The carbon dioxide evasion cycle of an intermittent first-order stream: contrasting water–air and soil–air exchange

Ephemeral streams and wetlands are characterized by complex cycles of submersion and emersion, which influence the greenhouse gas flux rates. In this study we quantify the spatiotemporal variability in CO₂ and CH₄ concentrations and fluxes of an intermittent first-order stream over three consecutive wet and dry cycles spanning 56 days, to assess how hydrologic phase transitions influence greenhouse gas evasion. Water column excess CO₂ ranged from ?11 to 1600 μM, and excess CH₄ from 1 to 15 μM. After accounting for temporal changes in the ratio of wet versus dry streambed hydraulic radius, total CO₂–C fluxes ranged from 12 to 156 mmol m^?2 day^?1, with an integrated daily mean of 61 ± 25 mmol m^?2 day^?1. Soil–air evasion rates were approximately equal to those of water–air evasion. Rainfall increased background water–air CO₂–C fluxes by up to 780% due to an increase in gas transfer velocity in the otherwise still waters. CH₄–C fluxes increased 19-fold over the duration of the initial, longer wet-cycle from 0.1 to 1.9 mmol m^?2 day^?1. Temporal shifts in water depth and site-specific ephemerality were key drivers of carbon dynamics in the upper Jamison Creek watercourse. Based on these findings, we hypothesise that the cyclic periodicity of fluxes of biogenic gases from frequently intermittent streams (wet and dry cycles ranging from days to weeks) and seasonally ephemeral watercourses (dry for months at a time) are likely to differ, and therefore these differences should be considered when integrating transient systems into regional carbon budgets and models of global change.     

Influence of weather variables on methane and carbon dioxide flux from a shallow pond

Freshwaters are important sources of the greenhouse gases methane (CH₄) and carbon dioxide (CO₂) to the atmosphere. Knowledge about temporal variability in these fluxes is very limited, yet critical for proper study design and evaluating flux data. Further, to understand the reasons for the variability and allow predictive modeling, the temporal variability has to be related to relevant environmental variables. Here we analyzed the effect of weather variables on CH₄ and CO₂ flux from a small shallow pond during a period of 4 months. Mean CH₄ flux and surface water CH₄ concentration were 8.0 [3.3–15.1] ± 3.1 mmol m^?2 day^?1 (mean [range] ± 1 SD) and 1.3 [0.3–3.5] ± 0.9 µM respectively. Mean CO₂ flux was 1.1 [?9.8 to 16.0] ± 6.9 mmol m^?2 day^?1. Substantial diel changes in CO₂ flux and surface water CH₄ concentration were observed during detailed measurements over a 24 h cycle. Thus diel patterns need to be accounted for in future measurements. Significant positive correlations of CH₄ emissions with temperature were found and could include both direct temperature effects as well as indirect effects (e.g. related to the growth season and macrophyte primary productivity providing organic substrates). CO₂ flux on the other hand was negatively correlated to temperature and solar radiation, presumably because CO₂ consumption by plants was higher relative to CO₂ production by respiration during warm sunny days. Interestingly, CH₄ fluxes were comparable to ponds with similar morphometry and macrophyte abundance in the tropics. We therefore hypothesize that CH₄ and CO₂ summer emissions from ponds could be more related to the morphometry and dominating primary producers rather than latitude per se. Data indicate that CH₄ emissions, given the system characteristic frameworks, is positively affected by increased temperatures or prolonged growth seasons.     

Productivity and Temperature as Drivers of Seasonal and Spatial Variations of Dissolved Methane in the Southern Bight of the North Sea

Dissolved CH₄ concentrations in the Belgian coastal zone (North Sea) ranged between 670 nmol l^?1 nearshore and 4 nmol l^?1 offshore. Spatial variations of CH₄ were related to sediment organic matter (OM) content and gassy sediments. In nearshore stations with fine sand or muddy sediments, the CH₄ seasonal cycle followed water temperature, suggesting methanogenesis control by temperature in these OM-rich sediments. In offshore stations with permeable sediments, the CH₄ seasonal cycle showed a yearly peak following the chlorophyll-a spring peak, suggesting that in these OM-poor sediments, methanogenesis depended on freshly produced OM delivery. This does not exclude the possibility that some CH₄ might originate from dimethylsulfide (DMS) or dimethylsulfoniopropionate (DMSP) or methylphosphonate transformations in the most offshore stations. Yet, the average seasonal CH₄ cycle was unrelated to those of DMS(P), very abundant during the Phaeocystis bloom. The annual average CH₄ emission was 126 mmol m^?2 y^?1 in the most nearshore stations (~4 km from the coast) and 28 mmol m^?2 y^?1 in the most offshore stations (~23 km from the coast), 1260–280 times higher than the open ocean average value (0.1 mmol m^?2 y^?1). The strong control of CH₄ by sediment OM content and by temperature suggests that marine coastal CH₄ emissions, in particular in shallow areas, should respond to future eutrophication and warming of climate. This is supported by the comparison of CH₄ concentrations at five stations obtained in March 1990 and 2016, showing a decreasing trend consistent with alleviation of eutrophication in the area.     

Methane and carbon dioxide emissions from inland waters in India – implications for large scale greenhouse gas balances

Inland waters were recently recognized to be important sources of methane (CH₄) and carbon dioxide (CO₂) to the atmosphere, and including inland water emissions in large scale greenhouse gas (GHG) budgets may potentially offset the estimated carbon sink in many areas. However, the lack of GHG flux measurements and well‐defined inland water areas for extrapolation, make the magnitude of the potential offset unclear. This study presents coordinated flux measurements of CH₄ and CO₂ in multiple lakes, ponds, rivers, open wells, reservoirs, springs, and canals in India. All these inland water types, representative of common aquatic ecosystems in India, emitted substantial amounts of CH₄ and a major fraction also emitted CO₂. The total CH₄ flux (including ebullition and diffusion) from all the 45 systems ranged from 0.01 to 52.1 mmol m^?2 d^?1, with a mean of 7.8 ± 12.7 (mean ± 1 SD) mmol m^?2 d^?1. The mean surface water CH₄ concentration was 3.8 ± 14.5 μm (range 0.03–92.1 μm ). The CO₂ fluxes ranged from ?28.2 to 262.4 mmol m^?2 d^?1 and the mean flux was 51.9 ± 71.1 mmol m^?2 d^?1. The mean partial pressure of CO₂ was 2927 ± 3269 μatm (range: 400–11 467 μatm). Conservative extrapolation to whole India, considering the specific area of the different water types studied, yielded average emissions of 2.1 Tg CH₄ yr^?1 and 22.0 Tg CO₂ yr^?1 from India's inland waters. When expressed as CO₂ equivalents, this amounts to 75 Tg CO₂ equivalents yr^?1 (53–98 Tg CO₂ equivalents yr^?1; ± 1 SD)_, with CH₄ contributing 71%. Hence, average inland water GHG emissions, which were not previously considered, correspond to 42% (30–55%) of the estimated land carbon sink of India. Thereby this study illustrates the importance of considering inland water GHG exchange in large scale assessments.     

Seasonal variability of methane in the rivers and lagoons of Ivory Coast (West Africa)

We report a data-set of dissolved methane (CH₄) in three rivers (Comoé, Bia and Tanoé) and five lagoons (Grand-Lahou, Ebrié, Potou, Aby and Tendo) of Ivory Coast (West Africa), during the four main climatic seasons (high dry season, high rainy season, low dry season and low rainy season). The surface waters of the three rivers were over-saturated in CH₄ with respect to atmospheric equilibrium (2221–38719%), and the seasonal variability of CH₄ seemed to be largely controlled by dilution during the flooding period. The strong correlation of CH₄ concentrations with the partial pressure of CO₂ (pCO₂) and dissolved silicate (DSi) confirm the dominance of a continental sources (from soils) for both CO₂ and CH₄ in these rivers. Diffusive air–water CH₄ fluxes ranged between 25 and 1187 μmol m^?2 day^?1, and annual integrated values were 288 ± 107, 155 ± 38, and 241 ± 91 μmol m^?2 day^?1 in the Comoé, Bia and Tanoé rivers, respectively. In the five lagoons, surface waters were also over-saturated in CH₄ (ranging from 1496 to 51843%). Diffusive air–water CH₄ fluxes ranged between 20 and 2403 μmol m^?2 day^?1, and annual integrated values were 78 ± 34, 338 ± 217, 227 ± 79, 330 ± 153 and 326 ± 181 μmol m^?2 day^?1 in the Grand-Lahou, Ebrié, Potou, Aby and Tendo lagoons, respectively. The largest CH₄ over-saturations were observed in the Tendo and Aby lagoons that are permanently stratified systems (unlike the other three lagoons), leading to anoxic bottom waters favorable for a large CH₄ production. In addition, these two stratified lagoons showed low pCO₂ values due to high primary production, which suggests an efficient transfer of organic matter across the pycnocline. As a result, the stratified Tendo and Aby lagoons were respectively, a low source of CO₂ to the atmosphere and a sink of atmospheric CO₂ while the other three well-mixed lagoons were strong sources of CO₂ to the atmosphere but less over-saturated in CH₄.     

Spatial and temporal dynamics of diffusive methane emissions in the Okavango Delta,northern Botswana,Africa

Global warming is associated with the continued increase in the atmospheric concentrations of greenhouse gases; carbon dioxide, methane (CH₄) and nitrous oxide. Wetlands constitute the largest single natural source of atmospheric CH₄ in the world contributing between 100 and 231 Tg year^?1 to the total budget of 503–610 Tg year^?1, approximately 60 % of which is emitted from tropical wetlands. We conducted diffusive CH₄ emission measurements using static chambers in river channels, floodplains and lagoons in permanent and seasonal swamps in the Okavango Delta, Botswana. Diffusive CH₄ emission rates varied between 0.24 and 293 mg CH₄ m^?2 h^?1, with a mean (±SE) emission of 23.2 ± 2.2 mg CH₄ m^?2 h^?1 or 558 ± 53 mg CH₄ m^?2 day^?1. These emission rates lie within the range reported for other tropical wetlands. The emission rates were significantly higher (P < 0.007) in permanent than in seasonal swamps. River channels exhibited the highest average fluxes at 31.3 ± 5.4 mg CH₄ m^?2 h^?1 than in floodplains (20.4 ± 2.5 mg CH₄ m^?2 h^?1) and lagoons (16.9 ± 2.6 mg CH₄ m^?2 h^?1). Diffusive CH₄ emissions in the Delta were probably regulated by temperature since emissions were highest (20–300 mg CH₄ m^?2 h^?1) and lowest (0.2–3.0 mg m^?2 h^?1) during the warmer-rainy and cooler winter seasons, respectively. Surface water temperatures between December 2010 and January 2012 varied from 15.3 °C in winter to 33 °C in summer. Assuming mean inundation of 9,000 km², the Delta’s annual diffusive emission was estimated at 1.8 ± 0.2 Tg, accounting for 2.8 ± 0.3 % of the total CH₄ emission from global tropical wetlands.     

Spatial variability of N2O, CH4 and CO2 fluxes within the Xilin River catchment of Inner Mongolia, China: a soil core study

In order to identify the effects of land-use/cover types, soil types and soil properties on the soil-atmosphere exchange of greenhouse gases (GHG) in semiarid grasslands as well as provide a reliable estimate of the midsummer GHG budget, nitrous oxide (N₂O), methane (CH₄) and carbon dioxide (CO₂) fluxes of soil cores from 30 representative sites were determined in the upper Xilin River catchment in Inner Mongolia. The soil N₂O emissions across all of the investigated sites ranged from 0.18 to 21.8 μg N m^-2 h^-1, with a mean of 3.4 μg N m^-2 h^-1 and a coefficient of variation (CV, which is given as a percentage ratio of one standard deviation to the mean) as large as 130%. CH₄ fluxes ranged from -88.6 to 2,782.8 μg C m^-2 h^-1 (with a CV of 849%). Net CH₄ emissions were only observed from cores taken from a marshland site, whereas all of the other 29 investigated sites showed net CH₄ uptake (mean: -33.3 μg C m^-2 h^-1). CO₂ emissions from all sites ranged from 3.6 to 109.3 mg C m^-2 h^-1, with a mean value of 37.4 mg C m^-2 h^-1 and a CV of 66%. Soil moisture primarily and positively regulated the spatial variability in N₂O and CO₂ emissions (R²?=?0.15–0.28, P?<?0.05). The spatial variation of N₂O emissions was also influenced by soil inorganic N contents (P?<?0.05). By simply up-scaling the site measurements by the various land-use/cover types to the entire catchment area (3,900 km²), the fluxes of N₂O, CH₄ and CO₂ at the time of sampling (mid-summer 2007) were estimated at 29 t CO₂-C-eq d^-1, -26 t CO₂-C-eq d^-1 and 3,223 t C d^-1, respectively. This suggests that, in terms of assessing the spatial variability of total GHG fluxes from the soils at a semiarid catchment/region, intensive studies may focus on CO₂ exchange, which is dominating the global warming potential of midsummer soil-atmosphere GHG fluxes. In addition, average GHG fluxes in midsummer, weighted by the areal extent of these land-use/cover types in the region, were approximately -30.0 μg C m^-2 h^-1 for CH₄, 2.4 μg N m^-2 h^-1 for N₂O and 34.5 mg C m^-2 h^-1 for CO₂.     

Carbon dioxide and methane emissions from interfluvial wetlands in the upper Negro River basin, Brazil

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. CO₂ emissions averaged 2193 mg C m^?2 day^?1. Methane was consumed in unflooded environments and emitted in flooded environments with average values of ?4.8 and 60 mg C m^?2 day^?1, respectively. Bubbles were emitted primarily during falling water periods when hydrostatic pressure at the sediment?Cwater interface declined. CO₂ and CH₄ emissions increased when dissolved O₂ 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 km²) emitted 1147 Gg C year^?1 as CO₂ and 31 Gg C year^?1 as CH₄. If these rates are extrapolated to the area occupied by hydromorphic soils in the upper Negro basin, 63 Tg C year^?1 of CO₂ and 1.7 Tg C year^?1 as CH₄ are estimated as the regional evasion to the atmosphere.     

CO2 dynamics along Danish lowland streams: water–air gradients, piston velocities and evasion rates

We measured CO₂ concentration and determined evasion rate and piston velocity across the water–air interface in flow-through chambers at eight stations along two 20 km long streams in agricultural landscapes in Zealand, Denmark. Both streams were 9–18-fold supersaturated in CO₂ with daily means of 240 and 340 μM in January–March and 130 and 180 μM in June–August. Annual CO₂ medians were 212 μM in six other streams and 460 μM in four groundwater wells, while seven lakes were weakly supersaturated (29 μM). Air concentrations immediately above stream surfaces were close to mean atmospheric conditions except during calm summer nights. Piston velocity from 0.4 to 21.6 cm h^?1 was closely related to current velocity permitting calculation of evasion rates for entire streams. CO₂ evasion rates were highest in midstream reaches (170–1,200 mmol m^?2 day^?1) where CO₂-rich soil water entered fast stream flow, while rates were tenfold lower (25–100 mmol m^?2 day^?1) in slow-flowing lower reaches. CO₂ evasion mainly derived from the input of CO₂ in soil water. The variability of CO₂ evasion along the two lowland streams covered much of the range in sub-Arctic and temperate streams reported previously. In budgets for the two stream catchments, loss of carbon from soils via the hydrological cycle was substantial (3.2–5.7 mmol m^?2 day^?1) and dominated by CO₂ consumed to form HCO₃ ^? by mineral dissolution (69–76%) and export of organic carbon (15–23%) relative to dissolved CO₂ export (7–9%).     

Effects of short-term drying and irrigation on CO2 and CH4 production and emission from mesocosms of a northern bog and an alpine fen

Atmospheric CO₂ and CH₄ exchange in peatlands is controlled by water table levels and soil moisture, but impacts of short periods of dryness and rainfall are poorly known. We conducted drying-rewetting experiments with mesocosms from an ombrotrophic northern bog and an alpine, minerotrophic fen. Efflux of CO₂ and CH₄ was measured using static chambers and turnover and diffusion rates were calculated from depth profiles of gas concentrations. Due to a much lower macroporosity in the fen compared to the bog peat, water table fluctuated more strongly when irrigation was stopped and resumed, about 11 cm in the fen and 5 cm in the bog peat. Small changes in air filled porosity caused CO₂ and CH₄ concentrations in the fen peat to be insensitive to changes in water table position. CO₂ emission was by a factor of 5 higher in the fen than in the bog mesocosms and changed little with water table position in both peats. This was probably caused by the importance of the uppermost, permanently unsaturated zone for auto- and heterotrophic CO₂ production, and a decoupling of air filled porosity from water table position. CH₄ emission was <0.4 mmol m^?2 day^?1 in the bog peat, and up to >12.6 mmol m^?2 day^?1 in the fen peat, where it was lowered by water table fluctuations. CH₄ production was limited to the saturated zone in the bog peat but proceeded in the capillary fringe of the fen peat. Water table drawdown partly led to inhibition of methanogenesis in the newly unsaturated zone, but CH₄ production appeared to continue after irrigation without time-lag. The identified effects of irrigation on soil moisture and respiration highlight the importance of peat physical properties for respiratory dynamics; but the atmospheric carbon exchange was fairly insensitive to the small-scale fluctuations induced.     

Carbon Dioxide and Methane Fluxes From Tree Stems,Coarse Woody Debris,and Soils in an Upland Temperate Forest

Forest soils and canopies are major components of ecosystem CO₂ and CH₄ fluxes. In contrast, less is known about coarse woody debris and living tree stems, both of which function as active surfaces for CO₂ and CH₄ fluxes. We measured CO₂ and CH₄ fluxes from soils, coarse woody debris, and tree stems over the growing season in an upland temperate forest. Soils were CO₂ sources (4.58 ± 2.46 µmol m^?2 s^?1, mean ± 1 SD) and net sinks of CH₄ (?2.17 ± 1.60 nmol m^?2 s^?1). Coarse woody debris was a CO₂ source (4.23 ± 3.42 µmol m^?2 s^?1) and net CH₄ sink, but with large uncertainty (?0.27 ± 1.04 nmol m^?2 s^?1) and with substantial differences depending on wood decay status. Stems were CO₂ sources (1.93 ± 1.63 µmol m^?2 s^?1), but also net CH₄ sources (up to 0.98 nmol m^?2 s^?1), with a mean of 0.11 ± 0.21 nmol m^?2 s^?1 and significant differences depending on tree species. Stems of N. sylvatica, F. grandifolia, and L. tulipifera consistently emitted CH₄, whereas stems of A. rubrum, B. lenta, and Q. spp. were intermittent sources. Coarse woody debris and stems accounted for 35% of total measured CO₂ fluxes, whereas CH₄ emissions from living stems offset net soil and CWD CH₄ uptake by 3.5%. Our results demonstrate the importance of CH₄ emissions from living stems in upland forests and the need to consider multiple forest components to understand and interpret ecosystem CO₂ and CH₄ dynamics.     

Release of CO2 and CH4 from lakes and drainage ditches in temperate wetlands

Shallow fresh water bodies in peat areas are important contributors to greenhouse gas fluxes to the atmosphere. In this study we determined the magnitude of CH₄ and CO₂ fluxes from 12 water bodies in Dutch wetlands during the summer season and studied the factors that might regulate emissions of CH₄ and CO₂ from these lakes and ditches. The lakes and ditches acted as CO₂ and CH₄ sources of emissions to the atmosphere; the fluxes from the ditches were significantly larger than the fluxes from the lakes. The mean greenhouse gas flux from ditches and lakes amounted to 129.1 ± 8.2 (mean ± SE) and 61.5 ± 7.1 mg m^?2 h^?1 for CO₂ and 33.7 ± 9.3 and 3.9 ± 1.6 mg m^?2 h^?1 for CH₄, respectively. In most water bodies CH₄ was the dominant greenhouse gas in terms of warming potential. Trophic status of the water and the sediment was an important factor regulating emissions. By using multiple linear regression 87% of the variation in CH₄ could be explained by PO₄ ^3? concentration in the sediment and Fe²⁺ concentration in the water, and 89% of the CO₂ flux could be explained by depth, EC and pH of the water. Decreasing the nutrient loads and input of organic substrates to ditches and lakes by for example reducing application of fertilizers and manure within the catchments and decreasing upward seepage of nutrient rich water from the surrounding area will likely reduce summer emissions of CO₂ and CH₄ from these water bodies.     

Soil N and C trace gas fluxes and microbial soil N turnover in a sessile oak (Quercus petraea (Matt.) Liebl.) forest in Hungary

During two intensive field campaigns in summer and autumn 2004 nitrogen (N₂O, NO/NO₂) and carbon (CO₂, CH₄) trace gas exchange between soil and the atmosphere was measured in a sessile oak (Quercus petraea (Matt.) Liebl.) forest in Hungary. The climate can be described as continental temperate. Fluxes were measured with a fully automatic measuring system allowing for high temporal resolution. Mean N₂O emission rates were 1.5 μg N m⁻² h⁻¹ in summer and 3.4 μg N m⁻² h⁻¹ in autumn, respectively. Also mean NO emission rates were higher in autumn (8.4 μg N m⁻² h⁻¹) as compared to summer (6.0 μg N m⁻² h⁻¹). However, as NO₂ deposition rates continuously exceeded NO emission rates (−9.7 μg N m⁻² h⁻¹ in summer and −18.3 μg N m⁻² h⁻¹ in autumn), the forest soil always acted as a net NO _x sink. The mean value of CO₂ fluxes showed only little seasonal differences between summer (81.1 mg C m⁻² h⁻¹) and autumn (74.2 mg C m⁻² h⁻¹) measurements, likewise CH₄uptake (summer: −52.6 μg C m⁻² h⁻¹; autumn: −56.5 μg C m⁻² h⁻¹). In addition, the microbial soil processes net/gross N mineralization, net/gross nitrification and heterotrophic soil respiration as well as inorganic soil nitrogen concentrations and N₂O/CH₄ soil air concentrations in different soil depths were determined. The respiratory quotient (ΔCO_{2 resp} ΔO_{2 resp}⁻¹) for the uppermost mineral soil, which is needed for the calculation of gross nitrification via the Barometric Process Separation (BaPS) technique, was 0.8978 ± 0.008. The mean value of gross nitrification rates showed only little seasonal differences between summer (0.99 μg N kg⁻¹ SDW d⁻¹) and autumn measurements (0.89 μg N kg⁻¹ SDW d⁻¹). Gross rates of N mineralization were highest in the organic layer (20.1–137.9 μg N kg⁻¹ SDW d⁻¹) and significantly lower in the uppermost mineral layer (1.3–2.9 μg N kg⁻¹ SDW d⁻¹). Only for the organic layer seasonality in gross N mineralization rates could be demonstrated, with highest mean values in autumn, most likely caused by fresh litter decomposition. Gross mineralization rates of the organic layer were positively correlated with N₂O emissions and negatively correlated with CH₄ uptake, whereas soil CO₂ emissions were positively correlated with heterotrophic respiration in the uppermost mineral soil layer. The most important abiotic factor influencing C and N trace gas fluxes was soil moisture, while the influence of soil temperature on trace gas exchange rates was high only in autumn.     

Gaseous and fluvial carbon export from an Amazon forest watershed

The transfer of carbon (C) from Amazon forests to aquatic ecosystems as CO₂ supersaturated in groundwater that outgases to the atmosphere after it reaches small streams has been postulated to be an important component of terrestrial ecosystem C budgets. We measured C losses as soil respiration and methane (CH₄) flux, direct CO₂ and CH₄ fluxes from the stream surface and fluvial export of dissolved inorganic C (DIC), dissolved organic C (DOC), and particulate C over an annual hydrologic cycle from a 1,319-ha forested Amazon perennial first-order headwater watershed at Tanguro Ranch in the southern Amazon state of Mato Grosso. Stream pCO₂ concentrations ranged from 6,491 to 14,976 ??atm and directly-measured stream CO₂ outgassing flux was 5,994 ± 677 g C m^?2 y^?1 of stream surface. Stream pCH₄ concentrations ranged from 291 to 438 ??atm and measured stream CH₄ outgassing flux was 987 ± 221 g C m^?2 y^?1. Despite high flux rates from the stream surface, the small area of stream itself (970 m², or 0.007% of watershed area) led to small directly-measured annual fluxes of CO₂ (0.44 ± 0.05 g C m² y^?1) and CH₄ (0.07 ± 0.02 g C m² y^?1) per unit watershed land area. Measured fluvial export of DIC (0.78 ± 0.04 g C m^?2 y^?1), DOC (0.16 ± 0.03 g C m^?2 y^?1) and coarse plus fine particulate C (0.001 ± 0.001 g C m^?2 y^?1) per unit watershed land area were also small. However, stream discharge accounted for only 12% of the modeled annual watershed water output because deep groundwater flows dominated total runoff from the watershed. When C in this bypassing groundwater was included, total watershed export was 10.83 g C m^?2 y^?1 as CO₂ outgassing, 11.29 g C m^?2 y^?1 as fluvial DIC and 0.64 g C m^?2 y^?1 as fluvial DOC. Outgassing fluxes were somewhat lower than the 40?C50 g C m^?2 y^?1 reported from other Amazon watersheds and may result in part from lower annual rainfall at Tanguro. Total stream-associated gaseous C losses were two orders of magnitude less than soil respiration (696 ± 147 g C m^?2 y^?1), but total losses of C transported by water comprised up to about 20% of the ± 150 g C m^?2 (±1.5 Mg C ha^?1) that is exchanged annually across Amazon tropical forest canopies.     

Methane fluxes from a patterned fen of the northeastern part of the La Grande river watershed, James Bay, Canada

In northeastern Canada, at the ecotonal limit of the forest tundra and lichen woodland, a rise of the regional water table in the peatland systems was registered since Little Ice Age resulting in increasing pool compartment at the expense of terrestrial surfaces. We hypothesized that, with a mean water table closer to peat surface and higher pool density, these ecosystems would be great CH₄ emitters. In summers 2009 and 2010, methane fluxes were measured in a patterned fen located in the northeastern portion of the La Grande river watershed to determine the contribution of the different microforms (lawns, hollows, hummocks, string, pools) to the annual CH₄ budget. Mean seasonal CH₄ fluxes from terrestrial microforms ranged between 12.9 and 49.4 mg m^?2 day^?1 in 2009 and 15.4 and 47.3 mg m^?2 day^?1 in 2010. Pool fluxes (which do not include ebullition fluxes) ranged between 102.6 and 197.6 mg CH₄ m^?2 day^?1 in 2009 and 76.5 and 188.1 mg CH₄ m^?2 day^?1 in 2010. Highest fluxes were measured in microforms with water table closer to peat surface but no significant relationship was observed between water table depth and CH₄ fluxes. Spatially weighted CH₄ budget demonstrates that, during the growing season, the studied peatland emitted 66 ± 31 in 2009 and 55 ± 26 mg CH₄ m^?2 day^?1 in 2010, 79 % of which is accounted by pool fluxes. In a context where climate projections predict greater precipitations in northeastern Canada, these results indicate that this type of peatlands could contribute to modify the methane balance in the atmosphere.     

Carbon flows in eutrophic Lake Rotsee: a <Superscript>13</Superscript>C-labelling experiment

The microbial segment of food webs plays a crucial role in lacustrine food-web functioning and carbon transfer, thereby influencing carbon storage and CO₂ emission and uptake in freshwater environments. Variability in microbial carbon processing (autotrophic and heterotrophic production and respiration based on glucose) with depth was investigated in eutrophic, methane-rich Lake Rotsee, Switzerland. In June 2011, ¹³C-labelling experiments were carried out at six depth intervals in the water column under ambient light as well as dark conditions to evaluate the relative importance of (chemo)autotrophic, mixotrophic and heterotrophic production. Label incorporation rates of phospholipid-derived fatty acid (PLFA) biomarkers allowed us to differentiate between microbial producers and calculate group-specific production. We conclude that at 6 m, net primary production (NPP) rates were highest, dominated by algal photoautotrophic production. At 10 m —the base of the oxycline— a distinct low-light community was able to fix inorganic carbon, while in the hypolimnion, heterotrophic production prevailed. At 2 m depth, high label incorporation into POC could only be traced to nonspecific PLFA, which prevented definite identification, but suggests cyanobacteria as dominating organisms. There was also depth zonation in extracellular carbon release and heterotrophic bacterial growth on recently fixed carbon. Large differences were observed between concentrations and label incorporation of POC and biomarkers, with large pools of inactive biomass settling in the hypolimnion, suggesting late-/post-bloom conditions. Net primary production (115 mmol C m^?2 d^?1) reached highest values in the epilimnion and was higher than glucose-based production (3.3 mmol C m^?2 d^?1, highest rates in the hypolimnion) and respiration (5.9 mmol C m^?2 d^?1, highest rates in the epilimnion). Hence, eutrophic Lake Rotsee was net autotrophic during our experiments, potentially storing large amounts of carbon.     

Leaf Litter Fuels Methanogenesis Throughout Decomposition in a Forested Peatland

Decomposing leaf litter is a large supply of energy and nutrients for soil microorganisms. How long decaying leaves continue to fuel anaerobic microbial activity in wetland ecosystems is poorly understood. Here, we compare leaf litter from 15 tree species with different growth forms (angiosperms and gymnosperms, deciduous, and longer life span), using litterbags positioned for up to 4 years in a forested peatland in New York State. Periodically, we incubated partially decayed residue per species with fresh soil to assess its ability to fuel microbial methane (CH₄) production and concomitant anaerobic carbon dioxide (CO₂) production. Decay rates varied by leaf type: deciduous angiosperm > evergreen gymnosperm > deciduous gymnosperm. Decay rates were slower in leaf litter with a large concentration of lignin. Soil with residue of leaves decomposed for 338 days had greater rates of CH₄ production (5.8 µmol g^?1 dry mass d^?1) than less decomposed (<0.42 µmol g^?1 dry mass d^?1) or more decomposed (2.1 µmol g^?1 dry mass d^?1) leaf residue. Species-driven differences in their ability to fuel CH₄ production were evident throughout the study, whereas concomitant rates of CO₂ production were more similar among species and declined with degree of decomposition. Methane production rates exhibited a positive correlation with pectin and the rate of pectin decomposition. This link between leaf litter decay rates, biochemical components in leaves, and microorganisms producing greenhouse gases should improve predictions of CH₄ production in wetlands.     

Net ecosystem carbon exchange and the greenhouse gas balance of tidal marshes along an estuarine salinity gradient

Tidal wetlands are productive ecosystems with the capacity to sequester large amounts of carbon (C), but we know relatively little about the impact of climate change on wetland C cycling in lower salinity (oligohaline and tidal freshwater) coastal marshes. In this study we assessed plant production, C cycling and sequestration, and microbial organic matter mineralization at tidal freshwater, oligohaline, and salt marsh sites along the salinity gradient in the Delaware River Estuary over four years. We measured aboveground plant biomass, carbon dioxide (CO₂) and methane (CH₄) exchange between the marsh and atmosphere, microbial sulfate reduction and methanogenesis in marsh soils, soil biogeochemistry, and C sequestration with radiodating of soils. A simple model was constructed to estimate monthly and annually integrated rates of gross ecosystem production (GEP), ecosystem respiration (ER) to carbon dioxide ( \( {\text{ER}}_{{{\text{CO}}_{2} }} \) ) or methane ( \( {\text{ER}}_{{{\text{CH}}_{4} }} \) ), net ecosystem production (NEP), the contribution of sulfate reduction and methanogenesis to ER, and the greenhouse gas (GHG) source or sink status of the wetland for 2 years (2007 and 2008). All three marsh types were highly productive but evidenced different patterns of C sequestration and GHG source/sink status. The contribution of sulfate reduction to total ER increased along the salinity gradient from tidal freshwater to salt marsh. The Spartina alterniflora dominated salt marsh was a C sink as indicated by both NEP (~140 g C m^?2 year^?1) and ²¹⁰Pb radiodating (336 g C m^?2 year^?1), a minor sink for atmospheric CH₄, and a GHG sink (~620 g CO_2-eq m^?2 year^?1). The tidal freshwater marsh was a source of CH₄ to the atmosphere (~22 g C–CH₄ m^?2 year^?1). There were large interannual differences in plant production and therefore C and GHG source/sink status at the tidal freshwater marsh, though ²¹⁰Pb radiodating indicated modest C accretion (110 g C m^?2 year^?1). The oligohaline marsh site experienced seasonal saltwater intrusion in the late summer and fall (up to 10 mS cm^?1) and the Zizania aquatica monoculture at this site responded with sharp declines in biomass and GEP in late summer. Salinity intrusion was also linked to large effluxes of CH₄ at the oligohaline site (>80 g C–CH₄ m^?2 year^?1), making this site a significant GHG source (>2,000 g CO_2-eq m^?2 year^?1). The oligohaline site did not accumulate C over the 2 year study period, though ²¹⁰Pb dating indicated long term C accumulation (250 g C m^?2 year^?1), suggesting seasonal salt-water intrusion can significantly alter C cycling and GHG exchange dynamics in tidal marsh ecosystems.     

Metabolic threshold and sulfide-buffering in diffusion controlled marine sediments impacted by continuous organic enrichment

The effects of organic enrichment on sediment biogeochemistry was studied in diffusion controlled sediment mesocosms, where labile organic matter (OM) (fish feed) pulses were added once a week to the sediment surface. Two types of sediments, differing mainly in content of reactive Fe, were used. The aim of this experiment was two-fold, (1) to evaluate the importance of Fe-driven sulfide buffering for sulfide accumulation in surface enriched sediments, and (2) to estimate the diagenetic capacity for degradation of labile OM near the sediment surface. The simulated OM loading rate of 375 mmol C m^?2 day^?1 led to a 5–6 times increase in CO₂-production and a 4–5 times increase in O₂-uptake. Sulfate reduction estimated by radiotracer experiments and CO₂-release was 105–131 mmol m^?2 day^?1, but accumulation of porewater sulfide was low in both sediment types. Instead 99% of sulfide was oxidized with O₂ at the sediment water interface in the low Fe treatment, whereas 46% of produced sulfide precipitated as Fe-S compound in the high Fe treatment resulting in significantly lower O₂-uptake. Furthermore, the accumulation of up to 30% of added OM by the end of the experiment indicated a saturation of the heterotrophic microbial communities in the upper enriched surface layer. These results suggest a maximum diagenetic capacity for OM degradation in the range of ~25 μmol C cm^?3 day^?1 or 260 mmol m^?2 day^?1 for the present sediment types.     

A modeling approach to assess the greenhouse gas risk in Koteshwar hydropower reservoir,India

Climate changes are likely to be significantly affected by reservoirs/lakes due to emission of greenhouse gas (GHG), change in the magnitude and seasonality of river runoffs and severe extreme events. In this study, a coupled GHG Risk Assessment Tool (GRAT) and Soil Water Assessment Tool (SWAT) are used to predict the GHG risk of Koteshwar reservoirs located in Uttarakhand, India. Before running the GRAT model, SWAT model was used to simulate the runoffs (one of the major input of GRAT). The model was calibrated (2004–09) and validated (2010–13) at Uttarkashi station using monthly discharge data. The model performance was checked by R², NSE, RSR, and p-value as 0.785, 0.60, 0.63, and 0.04, respectively, during calibration and 0.790, 0.66, 0.57, and 0.06 during validation and shows satisfactory model performance on monthly time step. Further, GRAT model is also applied and the results show that Koteshwar reservoir is found to be under high risk of CO₂ (CO₂ > 645 mg m^?2 d^?1) and medium risk of CH₄ (CH₄ < 45 mg m^?2 d^?1) till 2023. Subsequently, the GHG risk is minimized after passage of time over 100 years. These models may be used by the policy-makers to know the potential of GHG and its vulnerability to the reservoirs after the impoundment.