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An experimental system for sampling trace gas fluxes through seasonal snowpack was deployed at a subalpine site near treeline at Niwot Ridge, Colorado. The sampling manifold was in place throughout the entire snow-covered season for continuous air sampling with minimal disturbance to the snowpack. A series of gases (carbon dioxide, water vapor, nitrous oxide, nitric oxide, ozone, volatile organic compounds) was determined in interstitial air withdrawn at eight heights in and above the snowpack at ~hourly intervals. In this paper, carbon dioxide data from 2007 were used for evaluation of this technique. Ancillary data recorded inlcuded snow physical properties, i.e., temperature, pressure, and density. Various vertical concentration gradients were determined from the multiple height measurements, which allowed calculation of vertical gas fluxes through the snowpack using Fick’s 1st law of diffusion. Comparison of flux results obtained from different height inlet combinations show that under most conditions fluxes derived from individual gradient intervals agree with the overall median of all data within a factor of 1.5. Winds were found to significantly influence gas concentration and gradients in the snowpack. Under the highest observed wind conditions, concentration gradients and calculated fluxes dropped to as low as 13% of non-wind conditions. Measured differential pressure amplitude exhibited a linear relationship with wind speed. This suggests that wind speed is a sound proxy for assessing advection transport in the snow. Neglecting the wind-pumping effect resulted in considerable underestimation of gas fluxes. An analysis of dependency of fluxes on wind speeds during a 3-week period in mid-winter determined that over this period actual gas fluxes were most likely 57% higher than fluxes calculated by the diffusion method, which omits the wind pumping dependency. [ABSTRACT FROM AUTHOR]

Boreal forests are highly susceptible to wildfire, and post-fire changes in soil temperature and moisture have the potential to transform large areas of the landscape from a net sink to a net source of carbon (C). Understanding the ecological controls that regulate these disturbance effects is critical to developing models of ecosystem response to changes in fire frequency and severity. This paper combines laboratory and field measurements along a chronosequence of burned black spruce stands into regression analyses and models that assess relationships between moss succession, soil microclimate, decomposition, and C source-sink dynamics. Results indicate that post-fire changes in temperature and substrate quality increased decomposition in humic materials by a factor of 3.0 to 4.0 in the first 7 years after fire. Bryophyte species exhibited a distinct successional pattern in the first five decades after fire that corresponded to decreased soil temperature and increased C accumulation in organic soils. Potential rates of C exchange in mosses were greatest in early successional species and declined as the stand matured. Residual sources of CO2 (those not attributed to moss respiration or humic decomposition) increased as a function of stand age, reflecting increased contributions from roots as the stand recovered from disturbance. Together, the field measurements, laboratory experiments, and models provide strong evidence that interactions between moss and plant succession, soil temperature, and soil moisture largely regulate C source-sink dynamics from black spruce systems in the first century following fire disturbance. [ABSTRACT FROM AUTHOR]

This paper reviews the role of soil respiration in determining ecosystem carbon balance, and the conceptual basis for measuring and modeling soil respiration. We developed it to provide background and context for this special issue on soil respiration and to synthesize the presentations and discussions at the workshop. Soil respiration is the largest component of ecosystem respiration. Because autotrophic and heterotrophic activity belowground is controlled by substrate availability, soil respiration is strongly linked to plant metabolism, photosynthesis and litterfall. This link dominates both base rates and short-term fluctuations in soil respiration and suggests many roles for soil respiration as an indicator of ecosystem metabolism. However, the strong links between above and belowground processes complicate using soil respiration to understand changes in ecosystem carbon storage. Root and associated mycorrhizal respiration produce roughly half of soil respiration, with much of the remainder derived from decomposition of recently produced root and leaf litter. Changes in the carbon stored in the soil generally contribute little to soil respiration, but these changes, together with shifts in plant carbon allocation, determine ecosystem carbon storage belowground and its exchange with the atmosphere. Identifying the small signal from changes in large, slow carbon pools in flux dominated by decomposition of recent material and autotrophic and mycorrhizal respiration is a significant challenge. A mechanistic understanding of the belowground carbon cycle and of the response of different components to the environment will aid in identifying this signal. Our workshop identified information needs to help build that understanding: (1) the mechanisms that control the coupling of canopy and belowground processes; (2) the responses of root and heterotrophic respiration to environment; (3) plant carbon allocation patterns, particularly in different forest developmental stages, and in response to treatments (warming, CO2, nitrogen additions); and (4) coupling measurements of soil respiration with aboveground processes and changes in soil carbon. Multi-factor experiments need to be sufficiently long to allow the systems to adjust to the treatments. New technologies will be necessary to reduce uncertainty in estimates of carbon allocation, soil carbon pool sizes, and different responses of roots and microbes to environmental conditions. [ABSTRACT FROM AUTHOR]

The major greenhouse gases in streams and rivers, carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O), can contribute significantly to regional greenhouse gas (GHG) budgets, and each appears to be responding to multiple drivers. Recent work suggests that tropical water bodies may be hot spots of GHG emissions due to high primary productivity in their watersheds, but tropical streams and rivers have historically been underrepresented in studies of GHG concentration and emissions. We use a five-year record of weekly water chemistry and dissolved gas data from eight tropical watersheds of varying lithology and redox conditions in the Luquillo Mountains of Puerto Rico to examine controls on GHG variability and estimate gas flux. Streams were frequently supersaturated in all three gases indicating that streams in this tropical landscape are sources of GHGs to the atmosphere. Concentrations of CO2 and N2O were associated with lateral inputs from the surrounding landscape, whereas CH4 concentrations correlated with in-stream oxygen availability and lithology. Our results underscore the importance of including tropical sites in global syntheses and budgets and the role of both in-stream biological and physical processes as well as landscape attributes that contribute to the export of gases to the fluvial network and atmosphere. [ABSTRACT FROM AUTHOR]

Eutrophication has been identified as the primary cause of water quality deterioration in inland waters worldwide, often associated with algal blooms or fish kills. Eutrophication can be controlled through watershed management and in-lake measures. An extreme heatwave event, through its impact on mineralization rates and internal nutrient loading (phosphorus—P, and nitrogen—N), could counteract eutrophication control measures. We investigated how the effectiveness of a nutrient abatement technique is impacted by an extreme heatwave, and to what extent biogeochemical processes are modulated by exposure to heatwaves. To this end, we carried out a sediment-incubation experiment, testing the effectiveness of lanthanum-modified bentonite (LMB) in reducing nutrients and greenhouse gas emissions from eutrophic sediments, with and without exposure to an extreme heatwave. Our results indicate that the effectiveness of LMB may be compromised upon exposure to an extreme heatwave event. This was evidenced by an increase in concentration of 0.08 ± 0.03 mg P/L with an overlying water volume of 863 ± 21 mL, equalling an 11% increase, with effects lasting to the end of the experiment. LMB application generally showed no effect on nitrogen species, while the heatwave stimulated nitrification, resulting in ammonium loss and accumulation of dissolved oxidized nitrogen species as well as increased dissolved nitrous oxide concentrations. In addition, carbon dioxide (CO2)-equivalent was more than doubled during the heatwave relative to the reference temperature, and LMB application had no effect on mitigating them. Our sediment incubation experiment indicates that the rates of biogeochemical processes can be significantly accelerated upon heatwave exposure, resulting in a change in fluxes of nutrient and greenhouse gas between sediment and water. The current efforts in eutrophication control will face more challenges under future climate scenarios with more frequent and intense extreme events as predicted by the IPCC. [ABSTRACT FROM AUTHOR]

Ponds are active components of the global carbon cycle processing and emitting carbon dioxide and methane to the atmosphere. These common habitats frequently experience seasonal water table variations resulting in periodically air-exposed sediments. However, the influence of these events on both the system scale carbon balance and in-pond environmental conditions remains poorly studied. We took advantage of an extraordinarily warm and dry summer to quantify the CO2 efflux from air-exposed sediments and water surfaces in desiccating ponds on Öland, Sweden. Simultaneously, we modelled metabolism and measured environmental variables within the ponds. We found that air-exposed sediments had high CO2 effluxes greatly exceeding that from the water surfaces. Sediment water content influenced the temperature and strongly regulated the CO2 efflux gradually approaching zero as water evaporated. Within the desiccating ponds, respiration was generally higher than gross primary production, but was lower compared to the same ponds with higher water table. These findings highlight the role of periodically air-exposed pond sediments as sites of highly active carbon processes. Not only is this important for the system-scale carbon in ponds, but it may also influence the destiny of buried carbon in lakes subject to climate changes. The environmental conditions within desiccating ponds, most notably high water temperatures and poor oxygen conditions, further iterate the dynamics and extreme nature of ponds. [ABSTRACT FROM AUTHOR]

In the last 40 years, large areas of the Mau forest, the largest contiguous tropical montane forest in East Africa, have been cleared for agriculture. To date, there are no empirical data on how this land use change affects carbon dioxide (CO2) fluxes from soil respiration and soil methane (CH4) fluxes. This study reports measured annual soil CO2 and CH4 fluxes from the native Mau forest and previously forested lands converted to smallholder grazing land, smallholder and commercial tea plantations and eucalyptus plantations. Fluxes were measured weekly from August 2015 to August 2016 using the static chamber method. Grazing lands had the highest (p = 0.028) cumulative respiratory CO2 fluxes (25.6 ± 2.9 Mg CO2–C ha−1 year−1), whereas lowest fluxes were observed in commercial tea plantations (5.6 ± 0.5 Mg CO2–C ha−1 year−1). Soil respiratory CO2 fluxes were positively correlated with soil pH, but negatively correlated with soil C:N ratio. Annual soil fluxes were explained by soil pH, bulk density and the interaction between soil pH and C:N ratio. Most soils were sinks for atmospheric CH4 across all land use types. Methane uptake was highest for native forest sites (− 3.08 ± 0.35 to − 5.84 ± 0.61 kg CH4–C ha−1 year−1) and for eucalyptus plantations (− 3.43 ± 0.19 kg CH4–C ha−1 year−1). Uptake decreased significantly with increasing land use intensity (smallholder tea plantations: − 1.42 ± 0.09 kg CH4–C ha−1 year−1, commercial tea plantations: − 1.44 ± 0.29 kg CH4–C ha−1 year−1). Soils of smallholder grazing lands had the lowest CH4 uptake rates (− 0.36 ± 0.25 kg CH4–C ha−1 year−1). Annual CH4 uptake was negatively correlated with mean annual soil water-filled pore space (p < 0.01) and bulk density (p = 0.003) and decreased with increasing soil inorganic NH4+ concentrations (p = 0.03). Annual soil CH4 can be explained by mainly soil water content and bulk density and these factors are related to gas diffusion. Our study shows that converting tropical montane forests to managed land use types affects soil CO2 and CH4 fluxes. Specifically, the CH4 sink strength in managed land use types of these montane tropical soils was reduced to less than half of the sink strength in the native forest. Soil respiratory CO2 fluxes were also altered by land use with grazing lands emitting 3–4 times more CO2 than the other land use types. [ABSTRACT FROM AUTHOR]

Tropical peatland ecosystems are a significant component of the global carbon cycle and feature a range of distinct vegetation types, but the extent of links between contrasting plant species, peat biogeochemistry and greenhouse gas fluxes remains unclear. Here we assessed how vegetation affects small scale variation of tropical peatland carbon dynamics by quantifying in situ greenhouse gas emissions over 1 month using the closed chamber technique, and peat organic matter properties using Rock-Eval 6 pyrolysis within the rooting zones of canopy palms and broadleaved evergreen trees. Mean methane fluxes ranged from 0.56 to 1.2 mg m−2 h−1 and were significantly greater closer to plant stems. In addition, pH, ranging from 3.95 to 4.16, was significantly greater closer to stems. A three pool model of organic matter thermal stability (labile, intermediate and passive pools) indicated a large labile pool in surface peat (35-42%), with equivalent carbon stocks of 2236-3065 g m−2. Methane fluxes were driven by overall substrate availability rather than any specific carbon pool. No peat properties correlated with carbon dioxide fluxes, suggesting a significant role for root respiration, aerobic decomposition and/or methane oxidation. These results demonstrate how vegetation type and inputs, and peat organic matter properties are important determinants of small scale spatial variation of methane fluxes in tropical peatlands that are affected by climate and land use change. [ABSTRACT FROM AUTHOR]

Soil N availability for plants and microorganisms depends on the breakdown of soil polymers such as proteins into smaller, assimilable units by microbial extracellular enzymes. Changing climatic conditions are expected to alter protein depolymerization rates over the next decades, and thereby affect the potential for plant productivity. We here tested the effect of increased CO2 concentration, temperature, and drought frequency on gross rates of protein depolymerization, N mineralization, microbial amino acid and ammonium uptake using 15N pool dilution assays. Soils were sampled in fall 2013 from the multifactorial climate change experiment CLIMAITE that simulates increased CO2 concentration, temperature, and drought frequency in a fully factorial design in a temperate heathland. Eight years after treatment initiation, we found no significant effect of any climate manipulation treatment, alone or in combination, on protein depolymerization rates. Nitrogen mineralization, amino acid and ammonium uptake showed no significant individual treatment effects, but significant interactive effects of warming and drought. Combined effects of all three treatments were not significant for any of the measured parameters. Our findings therefore do not suggest an accelerated release of amino acids from soil proteins in a future climate at this site that could sustain higher plant productivity. [ABSTRACT FROM AUTHOR]

Coastal forested wetlands provide important ecosystem services such as carbon sequestration, nutrient retention, and flood protection, but they are also important sources of greenhouse gas emissions. Human appropriation of surface water and extensive ditching and draining of coastal plain landscapes are interacting with rising sea levels to increase the frequency and magnitude of saltwater incursion into formerly freshwater coastal wetlands. Both hydrologic change and saltwater incursion are expected to alter carbon and nutrient cycling in coastal forested wetlands. We performed a full factorial experiment in which we exposed intact soil cores from a coastal forested wetland to experimental marine salt treatments and two hydrologic treatments. We measured the resulting treatment effects on the emissions of carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) over 112 days. Salinity effects were compared across four treatments to isolate the effects of increases in ionic strength from the impact of adding a terminal electron acceptor (SO42−). We compared control treatments (DI addition), to artificial saltwater (ASW, target salinity of 5 parts per thousand) and to two treatments that added sulfate alone (SO42−, at the concentration found in 5 ppt saltwater) and saltwater with the sulfate removed (ASW-SO42−, with the 5 ppt target salinity maintained by adding additional NaCl). We found that all salt treatments suppressed CO2 production, in both drought and flooded treatments. Contrary to our expectations, CH4 fluxes from our flooded cores increased between 300 and 1200% relative to controls in the ASW and ASW-SO42− treatments respectively. In the drought treatments, we saw virtually no CH4 release from any core, while artificial seawater with sulfate increased N2O fluxes by 160% above DI control. In contrast, salt and sulfate decreased N2O fluxes by 72% in our flooded treatments. Our results indicate that salinization of forested wetlands of the coastal plain may have important climate feedbacks resulting from enhanced greenhouse gas emissions and that the magnitude and direction of these emissions are contingent upon wetland hydrology. [ABSTRACT FROM AUTHOR]

Soil respiration is the primary path by which CO2 fixed by land plants returns to the atmosphere. Estimated at approximately 75 x 1015 g C/yr, this large natural flux is likely to increase due changes in the Earth's condition. The objective of this paper is to provide a brief scientific review for policymakers who are concerned that changes in soil respiration may contribute to the rise in CO2 in Earth's atmosphere. Rising concentrations of CO2 in the atmosphere will increase the flux of CO2from soils, while simultaneously leaving a greater store of carbon in the soil. Traditional tillage cultivation and rising temperature increase the flux of CO2 from soils without increasing the stock of soil organic matter. Increasing deposition of nitrogen from the atmosphere may lead to the sequestration of carbon in vegetation and soils. The response of the land biosphere to simultaneous changes in all of these factors is unknown, but a large increase in the soil carbon pool seems unlikely to moderate the rise in atmospheric CO2 during the next century. [ABSTRACT FROM AUTHOR]

The stimulation of plant growth and biomass accumulation by elevated CO2 may be limited by soil nitrogen (N) availability. However, our understanding of the response of soil N cycling to elevated CO2 and when progressive N limitation occurs remains limited. Here, we used an open top chamber experiment to examine the effects of 10 years of elevated CO2 on ecosystem carbon (C) and N dynamics in a Quercus mongolica (oak) dominated system in northeastern China. Elevated CO2 increased oak biomass, C and N stocks and C/N by 26.4, 26.2, 16.5 and 8.6% respectively, which suggests increased plant N demand. Soil gross N mineralization, re-mineralization of microbial N and nitrification were accelerated likely due to increased photosynthesis (by 34.9%) and microbial biomass (by 24.2%) under elevated CO2. Thus, the supply of soil available N can sustain the tree growth stimulated by elevated CO2, and to date progressive N limitation has not happened. Nevertheless, both the annual increase of oak biomass, C and N stocks and C/N ratio and the seasonal variations of soil available N and microbial N concentrations, and net N transformation rates indicated that gradual N deficiency may be occurring and the CO2 fertilization effect has weakened with increasing treatment duration. [ABSTRACT FROM AUTHOR]

The Pantanal is the largest wetland in the world and yet little is known about the variability in carbon (C) dynamics across its flood seasons. We examined the effect of inundation on the C cycle in the 2013-2015 flood cycles illustrated by dissolved CO2, CH4, organic C (DOC) concentration measurements, and optical properties of dissolved organic matter (DOM) evaluated by absorbance and fluorescence spectroscopy with parallel factor analysis (PARAFAC). During the 2015 flood cycle, pCO2 varied between 5973 and 14,292 μatm, with pCH4 concentrations ranging between 2956 and 51,675 μatm respectively, with high temporal variability for both gases. The supersaturation of CO2 and CH4 in relation to the atmospheric equilibrium caused the system to behave as a net source of CO2 and CH4 to the atmosphere with evasion rates of 320 mg CO2 m−2 d−1 and 20 mg CH4 m−2 d−1, respectively. Mean DOC concentration was 7.0 ± 0.4 mg L−1 and did not differ between flood cycles. Higher concentrations of DOC were measured at the start (rising floodwaters) and at the end (receding floodwaters) of flood cycles, while lower DOC concentrations were observed during the peak flood. The PARAFAC analysis indicated the presence of five DOM components: humic (C1 and C2) and fulvic type material (C3) showed the highest relative abundance (68.5% of the total PARAFAC component fluorescence), as well as protein-like material (C4 and C5) derived from microorganisms. Our measured diffusive flux levels were below the range of emissions found for wetlands and floodplains for CO2, but were slightly higher for CH4 relative to other studies in lakes and seasonally flooded areas of the Pantanal. The large variations in concentrations of CO2, CH4 and DOC and the optical properties of DOM during the course of each flood cycle suggest a close relationship between carbon and water cycles in this tropical wetland. [ABSTRACT FROM AUTHOR]

Tidal freshwater ecosystems experience acute seawater intrusion associated with periodic droughts, but are expected to become chronically salinized as sea level rises. Here we report the results from an experimental manipulation in a tidal freshwater Zizaniopsis miliacea marsh on the Altamaha River, GA where diluted seawater was added to replicate marsh plots on either a press (constant) or pulse (2 months per year) basis. We measured changes in porewater chemistry (SO42−, Cl−, organic C, inorganic nitrogen and phosphorus), ecosystem CO2 and CH4 exchange, and microbial extracellular enzyme activity. We found that press (chronic) seawater additions increased porewater chloride and sulfate almost immediately, and ammonium and phosphate after 2-4 months. Chronic increases in salinity also decreased net ecosystem exchange, resulting in reduced CO2 and CH4 emissions from press plots. Our pulse treatment, designed to mimic natural salinity incursion in the Altamaha River (September and October), temporarily increased porewater ammonium concentrations but had few lasting effects on porewater chemistry or ecosystem carbon balance. Our findings suggest that long-term, chronic saltwater intrusion will lead to reduced C fixation and the potential for increased nutrient (N, P) export while acute pulses of saltwater will have temporary effects. [ABSTRACT FROM AUTHOR]

Peatland measurements of CO and CH flux were obtained at scales appropriate to the in situ biological community below the tree layer to demonstrate representativeness of the spruce and peatland responses under climatic and environmental change (SPRUCE) experiment. Surface flux measurements were made using dual open-path analyzers over an area of 1.13 m in daylight and dark conditions along with associated peat temperatures, water table height, hummock moisture, atmospheric pressure and incident radiation data. Observations from August 2011 through December 2014 demonstrated seasonal trends correlated with temperature as the dominant apparent driving variable. The S1-Bog for the SPRUCE study was found to be representative of temperate peatlands in terms of CO and CH flux. Maximum net CO flux in midsummer showed similar rates of C uptake and loss: daytime surface uptake was −5 to −6 µmol m s and dark period loss rates were 4-5 µmol m s (positive values are carbon lost to the atmosphere). Maximum midsummer CH-C flux ranged from 0.4 to 0.5 µmol m s and was a factor of 10 lower than dark CO-C efflux rates. Midwinter conditions produced near-zero flux for both CO and CH with frozen surfaces. Integrating temperature-dependent models across annual periods showed dark CO-C and CH-C flux to be 894 ± 34 and 16 ± 2 gC m y, respectively. Net ecosystem exchange of carbon from the shrub-forb- Sphagnum-microbial community (excluding tree contributions) ranged from −3.1 gCO-C m y in 2013, to C losses from 21 to 65 gCO-C m y for the other years. [ABSTRACT FROM AUTHOR]

Arbuscular mycorrhizal (AMF) function has mostly been studied from the plant perspective, but there is a shortage of empirical assessments of their ecosystem level impacts on soil carbon (C). Our understanding of the role of AMF on C processing belowground has been restricted mostly to fresh plant residues, not stabilized soil organic matter. The mechanisms by which elevated CO (eCO) alter soil C remain an open question but AMF likely play a role via C and nutrients, which could in turn, be plant species dependent. We assessed AMF as mediators of C processing in the rhizosphere of two grasses under eCO. We exposed a C and a C grass to a combination of ambient and eCO with and without modification of the AMF communities and using stable isotopes quantified the respiration of native soil C (as rhizosphere priming), its contribution to dissolved and microbial C and the final remaining C pool. The AMF treatment impacted soil C respiration under the C-plant and only under eCO. eCO suppressed decomposition (negative priming) but this effect disappeared when the AMF community was reduced. In contrast to studies of fresh plant residues suggesting that AMF can enhance C loss, our observations indicate that AMF may promote C storage in the soil organic matter pool. Results support that AMF can mediate the effect of eCO on soil C in the rhizosphere of some plant species, a potential mechanism explaining variation in impacts of eCO on soil C storage and C balances across species and ecosystems. [ABSTRACT FROM AUTHOR]

Quantifying rates of microbial carbon transformation in peatlands is essential for gaining mechanistic understanding of the factors that influence methane emissions from these systems, and for predicting how emissions will respond to climate change and other disturbances. In this study, we used porewater stable isotopes collected from both the edge and center of a thermokarst bog in Interior Alaska to estimate in situ microbial reaction rates. We expected that near the edge of the thaw feature, actively thawing permafrost and greater abundance of sedges would increase carbon, oxygen and nutrient availability, enabling faster microbial rates relative to the center of the thaw feature. We developed three different conceptual reaction networks that explained the temporal change in porewater CO, CH, δC-CO and δC-CH. All three reaction-network models included methane production, methane oxidation and CO production, and two of the models included homoacetogenesis-a reaction not previously included in isotope-based porewater models. All three models fit the data equally well, but rates resulting from the models differed. Most notably, inclusion of homoacetogenesis altered the modeled pathways of methane production when the reaction was directly coupled to methanogenesis, and it decreased gross methane production rates by up to a factor of five when it remained decoupled from methanogenesis. The ability of all three conceptual reaction networks to successfully match the measured data indicate that this technique for estimating in situ reaction rates requires other data and information from the site to confirm the considered set of microbial reactions. Despite these differences, all models indicated that, as expected, rates were greater at the edge than in the center of the thaw bog, that rates at the edge increased more during the growing season than did rates in the center, and that the ratio of acetoclastic to hydrogenotrophic methanogenesis was greater at the edge than in the center. In both locations, modeled rates (excluding methane oxidation) increased with depth. A puzzling outcome from the effort was that none of the models could fit the porewater dataset without generating 'fugitive' carbon (i.e., methane or acetate generated by the models but not detected at the field site), indicating that either our conceptualization of the reactions occurring at the site remains incomplete or our site measurements are missing important carbon transformations and/or carbon fluxes. This model-data discrepancy will motivate and inform future research efforts focused on improving our understanding of carbon cycling in permafrost wetlands. [ABSTRACT FROM AUTHOR]

During summer drought, Mediterranean fluvial networks are transformed into highly heterogeneous landscapes characterized by different environments (i.e., running and impounded waters, isolated river pools and dry beds). This hydrological setting defines novel biogeochemically active areas that could potentially increase the rates of carbon emissions from the fluvial network to the atmosphere. Using chamber methods, we aimed to identify hot spots for carbon dioxide (CO) and methane (CH) emissions from two typical Mediterranean fluvial networks during summer drought. The CO efflux from dry beds (mean ± SE = 209 ± 10 mmol CO m d) was comparable to that from running waters (120 ± 33 mmol m d) and significantly higher than from impounded waters (36.6 ± 8.5 mmol m d) and isolated pools (17.2 ± 0.9 mmol m d). In contrast, the CH efflux did not significantly differ among environments, although the CH efflux was notable in some impounded waters (13.9 ± 10.1 mmol CH m d) and almost negligible in the remaining environments (mean <0.3 mmol m d). Diffusion was the only mechanism driving CO efflux in all environments and was most likely responsible for CH efflux in running waters, isolated pools and dry beds. In contrast, the CH efflux in impounded waters was primarily ebullition-based. Using a simple heuristic approach to simulate potential changes in carbon emissions from Mediterranean fluvial networks under future hydrological scenarios, we show that an extreme drying out (i.e., a four-fold increase of the surface area of dry beds) would double the CO efflux from the fluvial network. Correspondingly, an extreme transformation of running waters into impounded waters (i.e., a twofold increase of the surface area of impounded waters) would triple the CH efflux. Thus, carbon emissions from dry beds and impounded waters should be explicitly considered in carbon assessments of fluvial networks, particularly under predicted global change scenarios, which are expected to increase the spatial and temporal extent of these environments. [ABSTRACT FROM AUTHOR]

During 2 years (2009, 2010), CO fluxes were measured in an oligotrophic fen located at the ecotonal limit of forest tundra and lichen woodland, in northeastern Canada. Peatlands in this region have registered a rise in surface moisture since Little Ice Age, allowing an increase in their pool size and density. In the studied peatland, 83 % of the surface is covered either with pools (42 %), hollows (28 %) and lawns (13 %) all of which present a mean water table higher than 7 cm. We evaluated how these conditions might influence the CO balance in these poor fens located in the northeastern section of the La Grande river watershed. During the two years measurements, results indicate a net CO source with 404 (±287) and 272 (±250) g CO myear emitted to the atmosphere throughout the growing seasons and winters. Modeled wintertime net ecosystem exchange resulted in a net loss of 150 (±36) and 150 (±37) g CO m year in 2009 and 2010 respectively, while 248 (±248) and 124 (±195) g CO m was emitted during 2009 and 2010 growing seasons. This high CO source can be attributed to three factors: (1) the length of the cold season which represented 210-214 days of net CO loss (2) a high pool/vegetated surface ratio since pools are a net source of CO and presented 38-40 % of the annual spatially weighted CO budget and (3) hydroclimatic conditions during the growing season as dryer and warmer conditions in 2009 reduced photosynthesis and increased respiration rates. [ABSTRACT FROM AUTHOR]

To gain a more mechanistic understanding of how soil organic matter (OM) characteristics can affect carbon mineralization in tidal freshwater wetlands, we conducted a long-term in situ field manipulation of OM type and monitored associated changes in carbon dioxide (CO) and methane (CH) production. In addition, we characterized microbial community structure and quantified the activity of several extracellular enzymes (EEA) involved in the acquisition of carbon, nitrogen, and phosphorus. Treatments included a plant litter addition, prepared using naturally-senescing vegetation from the site, and a compost amendment, designed to increase the concentration of aged, partially humified, OM. Both types of OM-amended soils had CO production rates 40-50 % higher than unamended control soils, suggesting that the added OM had inherently higher quality and/or availability than the native soil OM. Rates of CO production were not correlated with microbial community structure or EEA except a modest relationship with cellulose breakdown via the K of β-1,4-glucosidase. We interpret this lack of correlation to be a consequence of high functional redundancy of microorganisms that are capable of producing CO. Rates of CH production were also influenced by OM quality, increasing by an order of magnitude with plant litter additions relative to compost-amended and control soils. Unlike CO, rates of CH production were significantly correlated with the microbial community structure and with enzyme kinetic parameters (V and K) for both carbon (β-1,4-glucosidase, 1,4-β-cellobiosidase, and β- d-xylosidase) and nitrogen acquisition (leucyl aminopeptidase). The monophyletic nature of methanogenic archaea, combined with their reliance on a small select group of organic substrates produced via enzyme-mediated hydrolysis and subsequent bacterial fermentation, provides a basis for the strong links between microbial community structure, EEA, and CH production. Our results suggest that incorporating microbial community structure and EEA into conceptual models of wetland OM decomposition may enhance our mechanistic understanding of, and predictive capacity for, biogeochemical process rates. [ABSTRACT FROM AUTHOR]

Current estimates of CO 2 outgassing from Amazonian rivers and streams have considerable uncertainty since they are based on limited-time surveys of pCO 2 measurements along the Amazon mainstem and mouths of major tributaries, using conservative estimates of gas exchange velocities. In order to refine basin-scale CO 2 efflux estimates from Amazonian rivers, we present a long time (5-year) dataset of direct measurements of CO 2 fluxes, gas transfer velocities and pCO 2 measurements in seven representative rivers of the lowland Amazon basin fluvial network, six non-tidal (Negro, Solimões, Teles Pires, Cristalino, Araguaia and Javaés) and one tidal river (Caxiuanã), with sizes ranging from 4th to 9th order. Surveys were conducted from January 2006 to December 2010, in a total of 389 campaigns covering all stages of their hydrographs. CO 2 fluxes and gas transfer velocities ( k) were measured using floating chambers and pCO 2 was measured simultaneously by headspace extraction followed by gas chromatography analysis. Results show high CO 2 flux rate variability among rivers and hydrograph stages, ranging from −0.8 to 15.3 μmol CO 2 m −2 s −1, with unexpected negative fluxes in clear-water rivers during low waters. Non-tidal rivers showed marked seasonal CO 2 flux patterns, with significantly higher exchange during high waters. Seasonality was modulated by pCO 2, which was positive and strongly correlated with discharge. In these rivers k was well correlated with wind speed, which allowed the use of wind data to model k. We estimate a release of 360 ± 60 Tg C year −1 from Amazonian rivers and streams within a 1.47 million km 2 quadrant in the central lowland Amazon. Extrapolating these values to the basin upstream of Óbidos, results in an outgassing of 0.8 Pg C to the atmosphere each year. Our results are a step forward in achieving more accurate gas emission values for Amazonian rivers and their role in the annual carbon budget of the Amazon basin. [ABSTRACT FROM AUTHOR]

Although methanogenic pathways generally produce equimolar amounts of carbon dioxide and methane, CO concentrations are often reported to be higher than CH concentrations in both field and laboratory incubation studies of peat decomposition. In field settings, higher pore water concentrations of CO may result from the loss of methane by: (1) ebullition due to the low solubility of methane in pore water and (2) vascular-plant transport. Higher CO concentrations may also be caused by: (1) production of additional CO by high-molecular weight (HMW) organic matter (OM) fermentation and/or (2) respiration from non-methanogenic pathways. In this study of a peatland where advection and transverse dispersion were the dominant pore water solute transport mechanisms, an isotope-mass balance approach was used to determine the proportions of CO formed from non-fractionating OM respiration and HMW fermentation relative to CO production from methanogenesis. This approach also allowed us to estimate the loss of CH from the belowground system. The pathways of CO production varied with depth and surface vegetation type. In a Carex-dominated fen, methane production initially produced 40 % of the total CO and then increased to 90-100 % with increasing depth. In a Sphagnum-dominated bog, methanogenesis resulted in 60 % of total CO production which increased to 100 % at depth. Both bogs and fens showed 85-100 % of methane loss from pore waters. Our results indicate that the isotopic composition of dissolved CO is a powerful indicator to allow partitioning of the processes affecting peat remineralization and methane production. [ABSTRACT FROM AUTHOR]

Microbial oxidation in aerobic soils is the primary biotic sink for atmospheric methane (CH), a powerful greenhouse gas. Although tropical forest soils are estimated to globally account for about 28% of annual soil CH consumption (6.2 Tg CH year), limited data are available on CH exchange from tropical montane forests. We present the results of an extensive study on CH exchange from tropical montane forest soils along an elevation gradient (1,000, 2,000, 3,000 m) at different topographic positions (lower slope, mid-slope, ridge position) in southern Ecuador. All soils were net atmospheric CH sinks, with decreasing annual uptake rates from 5.9 kg CH-C ha year at 1,000 m to 0.6 kg CH-C ha year at 3,000 m. Topography had no effect on soil atmospheric CH uptake. We detected some unexpected factors controlling net methane fluxes: positive correlations between CH uptake rates, mineral nitrogen content of the mineral soil and with CO emissions indicated that the largest CH uptake corresponded with favorable conditions for microbial activity. Furthermore, we found indications that CH uptake was N limited instead of inhibited by NH. Finally, we showed that in contrast to temperate regions, substantial high affinity methane oxidation occurred in the thick organic layers which can influence the CH budget of these tropical montane forest soils. Inclusion of elevation as a co-variable will improve regional estimates of methane exchange in these tropical montane forests. [ABSTRACT FROM AUTHOR]

Extensive interfluvial wetlands occur in the upper Negro River basin (Brazil) and contain a mosaic of vegetation dominated by emergent grasses and sedges with patches of shrubs and palms. To characterize the release of carbon dioxide and methane from these habitats, diffusive and ebullitive emissions and transport through plant aerenchyma were measured monthly during 2005 in permanently and seasonally flooded areas. CO emissions averaged 2193 mg C m day. Methane was consumed in unflooded environments and emitted in flooded environments with average values of −4.8 and 60 mg C m day, respectively. Bubbles were emitted primarily during falling water periods when hydrostatic pressure at the sediment-water interface declined. CO and CH emissions increased when dissolved O decreased and vegetation was more abundant. Total area and seasonally varying flooded areas for two wetlands, located north and south of the Negro River, were determined through analysis of synthetic aperture radar and optical remotely sensed data. The combined areas of these two wetlands (3000 km) emitted 1147 Gg C year as CO and 31 Gg C year as CH. If these rates are extrapolated to the area occupied by hydromorphic soils in the upper Negro basin, 63 Tg C year of CO and 1.7 Tg C year as CH are estimated as the regional evasion to the atmosphere. [ABSTRACT FROM AUTHOR]

The ocean-atmosphere flux of carbon dioxide in subantarctic surface water (SASW) east of New Zealand has been determined using data from bi-monthly cruises on a time series transect for 8 years. The 60 km long transect extends from the coast (45.770°S 170.720°E) to a station at 45.833°S 171.500°E. Sea surface temperature, salinity, nutrient concentrations and pCO have been measured at a frequency of about once every 2 months from January 1998 until December 2005. Measured pCO exhibits a seasonal cycle with a maximum in late winter/spring, and a minimum in late summer/autumn, a mean 356 μatm, and an amplitude of 9 μatm. The magnitude of ΔpCO (the air-sea concentration gradient) has increased over the 8 years, primarily due to the increase in atmospheric CO concentration. The air-sea flux of CO was determined from wind speed data and ΔpCO. The uptake of atmospheric CO by SASW in the study area changed from + 1 and +82 mmol m in 1998 and 1999 respectively (ocean as source) to −870 and −510 mmol m in 2004 and 2005 (ocean as sink). These values are substantially less in magnitude than the value obtained from the Takahashi et al. (Deep-Sea Res II, ) flux climatology. [ABSTRACT FROM AUTHOR]

llochthonous and autochthonous organic matter deposited in benthic sediments are mineralized by microbial communities, resulting in release of nutrients to the water column. Lakes with different trophic states may have sediments with different carbon and nutrient concentration with consequently different microbial communities. Microbial diversity of surface sediments of three subtropical lakes of different trophic state was investigated by measuring catabolic response to a wide variety of carbon-substrates. Basal carbon dioxide and methane production rates were highest in Lake Apopka (hypereutrophic), followed by Lake Annie (oligo-mesotrophic) and Lake Okeechobee (eutrophic) sediments. The oligo-mesotrophic Lake Annie showed the highest metabolic quotient ( qCO; proportion of basal respiration per unit of microbial biomass, 0.008 ± 0.001) indicating inefficient use of energy. The low qCO found in Lake Apopka sediment indicated higher efficiency in using energy. Lake Okeechobee sediments had intermediary values of qCO (M9 0.005 ± 0.001; M17 0.006 ± 0.0003; KR 0.004 ± 0.001) as compared with other lakes (lake Apopka 0.004 ± 0.14). Lake Apopka's sediment catabolic diversity was higher than that observed in other sediments. Addition of organic electron donors to sediment samples from all lakes stimulated heterotrophic activity; however, the extent of the response varied greatly and was related to microbial biomass. The hypereutrophic Lake Apopka sediments had the highest respiration per unit of microbial biomass with the addition of electron donors indicating that these sediments respired most of the C added. These results showed that sediments with different biogeochemical properties had microbial communities with distinct catabolic responses to addition of the C sources. [ABSTRACT FROM AUTHOR]

Spring snowmelt in the arctic and boreal regions represents the most significant event in the hydrological year. We measured concentrations and fluxes of different carbon species in 2 small contrasting (control v drained) forested peatland catchments in E. Finland between April and June 2008 and compared these to long-term annual fluxes. Measurements were made using a combination of continuous sensors (CO, temperature, pH, discharge) and routine spot sampling (DOC, POC, DIC, CO, CH, NO). The highest concentrations of CO and CH in streamwater were observed under low flow conditions before the spring flood event, reflecting accumulation and downstream release of gaseous C at the end of the winter period. Over the length of the study mean CH concentrations were 10× higher in the drained site. The snowmelt event was associated with a dilution of DOC and CO, with the drained catchment showing a much flashier hydrological response compared to the control site, and post-event, a slower recovery in DOC and CO concentrations. Fluxes of all carbon species during the snowmelt event were significant and represented 37-45% of the annual flux. This highlights the challenge of quantifying aquatic C fluxes in areas with large temporal variability and suggests that inability to 'capture' the spring snowmelt event may lead to under-estimation of C fluxes in northern regions. [ABSTRACT FROM AUTHOR]

Considerable research has recently been devoted to understanding biogeochemical processes under winter snow cover, leading to enhanced appreciation of the importance of many winter ecological processes. In this study, a comprehensive investigation of the stable carbon isotope composition (δ13C) of CO2 within a high-elevation subalpine forest snowpack was conducted. Our goals were to study the δ13C of biological soil respiration under snow in winter, and to assess the relative importance of diffusion and advection (ventilation by wind) for gas transport within snow. In agreement with other studies, we found evidence of an active microbial community under a roughly 1-m deep snowpack during winter and into spring as it melted. Under-snow CO2 mole fractions were observed up to 3,500 μmol mol−1, and δ13C of CO2 varied from ~−22 to ~−8‰. The δ13C of soil respiration calculated from mixing relationships was −26 to −24‰, and although it varied in time, it was generally close to that of the bulk organic horizon (−26.0‰). Subnivean CO2 and δ13C were quite dynamic in response to changes in soil temperature, liquid water availability, and wind events. No clear biologically-induced isotopic changes were observed during periods when microbial activity and root/rhizosphere activity were expected to vary, although such changes cannot be eliminated. There was clear evidence of isotopic enrichment associated with diffusive transport as predicted by theory, but simple diffusive enrichment (4.4‰) was not observed. Instead, ventilation of the snowpack by sustained wind events in the forest canopy led to changes in the diffusively-enriched gas profile. The isotopic influence of diffusion on gases in the snowpack and litter was greatest at greater depths, due to the decreased relative contribution of advection at depth. There were highly significant correlations between the apparent isotopic content of respiration from the soil with wind speed and pressure. In summary, physical factors influencing gas transport substantially modified and potentially obscured biological factors in their effects on δ13C of CO2 within this subalpine forest snowpack. [ABSTRACT FROM AUTHOR]

Eddy covariance measurements of the surface energy balance and carbon dioxide exchange above high-elevation (3,480 m above sea level) alpine tundra located near Niwot Ridge, Colorado, were compared to simultaneous measurements made over an adjacent subalpine forest over two summers and one winter, from June 9, 2007 to July 3, 2008. The surface energy balance closure at the alpine site averaged 71 and 91%, winter and summer, respectively, due to the high wind speeds, short turbulent flux footprint, and relatively flat ridge-top location of the measurement site. Throughout the year, the alpine site was cooler with higher relative humidity, and had a higher horizontal wind speed, especially in winter, compared to the forest site. Wind direction was persistently downslope at the alpine site (summer and winter, day and night), whereas upslope winds were common at the forest site during summer daytime periods. The latent and sensible heat fluxes were consistently larger in magnitude at the forest site, with the largest differences during summer. The horizontal advective flux of CO2 at the alpine site averaged 6% of the net ecosystem exchange ( NEE) during summer nights (5% during summer daytime), and was small in relation to the high wind speeds, relatively flat site, and weak sources of CO2 upwind of the site. The magnitudes and diurnal behavior of the alpine NEE calculated using three methods; eddy-covariance, friction velocity filter, and with advection and storage calculations, gave similar results. The period of net CO2 uptake (negative NEE) was 100 days at the alpine site with a net uptake of 16 g C m−2, compared to 208 days at the forest site with a net uptake of 108 g C m−2, with initiation of net uptake coinciding with air temperatures reaching +10°C. Winter respiration loss at the alpine site was 164 g C m−2 over 271 days, compared to 52 g C m−2 over 175 days at the forest site, with the initiation of net loss coinciding with air temperatures reaching −10°C at each site. [ABSTRACT FROM AUTHOR]

The importance of snow and related cryospheric processes as an ecological factor has been recognized since at least the beginning of the twentieth century. Even today, however, many observations remain anecdotal. The research to date on cold-lands ecosystems results in scientists being unable to evaluate to what extent changes in the cryosphere will be characterized by abrupt changes in local and global biogeochemical cycles, and how these changes in seasonality may affect the rates and timing of key ecological processes. Studies of gas exchanges through snow have revealed that snow plays an important role in modulating wintertime soil biogeochemical processes, and that these can be the driving processes for gas exchange at the snow surface. Previous research has primarily focused on carbon dioxide, and resulted from episodic experiments at a number of snow-covered sites. Here we report new insights from several field sites on Niwot Ridge in the Colorado Rocky Mountains, including a dedicated snow gas flux research facility established at the 3340 m Soddie site. A novel in situ experimental system was developed at this site to continuously sample trace gases from above and within the snowpack for the duration of seasonal snow cover. The suite of chemical species investigated includes carbon dioxide, nitrous oxide, nitrogen oxides, ozone, and volatile inorganic and organic gases. Wintertime measurements have been supplemented by soil chamber experiments and eddy covariance measurements to allow assessment of the contribution of wintertime fluxes to annual biogeochemical budgets. This research has resulted in a plethora of new insight into the physics of gas transport through the snowpack, and the magnitude and the chemical and biogeochemical processes that control fluxes at the soil-snowpack and the snow-atmosphere interface. This article provides an overview of the history and evolution of this research, and highlights the findings from the ten articles that constitute this special issue. [ABSTRACT FROM AUTHOR]

Whole air drawn from four heights within the high elevation (3,340 m asl), deep, winter snowpack at Niwot Ridge, Colorado, were sampled into stainless steel canisters, and subsequently analyzed by gas chromatography for 51 volatile inorganic and organic gases. Two adjacent plots with similar snow cover were sampled, one over bare soil and a second one from within a snow-filled chamber where Tedlar/Teflon-film covered the ground and isolated it from the soil. This comparison allowed for studying effects from processes in the snowpack itself versus soil influences on the gas concentrations and fluxes within and through the snowpack. Samples were also collected from ambient air above the snow surface for comparison with the snowpack air. Analyzed gas species were found to exhibit three different kinds of behavior: (1) One group of gases, i.e., carbon dioxide (CO2), chloroform (CHCl3), dimethylsulfide (CH3)2S, carbondisulfide (CS2), and dichlorobromomethane (CHBrCl2), displayed higher concentrations inside the snow, indicating a formation of these species and release into the atmosphere. (2) A second group of compounds, including carbon monoxide (CO), carbonyl sulfide (COS), the hydrocarbons methane, ethane, ethyne, benzene, and the halogenated compounds methylchloride (CH3Cl), methylbromide (CH3Br), dibromomethane (CH2Br2), bromoform (CHBr3), tetrachloromethane (CCl4), CFC-11, CFC-12, HCFC-22, CFC-113, 1,2-dichloroethane, methylchloroform, HCFC-141b, and HCFC-142b, were found at lower concentrations in the snow, indicating that the snow and/or soil constitute a sink for these gases. (3) For 21 other gases absolute concentrations, respectively concentration gradients, were too low to unequivocally identify their uptake or release behavior. For gases listed in the first two groups, concentration gradients were incorporated into a snowpack gas diffusion model to derive preliminary estimates of fluxes at the snow-atmosphere interface. The snowpack gradient flux technique was found to offer a highly sensitive method for the study of these surface gas exchanges. Microbial activities below this deep, winter snowpack appear to be the driving mechanism behind these gas sources and sinks. Flux results were applied to a simple box model to assess the potential contribution of the snowpack uptake rates to atmospheric lifetimes of these species. [ABSTRACT FROM AUTHOR]

Carbon dioxide is taken up by agricultural crops and released soon after during the consumption of agricultural commodities. The global net impact of this process on carbon flux to the atmosphere is negligible, but impact on the spatial distribution of carbon dioxide uptake and release across regions and continents is significant. To estimate the consumption and release of carbon by humans over the landscape, we developed a carbon budget for humans in the United States. The budget was derived from food commodity intake data for the US and from algorithms representing the metabolic processing of carbon by humans. Data on consumption, respiration, and waste of carbon by humans were distributed over the US using geospatial population data with a resolution of ~ 450 x 450 m. The average adult in the US contains about 21 kg C and consumes about 67 kg C year-1 which is balanced by the annual release of about 59 kg C as expired CO2, 7 kg C as feces and urine, and less than 1 kg C as flatus, sweat, and aromatic compounds. In 2000, an estimated 17.2 Tg C were consumed by the US population and 15.2 Tg C were expired to the atmosphere as CO2. Historically, carbon stock in the US human population has increased between 1790 and 2006 from 0.06 Tg to 5.37 Tg. Displacement and release of total harvested carbon per capita in the US is nearly 12% of per capita fossil fuel emissions. Humans are using, storing, and transporting carbon about the Earth’s surface. Inclusion of these carbon dynamics in regional carbon budgets can improve our understanding of carbon sources and sinks. [ABSTRACT FROM AUTHOR]

The objective of this study was to evaluate the effect of N fertilization and the presence of N2 fixing leguminous trees on soil fluxes of greenhouse gases. For a one year period, we measured soil fluxes of nitrous oxide (N2O), carbon dioxide (CO2) and methane (CH4), related soil parameters (temperature, water-filled pore space, mineral nitrogen content, N mineralization potential) and litterfall in two highly fertilized (250 kg N ha−1 year−1) coffee cultivation: a monoculture (CM) and a culture shaded by the N2 fixing legume species Inga densiflora (CIn). Nitrogen fertilizer addition significantly influenced N2O emissions with 84% of the annual N2O emitted during the post fertilization periods, and temporarily increased soil respiration and decreased CH4 uptakes. The higher annual N2O emissions from the shaded plantation (5.8 ± 0.3 kg N ha−1 year−1) when compared to that from the monoculture (4.3 ± 0.1 kg N ha−1 year−1) was related to the higher N input through litterfall (246 ± 16 kg N ha−1 year−1) and higher potential soil N mineralization rate (3.7 ± 0.2 mg N kg−1 d.w. d−1) in the shaded cultivation when compared to the monoculture (153 ± 6.8 kg N ha−1 year−1 and 2.2 ± 0.2 mg N kg−1 d.w. d−1). This confirms that the presence of N2 fixing shade trees can increase N2O emissions. Annual CO2 and CH4 fluxes of both systems were similar (8.4 ± 2.6 and 7.5 ± 2.3 t C-CO2 ha−1 year−1, −1.1 ± 1.5 and 3.3 ± 1.1 kg C-CH4 ha−1 year−1, respectively in the CIn and CM plantations) but, unexpectedly increased during the dry season. [ABSTRACT FROM AUTHOR]

Methane (CH4) is the second most important greenhouse gas after carbon dioxide (CO2). To understand CH4 cycling, quantitative information about microbial CH4 oxidation in soils is essential. Field methods such as the gas push-pull test (GPPT) to quantify CH4 oxidation are often used in combination with specific inhibitors, such as acetylene (C2H2). Acetylene irreversibly binds to the enzyme methane monooxygenase, but little is known about recovery of CH4 oxidation activity after C2H2 inhibition in situ, which is important when performing several experiments at the same location. To assess recovery of CH4 oxidation activity following C2H2 inhibition, we performed a series of GPPTs over 8 weeks at two different locations in the vadose zone above a petroleum hydrocarbon-contaminated aquifer in Studen, Switzerland. After 4 weeks a maximum recovery of 30% and 50% of the respective initial activity was reached, with a subsequent slight drop in activity at both locations. Likely, CH4 oxidation activity and CH4 concentrations were too low to allow for rapid recovery following C2H2 inhibition at the studied locations. Therefore, alternative competitive inhibitors have to be evaluated for application in conjunction with GPPTs, especially for sites with low activity. [ABSTRACT FROM AUTHOR]

In order to evaluate the role of photochemistry in the carbon dioxide (CO2) generation from a 10-year-old boreal reservoir, the photomineralization of dissolved organic matter (DOM) was assessed and compared to a boreal river as well as to boreal and temperate lakes during July and August, 2003. Sterile water samples were irradiated by sunlight over the whole photoperiod and subsequently analyzed for CO2. Mean energy-normalized apparent photochemical yield of CO2 (an index of DOM photoreactivity normalized for the energy absorbed by samples) was significantly higher in the reservoir (27.7 ± 13.0 mg CO2·m−3·kJ−1) and the boreal river (35.8 ± 2.3 mg CO2·m−3·kJ−1) than in the boreal lakes (15.5 ± 5.1 mg CO2·m−3·kJ−1). The DOM photoreactivity of the temperate lakes (20.9 ± 8.1 mg CO2·m−3·kJ−1) was not statistically different from any type of boreal water bodies. There was no significant difference in either the integrated photoproduction of CO2 (273–433 mg CO2·m−2·d−1) or the potential photochemical contribution to CO2 diffusive fluxes (56–92%) among these water bodies. DOM photoreactivity was significantly affected by the cumulative hydrological residence time (CHRT) when considering the whole data set. However, when considering only the boreal water bodies, iron (Fe) and manganese (Mn) also intervened. The fact that DOM photoreactivity was related to CHRT as well as to Fe and Mn concentrations, which are respectively permanent and long-lasting features of the reservoir, suggests that the photoproduction of CO2 is not likely to decrease over time. This process may therefore play a substantial role in the long-term CO2 emissions from boreal reservoirs during the summer, its potential contribution to CO2 diffusive fluxes being estimated at 56 ± 29 %. [ABSTRACT FROM AUTHOR]

Termites and soil calcium carbonate are major factors in the global carbon cycle: termites by their role in decomposition of organic matter and methane production, and soil calcium carbonate by its storage of atmospheric carbon dioxide. In arid and semiarid soils, these two factors potentially come together by means of biomineralization of calcium carbonate by termites. In this study, we evaluated this possibility by testing two hypotheses. Hypothesis 1 states that termites biomineralize calcium carbonate internally and use it as a cementing agent for building aboveground galleries. Hypothesis 2 states that termites transport calcium carbonate particles from subsoil horizons to aboveground termite galleries where the carbonate detritus becomes part of the gallery construction. These hypotheses were tested by using (1) field documentation that determined if carbonate-containing galleries only occurred on soils containing calcic horizons, (2) 13C/12C ratios, (3) X-ray diffraction, (4) petrographic thin sections, (5) scanning electron microscopy, and (6) X-ray mapping. Four study sites were evaluated: a C4-grassland site with no calcic horizons in the underlying soil, a C4-grassland site with calcic horizons, a C3-shrubland site with no calcic horizons, and a C3-shrubland site with calcic horizons. The results revealed that carbonate is not ubiquitously present in termite galleries. It only occurs in galleries if subsoil carbonate exists within a depth of 100 cm. 13C/12C ratios of carbonate in termite galleries typically matched 13C/12C ratios of subsoil carbonate. X-ray diffraction revealed that the carbonate mineralogy is calcite in all galleries, in all soils, and in the termites themselves. Thin sections, scanning electron microscopy, and X-ray mapping revealed that carbonate exists in the termite gut along with other soil particles and plant opal. Each test argued against the biomineralization hypothesis and for the upward-transport hypothesis. We conclude, therefore, that the gallery carbonate originated from upward transport and that this CaCO3 plays a less active role in short-term carbon sequestration than it would have otherwise played if it had been biomineralized directly by the termites. [ABSTRACT FROM AUTHOR]

In forest ecosystems the single largest respiratory flux influencing net ecosystem productivity (NEP) is the total soil CO2 efflux; however, it is difficult to make measurements of this flux that are accurate at the ecosystem scale. We examined patterns of soil CO2 efflux using five different methods: auto-chambers, portable gas analyzers, eddy covariance along and two models parameterized with the observed data. The relation between soil temperature and soil moisture with soil CO2 effluxes are also investigated, both inter-annually and seasonally, using these observations/results. Soil respiration rates ( R soil) are greatest during the growing season when soil temperatures are between 15 and 25 °C, but some soil CO2 efflux occurs throughout the year. Measured soil respiration was sensitive to soil temperature, particularly during the spring and fall. All measurement methods produced similar annual estimates. Depending on the time of the year, the eddy covariance (flux tower) estimate for ecosystem respiration is similar to or slightly lower than estimates of annual soil CO2 efflux from the other methods. As the eddy covariance estimate includes foliar and stem respiration which the other methods do not; it was expected to be larger (perhaps 15–30%). The auto-chamber system continuously measuring soil CO2 efflux rates provides a level of temporal resolution that permits investigation of short- to longer term influences of factors on these efflux rates. The expense of building and maintaining an auto chamber system may not be necessary for those researchers interested in estimating R soil annually, but auto-chambers do allow the capture of data from all seasons needed for model parameterization. [ABSTRACT FROM AUTHOR]

The ability to predict stream alkalinity values over timescales shorter than monthly or annually is needed to understand the response of stream chemistry to acidic inputs which occur across short time scales (days). We develop and apply a coupled series of physically-based models which are able to predict daily stream alkalinity values by first calculating soil air CO2 concentrations. We apply the model to a 9 year record of discharge and stream chemistry from a small catchment in the Shenandoah National park of Virginia. We find that we are able to accurately predict the minimum daily stream alkalinity values for all years and we are able to accurately predict the entire annual cycle for 6 of the 9 years (Nash–Sutcliffe criterion equals 0.26). For the 3 years which we overpredict summer stream alkalinity, summer precipitation was greater than normal and much greater than the period for which the model was calibrated. [ABSTRACT FROM AUTHOR]

JGOFS results showed that the ocean is a major sink for the increasing atmospheric carbon dioxide resulting from human activity. However, the role of the coastal seas in the global carbon cycling is poorly understood. In the present work, the inorganic carbon (IC) in the Yangtze River Estuary and Jiaozhou Bay are studied as examples of offshore sediments. Sequential extraction was used to divide inorganic carbon in the sediments into five forms, NaCl form, NH3 H2O form, NaOH form, NH2OH HCl form and HCl form. Studied of their content and influencing factors were also showed that NaCl form < NH3 H2O form2OH HCl form3 H2O form, NaOH form and NH2OH HCl form may convert to HCl form ultimately. So every IC form has different contribution to carbon cycling. This study showed that the contribution of various form of IC to the carbon cycle is in the order of NaOH form>NH2OH HCl form>NH3 H2O form>NaCl form>HCl form, and that the contribution of HCl form contributes little to carbon cycling, HCl form may be one of end-result of atmospheric CO2. So Yangtze River estuary sediment may absorb at least about 40.96×1011 g atmospheric CO2 every year, which indicated that offshore sediment play an important role in absorbing atmospheric CO2. [ABSTRACT FROM AUTHOR]

Emissions of CO2 from soils make up one of the largest fluxes in the global C cycle, thus small changes in soil respiration may have large impacts on global C cycling. Anthropogenic additions of CO2 to the atmosphere are expected to alter soil carbon cycling, an important component of the global carbon budget. As part of the Duke Forest Free-Air CO2 Enrichment (FACE) experiment, we examined how forest growth at elevated (+200 ppmv) atmospheric CO2 concentration affects soil CO2 dynamics over 7 years of continuous enrichment. Soil respiration, soil CO2 concentrations, and the isotopic signature of soil CO2 were measured monthly throughout the 7 years of treatment. Estimated annual rates of soil CO2 efflux have been significantly higher in the elevated plots in every year of the study, but over the last 5 years the magnitude of the CO2 enrichment effect on soil CO2 efflux has declined. Gas well samples indicate that over 7 years fumigation has led to sustained increases in soil CO2 concentrations and depletion in the δ 13C of soil CO2 at all but the shallowest soil depths. [ABSTRACT FROM AUTHOR]

We used five analytical approaches to compare net ecosystem exchange (NEE) of carbon dioxide (CO2) from automated and manual static chambers in a peatland, and found the methods comparable. Once per week we sampled manually from 10 collars with a closed chamber system using a LiCor 6200 portable photosynthesis system, and simulated four photosynthetically active radiation (PAR) levels using shrouds. Ten automated chambers sampled CO2 flux every 3 h with a LiCor 6252 infrared gas analyzer. Results of the five comparisons showed (1) NEE measurements made from May to August, 2001 by the manual and automated chambers had similar ranges: −10.8 to 12.7 μmol CO2 m−2 s−1 and −17.2 to 13.1 μmol CO2 m−2 s−1, respectively. (2) When sorted into four PAR regimes and adjusted for temperature (respiration was measured under different temperature regimes), mean NEE did not differ significantly between the chambers ( p < 0.05). (3) Chambers were not significantly different in regression of ln( − respiration) on temperature. (4) But differences were found in the PAR vs. NEE relationship with manual chambers providing higher maximum gross photosynthesis estimates (GPmax), and slower uptake of CO2 at low PAR ( α) even after temperature adjustment. (5) Due to the high variability in chamber characteristics, we developed an equation that includes foliar biomass, water table, temperature, and PAR, to more directly compare automated and manual NEE. Comparing fitted parameters did not identify new differences between the chambers. These complementary chamber techniques offer a unique opportunity to assess the variability and uncertainty in CO2 flux measurements. [ABSTRACT FROM AUTHOR]

Soil CO2 flux can contribute as much as 60–80% of total ecosystem respiration in forests. Although considerable research has focused on quantifying this flux during the growing season, comparatively little effort has focused on non-growing season fluxes. We measured soil CO2 efflux through snow in 50 and ~300 year old subalpine forest stands near Fraser CO. Our objectives were to quantify seasonal patterns in wintertime soil CO2 flux; determine if differences in soil CO2 flux between the two forest ages during the growing season persist during winter; and to quantify the sample size necessary to discern treatment differences. Soil CO2 flux during the 2002–2003 and 2003–2004 snow season averaged 0.31 and 0.35 μmols m−2 s−1 for the young and old forests respectively; similar to the relative difference observed during summer. There was a significant seasonal pattern of soil CO2 flux during the winter with fluxes averaging 0.22 μmols m−2 s−1 in December and January and increasing to an average of 0.61 μmols m−2 s−1 in May. Within-plot variability for measurements used in calculating flux was low. The coefficients of variation (CV) for CO2 concentration, snowpack density, and snow depth were 17, 8 and 14%, respectively, yielding a CV for flux measurements within-plot of 29%. A within plot CV of 29% requires 8 sub-samples per plot to estimate the mean flux with a standard error of ±10% of the mean. Variability in CO2 flux estimates among plots (size = 400 m2) was similar to that within plot and was also low (CV = ~28%). With a CV of 28% among plots, ten plots per treatment would have a 50% probability of detecting a 25% difference in treatment means for α = 0.05. [ABSTRACT FROM AUTHOR]

In an old growth coniferous forest located in the central Cascade Mountains, Oregon, we added or removed aboveground litter and terminated live root activity by trenching to determine sources of soil respiration. Annual soil efflux from control plots ranged from 727 g C m−2 year−1 in 2002 to 841 g C m−2 year−1 in 2003. We used aboveground litter inputs (149.6 g C m−2 year−1) and differences in soil CO2 effluxes among treatment plots to calculate contributions to total soil efflux by roots and associated rhizosphere organisms and by heterotrophic decomposition of organic matter derived from aboveground and belowground litter. On average, root and rhizospheric respiration ( Rr) contributed 23%, aboveground litter decomposition contributed 19%, and belowground litter decomposition contributed 58% to total soil CO2 efflux, respectively. These values fall within the range of values reported elsewhere, although our estimate of belowground litter contribution is higher than many published estimates, which we argue is a reflection of the high degree of mycorrhizal association and low nutrient status of this ecosystem. Additionally, we found that measured fluxes from plots with doubled needle litter led to an additional 186 g C m−2 year−1 beyond that expected based on the amount of additional carbon added; this represents a priming effect of 187%, or a 34% increase in the total carbon flux from the plots. This finding has strong implications for soil C storage, showing that it is inaccurate to assume that increases in net primary productivity will translate simply and directly into additional belowground storage. [ABSTRACT FROM AUTHOR]

The question of how to extrapolate point measurements of soil CO2 processes to coarser scales remains unanswered because we know little about the spatial and temporal variability in the CO2 concentration of soil air. In this work, we describe a series of simple physically-based models that simulate soil temperature, soil tension, and soil CO2 processes. We apply these models to simulate the spatial and temporal dynamics of soil CO2 concentrations throughout a small catchment in the Virginia Blue Ridge. Output from the simulations is compared with field measurements. We find that despite some model deficiencies, we are able to simulate the gross patterns through space and time of soil air CO2 concentration. During the growing season when soil temperature is high, we find that soil water status is the limiting control on soil respiration and CO2 concentration. We also find that soil CO2 concentration can be high despite low respiration values due to decreased soil diffusivity as moisture fills pore spaces. [ABSTRACT FROM AUTHOR]

Streams draining peatland systems contain a number of different C-species, all of which are linked either directly or indirectly to the cycling of C in the terrestrial environment. Concentrations and fluxes of dissolved, particulate and gaseous forms of carbon were measured along a network of streams draining an acidic peatland catchment (46.3 km2) in NE Scotland. The main aim was to identify sources and sinks of all the major forms of C in the drainage network and use this to develop a conceptual understanding of the evolution of streamwater chemistry along a peatland stream continuum. The investigation included a small-scale intra-catchment study of three contiguous sites in a 1.3 km2 headwater catchment (Brocky Burn) and a larger scale integrated study of seven sites. Mean annual fluxes of the main carbon species varied from 115–215 (DOC), 8.15–97.0 (POC), 0.32–6.90 ($HCO3-$-C) and 2.62–10.4 (free CO2-C) kg C ha-1 year-1; all contributed to the overall carbon flux to varying degrees. Methane-C was only measurable at sites within areas of deep peat (<0.01–0.09 kg C ha-1 year-1). Downstream spatial changes in the intra-catchment study (Brocky Burn) were characterised by a decrease in DOC, CO2-C and CH4-C and an increase in POC fluxes over a distance of 1.1 km from the Upper to the Lower sites. In the context of the integrated catchment study estimated losses and gains of carbon from the water column showed no net change in DOC, a large decrease in POC (-55%) and a slight increase in $HCO3-$-C (+7.7%) and CO2-C (+4.5%). A significant decrease in the CO2-C flux: HCO3-C flux ratio with distance downstream from the stream source, illustrates the importance of outgassing of CO2 from streams draining peatland C reservoirs. These data are interpreted in the context of losses and gains of the various components of the aquatic C flux along the peatland stream continuum. [ABSTRACT FROM AUTHOR]

The effects of elevated CO2 on nutrient cycling and selected belowground processes in the closed-canopy sweetgum plantation were assessed as part of a free-air CO2 enrichment (FACE) experiment at Oak Ridge, Tennessee. We hypothesized that nitrogen (N) constraints to growth response to elevated CO2 would be mitigated primarily by reduced tissue concentrations (resulting in increased biomass production per unit uptake) rather than increased uptake. Conversely, we hypothesized that the constraints of other nutrients to growth response to elevated CO2 would be mitigated primarily by increased uptake because of adequate soil supplies. The first hypothesis was not supported: although elevated CO2 caused reduced foliar N concentrations, it also resulted in increased uptake and requirement of N, primarily because of greater root turnover. The additional N uptake with elevated CO2 constituted between 10 and 40% of the estimated soil mineralizeable N pool. The second hypothesis was largely supported: elevated CO2 had no significant effects on tissue concentrations of P, K, Ca, or Mg and caused significantly increased uptake and requirement of K, Ca, and Mg. Soil exchangeable pools of these nutrients are large and should pose no constraint to continued growth responses. Elevated CO2 also caused increased microbial biomass, reduced N leaching and increased P leaching from O horizons (measured by resin lysimeters), reduced soil solution $NH4+$, $SO42-$, and Ca2+ concentrations, and increased soil solution pH. There were no statistically significant treatment effects on soil nutrient availability as measured by resin capsules, resin stakes, or in situ incubations. Despite significantly lower litterfall N concentrations in the elevated CO2 treatment, there were no significant treatment effects on translocation or forest floor biomass or nutrient contents. There were also no significant treatment effects on the rate of decomposition of fine roots. In general, the effects of elevated CO2 on nutrient cycling in this study were not large; future constraints on growth responses imposed by N limitations will depend on changes in N demand, atmospheric N deposition, and soil mineralization rates. [ABSTRACT FROM AUTHOR]

Examines diel variation in lacunal CH[sub 4] and CO[sub 2] concentration and σ[sup 13]C in phragmites australis. Typification of through-flow convective plant patterns through diurnal patterns of variation in lacunal gas concentrations and isotopic fractionation; Transportation of CH[sub 4] from the sediments to the atmosphere via the lacunal plant spaces; Association of isotopic fractionations in aquatic plants to gas transport.

Discusses satisfactorily seasonal and inter-annual variability of carbon dioxide in the Scheldt plume off the Belgian coast. Dynamics of carbon dioxide; Air-water fluxes of carbon dioxide; Net annual source and sink of atmospheric carbon dioxide; Annual net emission of carbon dioxide.

Focuses on the carbon dioxide concentrations in perennially ice-covered lakes of Taylor Valley, Antarctica. Details on the carbon dioxide partial pressure values; Loss of carbon dioxide annually to primary productivity in the uppermost meters of the water column; Use of preliminary respiration rates for obtaining an annual inorganic carbon balance.

The partial pressures Of CO2 (pCO2) and CH4 (pCH4) in streams are not only governed by in stream processes, but also by transformations occurring in soil and groundwater ecosystems. As such, stream water pCO2 and pCH4 can provide a tool to assess ecosystem respiration and anaerobic metabolism throughout drainage basins. We conducted three surveys sampling the gas content of streams in eastern Tennessee and western North Carolina to assess factors regulating ecosystem metabolism in catchments with contrasting geomorphologies, elevations and soil organic matter storage. In our first survey, the influence of drainage basin geomorphology on ecosystem respiration was examined by sampling streams draining catchments underlain by either shale or dolomite. Geomorphology is influenced by geology with shale catchments having shallower soils, broader, unconstrained valley floors compared with dolomite catchments. pCO2 varied little between catchment typesbut increased from an average of 3340 ppmv in spring to 9927 ppmv in summer or 9.3 and 28 times atmospheric equilibrium (pCO2(equilib)), respectively. In contrast, pCH4 was over twice as high in streams draining shale catchments (306 ppmv; pCH4(equilib) = 116) compared with more steeply incised dolomite basins(130 ppmv; pCH4(equilib) = 51). Using the ratio of pCH4:pCO2 as an index of anaerobic metabolism, shale catchments had nearly twice as much anaerobiosis (pCH4:pCO2 = 0.046) than dolomite drainages (pCH4:pCO2 = 0.024). In our second survey, streams were sampled along an elevational gradient (525 to 1700 m) in the Great Smoky Mountains National Park, USA where soil organic matter storage increases with elevation. pCO2 did not vary between streams but increased from 5340 ppmv (pCO2(equilib) = 15) to 8565 ppmv (pCO2(equilib) = 24) from spring to summer, respectively. During spring pCH4 was low and constant across streams, but during summer increased with elevation ranging from 17 to 2068 ppmv (pCH4(equilib) = 10 to 1216). The contribution of anaerobiosis to total respiration was constant during spring (pCH4:pCO2 = 0.017) but during summer increased with elevation from 0.002 at 524 m to 0.289 at 1286 m. In our last survey, we examined how pCO2 and pCH4 changed with catchment size along two rivers (ca. 60 km stretches in both rivers corresponding to increases in basin size from 1.7–477 km2 and 2.5–275 km2). pCO2 and pCH4 showed opposite trends, with pCO2 decreasing ca. 50% along the rivers, whereas pCH4 roughly doubled in concentration downstream. These opposing shifts resulted in a nearly five-fold increase of pCH2:pCO2 along the rivers from a low of 0.012 in headwaters to a high of 0.266 65-km downstream. pCO2 likely declines moving downstream as groundwater influences on stream chemistry decreases, whereas pCH4 may increase as the prevalence of anoxia in rivers expands due to finer-grained sediments and reduced hydrologic exchange with oxygenated surface water. [ABSTRACT FROM AUTHOR]