed time intervals -as is commonly done during manual field campaigns- had limitations to reproduce statistical properties, temporal dependence, annual budgets, and associated uncertainty, when compared with information derived from continuous measurements (i.e., automated hourly measurements) for all soil GHG fluxes. We present a novel approach (i.e., temporal univariate Latin Hypercube sampling) that can be applied to optimize monitoring efforts of GHG fluxes across time. We suggest that multiple GHG fluxes should not be simultaneously measured at few fixed time intervals (especially once a month), but an optimized sampling approach can be used to reduce bias and uncertainty. These results have implications for assessing GHG fluxes from soils and consequently reduce uncertainty on the role of soils in nature-based solutions. [ABSTRACT FROM AUTHOR]

Tidal wetlands are comprised of complex interdependent pathways where measurements of carbon exchange are often scale dependent. Common data collection methods (i.e., chambers and eddy covariance) are inherently constrained to different spatial and temporal scales which could generate biased information for applications of carbon accounting, identifying functional relationships and predicting future responses to climate change. Consequently, it is needed to systematically evaluate measurements derived from multiple approaches to identify differences and how techniques complement each other to reconcile interpretations. To accomplish this, we tested ecosystem‐scale eddy covariance with plot‐scale chamber measurements within a temperate salt marsh. We found good agreement (R2 = 0.71–0.95) when comparing measurements of CH4 emissions and CO2 exchange but this agreement was dependent upon canopy phenology with discrepancies mainly arising during senescence and dormancy phenophases. The environmental drivers for CH4 and CO2 fluxes were mostly preserved across different measurement techniques, but the number of drivers increases while their individual strength decreases at the ecosystem scale. Empirical upscaling models parameterized with chamber measurements overestimated annual net ecosystem exchange (NEE; 108%) and gross primary production (GPP; 12%) while underestimating ecosystem respiration (Reco; 14%) and CH4 emissions (69%) compared to eddy covariance measurements. Our results suggest that the environmental complexity of CH4 and CO2 fluxes in salt marshes may be underestimated by chamber‐based measurements, and highlights how different techniques are complementary while considering limitations at each level of measurement. Plain Language Summary: Tidal wetlands are important landscape features which play a significant role in regional carbon cycling between land, ocean and the atmosphere. These ecosystems are very productive, with high rates of photosynthesis during the growing season, but they also emit methane and carbon dioxide which can offset carbon gains. Therefore, it is important to accurately measure and understand what factors regulate carbon cycling processes to better characterize how these systems function, calculate carbon budgets and help predict possible reposes to climate change. We compare several common measurement techniques including chamber and eddy covariance which incorporate information across different scales to evaluate the level of agreement and how these techniques can be combined to increase our understanding of ecosystem functionality. We found that techniques agree well when comparing time‐matched measurement windows but different plant development stages across the ecosystem likely impacted agreement during autumn and early winter. We found that primary factors regulating fluxes were similar across techniques but when considering data collected over more hours of the day with eddy covariance, additional factors were identified indicating measurements from chambers which are limited in both coverage across space and time may present an oversimplified view within tidal wetlands. Key Points: Plot‐level and eddy covariance measurements had good overall agreement, but the strength of agreement was dependent upon plant phenologyThe relevance and influence of environmental drivers varied for all fluxes across different techniquesModels parameterized with plot‐level data overestimated ecosystem‐scale net ecosystem exchange but underestimated CO2 and CH4 emissions [ABSTRACT FROM AUTHOR]

Over the last 100 years a shifting human population has seen large proportions of people relocating from rural agricultural regions to urban and suburban areas. Extensive development within these modified lands has altered soils and natural vegetative cover which directly affects many biophysical processes. The current study examines over 4 years of continuous (i.e., hourly) CO2 measurements within a suburban forest floor and lawn to elucidate daily, seasonal and annual patterns of biogenic CO2 emissions while identifying the importance of forcing biophysical drivers (e.g., temperature, moisture). On an annual basis, the suburban forest floor and lawn emitted 587 ± 15.5 g C m−2 yr−1 and 1220 ± 31.5 g C m−2 yr−1, respectively. Overall, temperature was a primary driver of CO2 emissions within both forest and lawn, yet the forest was more complex with additional influence from tree canopy phenology and soil moisture. Diurnal CO2 emission patterns were different between land use types in their response to temperature as we found a counterclockwise hysteresis in forest and an in-sync response in the lawn. In all cases, the strength of biophysical drivers and observed CO2 emission rates were different depending on land use type and throughout the different seasons. Our findings provide insights for modeling and benchmarking biogenic CO2 dynamics across suburban ecosystems. [ABSTRACT FROM AUTHOR]

Coastal salt marshes store large amounts of carbon but the magnitude and patterns of greenhouse gas (GHG; i.e., carbon dioxide (CO2) and methane (CH4)) fluxes are unclear. Information about GHG fluxes from these ecosystems comes from studies of sediments or at the ecosystem‐scale (eddy covariance) but fluxes from tidal creeks are unknown. We measured GHG concentrations in water, water quality, meteorological parameters, sediment CO2 efflux, ecosystem‐scale GHG fluxes, and plant phenology; all at half‐hour intervals over 1 year. Manual creek GHG flux measurements were used to calculate gas transfer velocity (k) and parameterize a model of water‐to‐atmosphere GHG fluxes. The creek was a source of GHGs to the atmosphere where tidal patterns controlled diel variability. Dissolved oxygen and wind speed were negatively correlated with creek CH4 efflux. Despite lacking a seasonal pattern, creek CO2 efflux was correlated with drivers such as turbidity across phenological phases. Overall, nighttime creek CO2 efflux (3.6 ± 0.63 μmol/m2/s) was at least 2 times higher than nighttime marsh sediment CO2 efflux (1.5 ± 1.23 μmol/m2/s). Creek CH4 efflux (17.5 ± 6.9 nmol/m2/s) was 4 times lower than ecosystem‐scale CH4 fluxes (68.1 ± 52.3 nmol/m2/s) across the year. These results suggest that tidal creeks are potential hotspots for CO2 emissions and could contribute to lateral transport of CH4 to the coastal ocean due to supersaturation of CH4 (>6,000 μmol/mol) in water. This study provides insights for modeling GHG efflux from tidal creeks and suggests that changes in tide stage overshadow water temperature in determining magnitudes of fluxes. Key Points: A tidal creek was a hotspot for CO2 efflux compared to the surrounding wetlandChanges in tide stage, not water temperature variability, regulated diel creek CO2 and CH4 effluxThe relative influence of nontidal drivers of creek CO2 and CH4 efflux varied by plant phenological phases [ABSTRACT FROM AUTHOR]

Tree stems exchange carbon dioxide, methane, and nitrous oxide with the atmosphere. The biophysical mechanisms controlling these fluxes are not fully understood, and consequently are not included in process-based models. We highlight advances and opportunities that will allow quantification of the role of these plant structures in the local-to-global balance of greenhouse gases (GHGs). [ABSTRACT FROM AUTHOR]

Forest soils are an important component of CO2 and CH4 fluxes at the global scale, but the magnitude of these fluxes varies greatly in space and time within a landscape. Understanding the spatial and temporal distributions of these fluxes across complex landscapes remains a major challenge for researchers and land managers alike. We investigated the spatiotemporal variability of soil-atmosphere CO2 and CH4 fluxes and the relationships of these fluxes to chemical and physical soil properties distributed across a topographically-heterogeneous landscape. Soil CO2 and CH4 fluxes were measured along with soil temperature, moisture, bulk density, texture, carbon, sorption capacity, and dissolved organic matter quality over 2 years along hillslope transects spanning valley bottom, transition zone, and upland landscape positions in a temperate forest watershed. Transition zone soil CO2 efflux was 54-160% higher than low-lying valley bottoms, and 15-54% higher than uplands. Net seasonal CH4 uptake was 58-150% higher in transition zone soils than in uplands, while valley bottoms were occasionally large net sources (up to 19 nmol CH4 m−2 s−1). Soil CO2 efflux and net CH4 uptake were both positively associated with seasonal temperature, and were highest in soils with relatively high carbon and clay content, and relatively low bulk density, moisture, and sorption capacity. We concluded that: (1) transition zone soils act as landscape hotspots for net CH4 uptake in addition to CO2 efflux, and (2) that this spatial distribution is more consistent across seasons for net CH4 uptake than for CO2 efflux. [ABSTRACT FROM AUTHOR]

Incorporation of silica-rich rice husk residue into flooded paddy soil decreases arsenic uptake by rice. However, the impact of this practice on soil greenhouse gas (GHG) emissions and elemental cycling is unresolved particularly as amended soils experience recurrent flooding and drying cycles. We evaluated the impact of pre-incubated silica-rich rice residue incorporation to soils on pore water chemistry and soil GHG fluxes (i.e., CO 2 , CH 4 , N 2 O) over a flooding and drying cycle typical of flooded rice cultivation. Soils pre-incubated with rice husk had 4-fold higher pore water Si than control and 2-fold higher than soils pre-incubated with rice straw, whereas the pore water As and Fe concentrations in soils amended with pre-incubated straw and husk were unexpectedly similar (maximum ~ 0.85 μM and ~ 450 μM levels, respectively). Pre-incubation of residues did not affect Si but did affect the pore water levels of As and Fe compared to previous studies using fresh residues where straw amended soils had higher As and Fe in pore water. The global warming potential (GWP) of soil GHG emissions decreased in the order straw (612 ± 76 g CO 2 -eq m − 2 ) > husk (367 ± 42 g CO 2 -eq m − 2 ) > ashed husk = ashed straw (251 ± 26 and 278 ± 28 g CO 2 -eq m − 2 ) > control (186 ± 23 g CO 2 -eq m − 2 ). The GWP increase due to pre-incubated straw amendment was due to: a) larger N 2 O fluxes during re-flooding; b) smaller contributions from larger CH 4 fluxes during flooded periods; and c) higher CH 4 and CO 2 fluxes at the onset of drainage. In contrast, the GWP of the husk amendment was dominated by CO 2 and CH 4 emissions during flooded and drainage periods, while ashed amendments increased CO 2 emissions particularly during drainage. This experiment shows that ashed residues and husk addition minimizes GWP of flooded soils and enhances pore water Si compared to straw addition even after pre-incubation. [ABSTRACT FROM AUTHOR]

Soil CO2 efflux is the primary source of CO2 emissions from terrestrial ecosystems to the atmosphere. The rates of this flux vary in time and space producing hot moments (sudden temporal high fluxes) and hot spots (spatially defined high fluxes), but these high reaction rates are rarely studied in conjunction with each other. We studied temporal and spatial variation of soil CO2 efflux in a water-limited Mediterranean ecosystem in Baja California, Mexico. Soil CO2 efflux increased 522% during a hot moment after rewetting of soils following dry summer months. Monthly precipitation was the primary driver of the seasonal trend of soil CO2 efflux (including the hot moment) and through changes in soil volumetric water content (VWC) it influenced the relationship between CO2 efflux and soil temperature. Geostatistical analyses showed that the spatial dependence of soil CO2 efflux changed between two contrasting seasons (dry and wet). During the dry season high soil VWC was associated with high soil CO2 efflux, and during the wet season the emergence of a hot spot of soil CO2 efflux was associated with higher root biomass and leaf area index. These results suggest that sampling designs should accommodate for changes in spatial dependence of measured variables. The spatio-temporal relationships identified in this study are arguably different from temperate ecosystems where the majority of soil CO2 efflux research has been done. This study provides evidence of the complexity of the mechanisms controlling the spatio-temporal variability of soil CO2 efflux in water-limited ecosystems. [ABSTRACT FROM AUTHOR]

Summary [ABSTRACT FROM AUTHOR]

• Plant phenological phases influence NEE and CH 4 exchange • CO 2 and CH 4 emissions during senescence and dormancy overshadowed annual carbon uptakes • Light availability partially explained NEE variability • Lower water table level increased ecosystem-scale CH 4 emissions • This temperate tidal salt marsh was a net source of carbon to the atmosphere Salt marshes are large carbon reservoirs as part of blue carbon ecosystems. Unfortunately, there is limited information about the net ecosystem (NEE) and methane (CH 4) exchange between salt marshes and the atmosphere to fully understand their carbon dynamics. We tested the influence of biophysical drivers by plant phenological phases (i.e., Greenup, Maturity, Senescence and Dormancy) on NEE and CH 4 exchange in a grass-dominated temperate tidal salt marsh. We used three years of data derived from eddy covariance, PhenoCam (to measure vegetation phenology), and ancillary meteorological and water/soil variables. Overall, NEE showed significant differences among all phenological phases (p < 0.05), while CH 4 exchange had significant differences among all phases except for Greenup and Dormancy. Net CO 2 uptake was higher across Maturity (-61 g C-CO 2 m2), while CO 2 emissions were higher during Dormancy (182 g C-CO 2 m2). The lower but constant CO 2 emissions during Dormancy overshadowed the CO 2 uptake during the growing season and contributed to >72% of the annual CO 2 emissions in this ecosystem. Net CH 4 emissions were higher during Maturity (3.7 g C-CH 4 m2) and Senescence (4.2 g C-CH 4 m2). Photosynthetically active radiation (PAR) substantially influenced (r2 > 0.57) daytime NEE across phenological phases, but a combination of variables including water table level (WTL), water temperature and atmospheric pressure were relevant to explain CH 4 exchange. The study site was an overall net carbon source to the atmosphere with annual emissions of 13-201 g C-CO 2 m−2yr−1 and 8.5-15.2 g C-CH 4 m−2yr−1. Our findings provide insights on: a) the role of plant phenological phases on ecosystem-scale CO 2 and CH 4 fluxes; b) challenges for modeling ecosystem-scale CO 2 and CH 4 fluxes in salt marshes; and c) the potential net loss of carbon to the atmosphere that should be considered for carbon management and accounting in these ecosystems. [ABSTRACT FROM AUTHOR]

Highlights • Soil flux predictions closely matched observations across seasons. • We used a machine learning approach to upscale soil fluxes and estimate uncertainty. • Temperature was positively related to CO 2 efflux and CH 4 uptake. • CH 4 fluxes had bi-directional responses to seasonal precipitation patterns. Abstract Upscaling soil-atmosphere greenhouse gas (GHG) fluxes across complex landscapes is a major challenge for environmental scientists and land managers. This study employs a quantile-based digital soil mapping approach for estimating the spatially continuous distributions (2 m spatial resolution) and uncertainties of seasonal mean mid-day soil CO 2 and CH 4 fluxes. This framework was parameterized using manual chamber measurements collected over two years within a temperate forested headwater watershed. Model accuracy was highest for early (r2 = 0.61) and late summer (r2 = 0.64) for CO 2 and CH 4 fluxes. Model uncertainty was generally lower for predicted CO 2 fluxes than CH 4 fluxes. Within the study area, predicted seasonal mean CO 2 fluxes ranged from 0.17 to 0.58 μmol m−2 s−1 in winter, and 1.4 to 5.1 μmol m−2 s−1 in early summer. Predicted CH 4 fluxes across the study area ranged from −0.52 to 0.02 nmol m−2 s−1 in winter, and −2.1 to 0.61 nmol m−2 s−1 in early and late summer. The models estimated a per hectare net GHG potential ranging from 0.44 to 4.7 kg CO 2 eq. hr−1 in winter and early summer, with an estimated 0.4 to 1.5% of emissions offset by CH 4 uptake. Flux predictions fell within ranges reported in other temperate forest systems. Soil CO 2 fluxes were more sensitive to seasonal temperature changes than CH 4 fluxes, with significant temperature relationships for soil CO 2 emissions and CH 4 uptake in pixels with high slope angles. In contrast, soil CH 4 fluxes from flat low-lying areas near the stream network within the watershed were significantly correlated to seasonal precipitation. This study identified key challenges for modeling high spatial resolution soil CO 2 and CH 4 fluxes, and suggests a larger spatial heterogeneity and complexity of underlying processes that govern CH 4 fluxes. [ABSTRACT FROM AUTHOR]