Carbon Dioxide and Methane Production in Small Reservoirs Flooding Upland Boreal Forest

Wetland microtopography alters response of potential net CO2 and CH4 production to temperature and moisture: Evidence from a laboratory experiment

The greenhouse gases methane (CH4) and carbon dioxide (CO2) are end products of microbial anaerobic degradation of organic matter (OM) in lake sediments. Although previous research has shown that phytoplankton lipid content influences sediment methanogenesis, current understanding on how OM quality affects methanogenesis is still limited. Such information is needed to more accurately assess how lake greenhouse gas emissions may change in response to anthropogenic activities. We cultured 11 phytoplankton species from five classes and studied how taxonomic identity, C : N ratio, lipid content, and fatty acid composition of phytoplankton biomass affects the CH4 and net CO2 production in anaerobic lake sediments with an incubation experiment that lasted > 100 d. The carbon‐normalized potential CH4 (0.09–0.23 μmol mg C−1 d−1) and net CO2 (0.09–0.28 μmol mg C−1 d−1) production rates were not related to phytoplankton taxonomic affiliation (e.g., class, species), C : N ratio, or fatty acid composition of algal biomass. Methane or net CO2 production potentials did not increase with higher lipid content (10–30%); however, total fatty acid content had a weak correlation with CH4 production potential. In contrast to previous research, our results suggest that lipid content is of minor importance in determining methanogenesis rates from the biomass of multispecies phytoplankton communities settling on sediments. The decrease in CO2 concentration and the correlation between stable carbon isotope signatures of CH4 and molar ratio of CH4 and CO2 at the end of the experiment may indicate that importance of hydrogenotrophic methanogenesis, which uses CO2 when other substrates become limiting, increased during the long incubation.

The FLooded Uplands Dynamics EXperiment (FLUDEX) was initiated to quantify carbon dioxide (CO2) and methane (CH4) production in boreal reservoirs, and to better understand underlying biogeochemical processes (dissolved inorganic carbon [DIC] production, net primary production [NPP], methanogenesis, and CH4 oxidation) governing CO2 and CH4 production in flooded boreal landscapes. The study experimentally flooded three upland boreal forest sites with different organic carbon (OC) storage in soils and vegetation over three seasons (June to September 1999–2001). Mass budgets of all reservoir inorganic carbon (inorganic C) and CH4 inputs and outputs were calculated to quantify net reservoir CO2 and CH4 production, and isotopic ratio mass budgets were calculated to quantify biogeochemical processes controlling net reservoir CO2 and CH4 production.The three reservoirs produced both CO2 and CH4 during each of the three flooding seasons, but neither CO2 nor CH4 production was related to overall mass of flooded OC. Net reservoir CO2 production in the second and third flooding seasons (408 to 479 kg C ha−1) was lower than in the first flooding season (703 to 797 kg C ha−1), while reservoir CH4 production steadily increased with each successive flooding season (from 3.2 to 4.6 kg C ha−1 in 1999 to 29.7 to 35.2 kg C ha−1 in 2001). Over three flooding seasons, NPP ranged from 77 to 273 kg C ha–1 and consumed 15 to 40% of gross reservoir CO2 production. CH4 oxidation was negligible during the first flooding season, but reduced gross reservoir CH4 production by 50% during the second flooding season, and by 70 to 88% during the third flooding season. However, despite decreases in net reservoir CO2 production and increases in CH4 oxidation over the study period, the overall total global warming potential (GWP) of the FLUDEX reservoirs remained constant due to successive increases in net CH4 production.

Greenhouse gas production and consumption in High Arctic deserts

The mechanisms, pathways, and rates of CO2 and CH4 production are central to understanding carbon cycling and greenhouse gas flux in wetlands. Thawing permafrost regions are of particular interest because they are disproportionally affected by climate warming and store large reservoirs of organic C that may be readily converted to CO2 and CH4 upon thaw. This conversion is accomplished by a community of microorganisms interacting in complex ways to transform large organic compounds into fatty acids and ultimately CO2 and CH4. While the central role of microbes in this process is well-known, geochemical rate models rarely integrate microbiological information. Herein, we expanded the geochemical rate model of Neumann et al., (2016, Biogeochemistry 127: 57–87) to incorporate a Bayesian probability analysis and applied the result to quantifying rates of CO2, CH4, and acetate production in closed-system incubations of peat collected from three habitats along a permafrost thaw gradient. The goals of this analysis were twofold. First, we integrated microbial community analyses with geochemical rate modeling by using microbial data to inform the best model choice among equally mathematically feasible model variants. Second, based on model results, we described changes in organic carbon transformation among habitats to understand the changing pathways of greenhouse gas production along the permafrost thaw gradient. We found that acetoclasty, hydrogenotrophy, CO2 production, and homoacetogenesis were the important reactions in this system, with little evidence for anaerobic CH4 oxidation. There was a distinct transition in the reactions across the thaw gradient. The collapsed palsa stage presents an initial disequilibrium where the abrupt (physically and temporally) change in elevation introduces freshly fixed carbon into anoxic conditions and the fermentation products build up over time as the system transitions through the acid phase and electron acceptors are depleted. In the bog, fermentation slows, while methanogenesis increases. In the fully-thawed fen, most of the terminal electron acceptors are depleted and the system becomes increasingly methanogenic. This suggests that as permafrost regions thaw and dry palsas transition into wet fens, CH4 emissions will rise, increasing the warming potential of these systems and accelerating climate warming feedbacks.

Flooding boreal forest ecosystems for hydroelectric power generation may release substantial amounts of carbon (C) to the atmosphere, contributing to global warming. The objectives of this study were to evaluate CO2 and CH4 production rates using spring/fall (14 °C) and summer (21 °C) temperatures under non-flooded and flooded conditions. incubation temperatures represented the mean annual air temperature in May and September (14 °C), and in July (21 °C). Greenhouse gas production rates were quantified using laboratory incubations of 2 contrasting soil types (Humo-Ferric Podzol [very dry, mineral] and a Histic Folisol [moist, organic]) collected at the experimental lakes area in northwestern Ontario, Canada. The mean production rate of CO2 and CH4 in the headspace of the incubation jars was significantly influenced by temperature, flooding and soil type. results showed that the mean CO2 (65 [Podzol]; 43 [Folisol]) and CH4 (0.06 [Podzol]; 0.06 [Folisol]) production rates (μg·g−1·−1) were significantly higher (P < 0.05) at 21 °C and in flooded treatments from both soil types. The greatest CO2 and CH4 production rates (μg·g−1·d−1) occurred from the Folisol (110 [CO2]; 0.03 [CH4]) and the L and FH horizons of the Podzol (250 [CO2]; 0.05 [CH4]). Q10 values showed that decomposition of soil organic matter was more temperature dependent in non-flooded treatments, with values ranging from 1.57 to 5.03, than in flooded treatments (1.31 to 3.58). The greatest loss of soil organic carbon relative to the original C content (g-m−2) occurred in the Ah horizon of the Podzol in non-flooded (0.01% [14 °C]; 0.02% [21°C]) and flooded (0.02% [14 °C]; 0.03% [21 °C]) treatments and at both temperatures. Information presented in this paper helps to evaluate how 2 contrasting soil types (Podzol and Folisol) responded to flooding and provided further insight into the dynamics of greenhouse gas (GHG) production rates as a result of hydroelectric reservoir creation. This will aid in future planning, construction, and management of hydroelectric reservoirs to help minimize GHG emissions and boreal forest disturbance.

Permafrost thaw leads to thermokarst lake formation and talik growth tens of meters deep, enabling microbial decomposition of formerly frozen organic matter (OM). We analyzed two 17-m-long thermokarst lake sediment cores taken in Central Yakutia, Russia. One core was from an Alas lake in a Holocene thermokarst basin that underwent multiple lake generations, and the second core from a young Yedoma upland lake (formed ~70years ago) whose sediments have thawed for the first time since deposition. This comparison provides a glance into OM fate in thawing Yedoma deposits. We analyzed total organic carbon (TOC) and dissolved organic carbon (DOC) content, n-alkane concentrations, and bacterial and archaeal membrane markers. Furthermore, we conducted 1-year-long incubations (4°C, dark) and measured anaerobic carbon dioxide (CO2 ) and methane (CH4 ) production. The sediments from both cores contained little TOC (0.7±0.4 wt%), but DOC values were relatively high, with the highest values in the frozen Yedoma lake sediments (1620mgL-1 ). Cumulative greenhouse gas (GHG) production after 1year was highest in the Yedoma lake sediments (226±212µgCO2 -Cg-1 dw, 28±36µg CH4 -Cg-1 dw) and 3 and 1.5 times lower in the Alas lake sediments, respectively (75±76µgCO2 -Cg-1 dw, 19±29µg CH4 -Cg-1 dw). The highest CO2 production in the frozen Yedoma lake sediments likely results from decomposition of readily bioavailable OM, while highest CH4 production in the non-frozen top sediments of this core suggests that methanogenic communities established upon thaw. The lower GHG production in the non-frozen Alas lake sediments resulted from advanced OM decomposition during Holocene talik development. Furthermore, we found that drivers of CO2 and CH4 production differ following thaw. Our results suggest that GHG production from TOC-poor mineral deposits, which are widespread throughout the Arctic, can be substantial. Therefore, our novel data are relevant for vast ice-rich permafrost deposits vulnerable to thermokarst formation.

Increasing global temperatures are changing the balance between carbon sequestration and its microbial processing in wetlands, making the tracking of these processes important. We used detrital carbon stable isotopes (δ13C) to trace aerobic decomposition and CH4 production in two experiments conducted in Alaskan wetlands. In laboratory bottle incubations, larger decreases in detritus δ13C corresponded to higher net CH4 and CO2 production rates. Because net CH4 production was the stronger predictor and its effect was negative, we hypothesize that decreases in δ13C trace concurrent CH4 production and oxidation. In a field experiment, decreases in detritus δ13C were not correlated with aerobic decomposition rates, but were positively correlated with CH4 production potentials as estimated from bottle incubations. We hypothesize that the positive relationship reflects only CH4 production, rather than concurrent production and oxidation. Although CH4 production rates were correlated with changes in detrital δ13C in both experiments, the direction of this relationship differed between laboratory and field with important consequences for the scale of ecological experiments. Our study demonstrates that CH4 cycling can create distinct patterns in δ13C of wetland detritus. Future studies should conduct explicit mass balance experiments to clarify mechanisms and determine the importance of scale in shaping isotopic patterns.

Soil respiration (SR) plays an important role in the global carbon cycle. The widespread and continued conversion of tropical forests to plantations is expected to drastically alter CO2 production in soil, with significant consequences for atmospheric concentrations of this crucial greenhouse gas. In Southeast Asia, rubber plantations are among the most widespread monoculture tree plantations. However, knowledge of how SR differs in rubber plantations compared to natural forests is scarce. In this study, surface CO2 fluxes and soil CO2 concentrations (at 5 cm, 10 cm, 30 cm and 70 cm depths) were measured at regular intervals over a one-year period along slopes at three sites in paired natural tropical forests and mature rubber plantations. Annual surface soil CO2 fluxes were 15% lower in the rubber plantations than in natural forest. This difference was due to substantially lower SR during the dry season in rubber plantations compared to natural forest. During the wet season, SR did not differ significantly between rubber plantations and natural forests. In rubber plantations, soil moisture increased from lower and middle to upper slope positions, but this did not significantly impact SR. Throughout the year, net CO2 production per unit volume in the topsoil (2.5–7.5 cm) exceeded by 2–3 orders of magnitude net CO2 production in the subsoil (7.5–50 cm). However, CO2 originating from 5 cm depth and below in both land cover types could only explain up to 30% of the aboveground measured CO2 flux, indicating that>70% of the total CO2 respired and emitted to the atmosphere originated from the uppermost few cm of the soil. Net CO2 production at different soil depths did not differ significantly between rubber plantations and natural forests. Our results indicate that SR characteristics in mature rubber plantation and natural forest were broadly similar, although dry season soil surface CO2 fluxes were lower in rubber plantations. However, further information on the drivers of CO2 production in the uppermost topsoil layers which are responsible for most CO2 emissions is needed to understand the extent to which these results are generalisable.

Past water management affected GHG production and microbial community pattern in Italian rice paddy soils

Deep carbon pool in permafrost regions is an important component of the global terrestrial carbon cycle. However, the greenhouse gas production from deep permafrost soils is not well understood. Here, using soils collected from 5-m deep permafrost cores from meadow and wet meadow on the northern Qinghai-Tibetan Plateau (QTP), we investigated the effects of temperature on CO2 and N2O production under aerobic incubations and CH4 production under anaerobic incubations. After a 35-day incubation, the CO2, N2O and CH4 production at −2 °C to 10 °C were 0.44~2.12 mg C-CO2/g soil C, 0.0027~0.097 mg N-N2O/g soil N, and 0.14~5.88 μg C-CH4/g soil C, respectively. Greenhouse gas production in deep permafrost is related to the C:N ratio and stable isotopes of soil organic carbon (SOC), whereas depth plays a less important role. The temperature sensitivity (Q10) values of the CO2, N2O and CH4 production were 1.67–4.15, 3.26–5.60 and 5.22–10.85, without significant differences among different depths. These results indicated that climate warming likely has similar effects on gas production in deep permafrost and surface soils. Our results suggest that greenhouse gas emissions from both the deep permafrost and surface soils to the air will increase under future climate change.

Despite the rising interest in understanding how climate change could affect the emissions of greenhouse gases (GHGs) from river systems, including floodplains, we still lack a mechanistic understanding of how changing environmental conditions, such as moisture and nutrient availability, limit the temperature responses of GHG production in floodplain sediments. To examine the environmental co-limitations on the temperature responses of three major GHGs (CO2, CH4, and N2O) produced in floodplain sediments, sediments from a constructed wetland on the floodplain of the lower Han River were incubated for 24 d at four temperatures spanning 4–28 ℃, under three conditions (closed, open/wetting, and open/drying). The net production of all three GHGs exhibited nonlinear temperature responses with gas-specific patterns and magnitudes of response varying over the incubation period. During the later incubation phase, positive temperature responses were weakened for the net production of CO2 and CH4 in the dried treatments, whereas a similar weakening occurred for N2O production in the wet treatments. This, combined with incubation-induced changes in dissolved organic carbon and its fluorescence components, indicated the lack or excess of moisture and associated changes in O2 and organic carbon availability as critical co-limiting factors for the temperature responses of GHG production. Warming decreased δ13C in the CH4 emitted from wet and hypoxic sediments, implying a stronger warming effect on CH4 production over oxidation. Unlike many studies assuming a consistent relationship between temperature and GHG production in sediments irrespective of other environmental conditions, our results suggest that warming effects on the GHG emissions from floodplain sediments would depend on the balance between gas production and consumption under the prevailing constraints of moisture, O2, and labile carbon availability.

The study of greenhouse gas (GHG) fluxes in terrestrial ecosystems is becoming increasingly important as the observed rise in global temperature and increased frequency of extreme weather events are attributed by the majority of climate experts to increased atmospheric GHG concentrations. Adequate and comprehensive knowledge of surface GHG fluxes is important for obtaining reliable information on CO2 and other GHG fluxes at regional and global scales, as well as for preparing reports on national GHG emissions and removals. The need to obtain accurate estimates of GHG fluxes at regional and global scales has led to the development of innovative mathematical models of varying complexity. These models can be divided into forward and inverse models. Forward algorithms provide the ability to estimate GHG fluxes when sufficient information on the structure of GHG sources and sinks is available. Inverse algorithms allow the retrieval of surface fluxes using remote sensing data. The most promising way to study high resolution fluxes over areas with complex topography and mosaic vegetation patterns is the use of unmanned aerial vehicles (UAVs).In our study, we proposed and tested a forward and inverse model for estimating GHG fluxes over an inhomogeneous underlying surface. The forward model is based on the RANS hydrodynamic model to calculate the wind velocity and turbulence coefficient, and the solution of the advection-diffusion equation to find a three-dimensional distribution of GHG concentrations. The GHG fluxes at the specified height above the ground surface are then calculated using the obtained concentration distribution and turbulence coefficient. The inverse algorithm is based on minimizing a cost functional, defined as the root mean square deviation of the modeled concentration field from the measured data. Concentration measurements at multiple (at least two) levels can be performed using UAV-based gas analyzers.Three experimental sites selected for our modeling study differ in geographic location, topography, and vegetation heterogeneity. These sites are: i) swampy and forested areas of the "Mukhrino" carbon supersite (Khanty-Mansiysk Autonomous Okrug, Russia, 60°53'20" N, 68°42'10" E), ii) the Roshni-Chu mountain forest site, which is part of the "Way Carbon" supersite (Chechen Republic, Russia, 43°2'59" N, 45°25'32" E), iii) the mixed forest experimental site "Lyali" (Komi Republic, Russia, 62°16'28" N, 50°39'54" E). For our numerical experiments we used measured data on surface topography, LAI, soil respiration, air temperature, prevailing wind direction, vertical canopy CO2 concentration profile and CO2 fluxes measured by eddy covariance technique.The model results show a rather good agreement with the measured data and could help to interpret the experimentally observed dependence of CO2 fluxes on wind direction in areas with an inhomogeneous underlying surface.

The production and consumption of the greenhouse gases (GHGs) methane (CH4), carbon dioxide (CO2) and nitrous oxide (N2O) in soil profile are poorly understood. This work sought to quantify the GHG production and consumption at seven depths (0–30, 30–60, 60–90, 90–150, 150–200, 200–250 and 250–300 cm) in a long-term field experiment with a winter wheat-summer maize rotation system, and four N application rates (0; 200; 400 and 600 kg N ha−1 year−1) in the North China Plain.The gas samples were taken twice a week and analyzed by gas chromatography. GHG production and consumption in soil layers were inferred using Fick’s law. Results showed nitrogen application significantly increased N2O fluxes in soil down to 90 cm but did not affect CH4 and CO2 fluxes. Soil moisture played an important role in soil profile GHG fluxes; both CH4 consumption and CO2 fluxes in and from soil tended to decrease with increasing soil water filled pore space (WFPS). The top 0–60 cm of soil was a sink of atmospheric CH4, and a source of both CO2 and N2O, more than 90% of the annual cumulative GHG fluxes originated at depths shallower than 90 cm; the subsoil (>90 cm) was not a major source or sink of GHG, rather it acted as a ‘reservoir’. This study provides quantitative evidence for the production and consumption of CH4, CO2 and N2O in the soil profile.

Chapter 7 - Impact of climate change and related disturbances on CO2 and CH4 cycling in coastal wetlands