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Methane (CH 4 ) is a potent greenhouse gas (GHG) with atmospheric concentrations that have nearly tripled since pre-industrial times. Wetlands account for a large share of global CH 4 emissions, yet the magnitude and factors controlling CH 4 fluxes in tidal wetlands remain uncertain. We synthesized CH 4 flux data from 100 chamber and 9 eddy covariance (EC) sites across tidal marshes in the conterminous United States to assess controlling factors and improve predictions of CH 4 emissions. This effort included creating an open-source database of chamber-based GHG fluxes (https://doi.org/10.25573/serc.14227085). Annual fluxes across chamber and EC sites averaged 26 ± 53 g CH 4 m -2  year -1 , with a median of 3.9 g CH 4 m -2  year -1 , and only 25% of sites exceeding 18 g CH 4 m -2  year -1 . The highest fluxes were observed at fresh-oligohaline sites with daily maximum temperature normals (MATmax) above 25.6°C. These were followed by frequently inundated low and mid-fresh-oligohaline marshes with MATmax ≤25.6°C, and mesohaline sites with MATmax >19°C. Quantile regressions of paired chamber CH 4 flux and porewater biogeochemistry revealed that the 90th percentile of fluxes fell below 5 ± 3 nmol m -2  s -1 at sulfate concentrations >4.7 ± 0.6 mM, porewater salinity >21 ± 2 psu, or surface water salinity >15 ± 3 psu. Across sites, salinity was the dominant predictor of annual CH 4 fluxes, while within sites, temperature, gross primary productivity (GPP), and tidal height controlled variability at diel and seasonal scales. At the diel scale, GPP preceded temperature in importance for predicting CH 4 flux changes, while the opposite was observed at the seasonal scale. Water levels influenced the timing and pathway of diel CH 4 fluxes, with pulsed releases of stored CH 4 at low to rising tide. This study provides data and methods to improve tidal marsh CH 4 emission estimates, support blue carbon assessments, and refine national and global GHG inventories., (© 2024 Smithsonian Institution and The Author(s). Global Change Biology published by John Wiley & Sons Ltd. This article has been contributed to by U.S. Government employees and their work is in the public domain in the USA.)

Belowground ecosystem processes can be highly variable and difficult to predict using microbial community data. Here, we argue that this stems from at least three issues: (a) complex covariance structure of samples (with environmental conditions or spatial proximity) can make distinguishing biotic drivers a challenge; (b) communities can control ecosystem processes through multiple mechanisms, making the identification of these controls a challenge; and (c) ecosystem function assessments can be broad in physiological scale, encapsulating multiple processes with unique microbially mediated controls. We test these assertions using methane (CH 4 )-cycling processes in soil samples collected along a wetland-to-upland habitat gradient in the Congo Basin. We perform our measurements of function under controlled laboratory conditions and statistically control for environmental covariates to aid in identifying biotic drivers. We divide measurements of microbial communities into four attributes (abundance, activity, composition, and diversity) that represent different forms of community control. Lastly, our process measurements differ in physiological scale, including broader processes (gross methanogenesis and methanotrophy) that involve more mediating groups, to finer processes (hydrogenotrophic methanogenesis and high-affinity CH 4 oxidation) with fewer mediating groups. We observed that finer scale processes can be more readily predicted from microbial community structure than broader scale processes. In addition, the nature of those relationships differed, with broad processes limited by abundance while fine-scale processes were associated with diversity and composition. These findings demonstrate the importance of carefully defining the physiological scale of ecosystem function and performing community measurements that represent the range of possible controls on ecosystem processes., (© 2020 John Wiley & Sons Ltd.)

Understanding the dynamics of methane (CH4 ) emissions is of paramount importance because CH4 has 25 times the global warming potential of carbon dioxide (CO2 ) and is currently the second most important anthropogenic greenhouse gas. Wetlands are the single largest natural CH4 source with median emissions from published studies of 164 Tg yr(-1) , which is about a third of total global emissions. We provide a perspective on important new frontiers in obtaining a better understanding of CH4 dynamics in natural systems, with a focus on wetlands. One of the most exciting recent developments in this field is the attempt to integrate the different methodologies and spatial scales of biogeochemistry, molecular microbiology, and modeling, and thus this is a major focus of this review. Our specific objectives are to provide an up-to-date synthesis of estimates of global CH4 emissions from wetlands and other freshwater aquatic ecosystems, briefly summarize major biogeophysical controls over CH4 emissions from wetlands, suggest new frontiers in CH4 biogeochemistry, examine relationships between methanogen community structure and CH4 dynamics in situ, and to review the current generation of CH4 models. We highlight throughout some of the most pressing issues concerning global change and feedbacks on CH4 emissions from natural ecosystems. Major uncertainties in estimating current and future CH4 emissions from natural ecosystems include the following: (i) A number of important controls over CH4 production, consumption, and transport have not been, or are inadequately, incorporated into existing CH4 biogeochemistry models. (ii) Significant errors in regional and global emission estimates are derived from large spatial-scale extrapolations from highly heterogeneous and often poorly mapped wetland complexes. (iii) The limited number of observations of CH4 fluxes and their associated environmental variables loosely constrains the parameterization of process-based biogeochemistry models., (© 2012 Blackwell Publishing Ltd.)

UNLABELLED: Wetlands cover a small portion of the world, but have disproportionate influence on global carbon (C) sequestration, carbon dioxide and methane emissions, and aquatic C fluxes. However, the underlying biogeochemical processes that affect wetland C pools and fluxes are complex and dynamic, making measurements of wetland C challenging. Over decades of research, many observational, experimental, and analytical approaches have been developed to understand and quantify pools and fluxes of wetland C. Sampling approaches range in their representation of wetland C from short to long timeframes and local to landscape spatial scales. This review summarizes common and cutting-edge methodological approaches for quantifying wetland C pools and fluxes. We first define each of the major C pools and fluxes and provide rationale for their importance to wetland C dynamics. For each approach, we clarify what component of wetland C is measured and its spatial and temporal representativeness and constraints. We describe practical considerations for each approach, such as where and when an approach is typically used, who can conduct the measurements (expertise, training requirements), and how approaches are conducted, including considerations on equipment complexity and costs. Finally, we review key covariates and ancillary measurements that enhance the interpretation of findings and facilitate model development. The protocols that we describe to measure soil, water, vegetation, and gases are also relevant for related disciplines such as ecology. Improved quality and consistency of data collection and reporting across studies will help reduce global uncertainties and develop management strategies to use wetlands as nature-based climate solutions. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1007/s13157-023-01722-2.

Understanding the dynamics of peatland methane (CH4) emissions and quantifying sources of uncertainty in estimating peatland CH4 emissions are critical for mitigating climate change. The relative contributions of CH4 emission pathways through ebullition, plant-mediated transport, and diffusion, together with their different transport rates and vulnerability to oxidation, determine the quantity of CH4 to be oxidized before leaving the soil. Notwithstanding their importance, the relative contributions of the emission pathways are highly uncertain. In particular, the ebullition process is more uncertain and can lead to large uncertainties in modeled CH4 emissions. To improve model simulations of CH4 emission and its pathways, we evaluated two model structures: (1) the ebullition bubble growth volume threshold approach (EBG) and (2) the modified ebullition concentration threshold approach (ECT) using CH4 flux and concentration data collected in a peatland in northern Minnesota, USA. When model parameters were constrained using observed CH4 fluxes, the CH4 emissions simulated by the EBG approach (RMSE = 0.53) had a better agreement with observations than the ECT approach (RMSE = 0.61). Further, the EBG approach simulated a smaller contribution from ebullition but more frequent ebullition events than the ECT approach. The EBG approach yielded greatly improved simulations of pore water CH4 concentrations, especially in the deep soil layers, compared to the ECT approach. When constraining the EBG model with both CH4 flux and concentration data in model–data fusion, uncertainty of the modeled CH4 concentration profiles was reduced by 78 % to 86 % in comparison to constraints based on CH4 flux data alone. The improved model capability was attributed to the well-constrained parameters regulating the CH4 production and emission pathways. Our results suggest that the EBG modeling approach better characterizes CH4 emission and underlying mechanisms. Moreover, to achieve the best model results both CH4 flux and concentration data are required to constrain model parameterization. [ABSTRACT FROM AUTHOR]

Understanding the dynamics of peatland methane (CH4) emissions and quantifying sources of uncertainty in estimating peatland CH4 emissions are critical for mitigating climate change. The relative contributions of CH4 emission pathways through ebullition, plant-mediated transport, and diffusion together with their different transport rates and vulnerability to oxidation determine the quantity of CH4 to be oxidized before leaving the soil. Notwithstanding their importance, the relative contributions of the emission pathways have not been well characterized by experiments or modeling approaches. In particular, the ebullition process is more uncertain and can lead to large uncertainties in modeled CH4 emissions. To improve model simulations of CH4 emission and its pathways, we evaluated two model structures: 1) the Ebullition Bubble Growth volume threshold approach (EBG) and 2) the modified Ebullition Concentration Threshold approach (ECT) using CH4 flux and concentration data collected in a peatland in northern Minnesota, USA. When model parameters were constrained using observed CH4 fluxes, the CH4 emissions simulated by the EBG approach (RMSE = 0.53) had a better agreement with observations than the ECT approach (RMSE = 0.61). Further, the EBG approach simulated a smaller contribution from ebullition but more frequent ebullition events than the ECT approach. The EBG approach yielded greatly improved simulations of pore water CH4 concentrations, especially in the deep soil layers, compared to the ECT approach. When constraining the EBG model with both CH4 flux and concentration data in model-data fusion, uncertainty of the modeled CH4 concentration profiles was reduced by 78 to 86 % in comparison to constraints based on CH4 flux data alone. The improved model capability was attributed to the well-constrained parameters regulating the CH4 production and emission pathways. Our results suggest that the EBG modeling approach better characterizes CH4 emission and underlying mechanisms. Moreover, to achieve the best model results both CH4 flux and concentration data are required to constrain model parameterization. [ABSTRACT FROM AUTHOR]

Peatlands are one of the largest natural sources for atmospheric methane (CH4), a potent greenhouse gas. Climate warming and elevated atmospheric carbon dioxide (CO2) are two important environmental factors that have been confirmed to stimulate peatland CH4 emissions; however, the mechanisms underlying enhanced emissions remain elusive. A data‐model integration approach was applied to understand the CH4 processes in a northern temperate peatland under a gradient of warming and doubled atmospheric CO2 concentration. We found that warming and elevated CO2 stimulated CH4 emissions through different mechanisms. Warming initially stimulated but then suppressed vegetative productivity while stimulating soil organic matter (SOM) mineralization and dissolved organic carbon (DOC) fermentation, which led to higher acetate production and enhanced acetoclastic and hydrogenotrophic methanogenesis. Warming also enhanced surface CH4 emissions, which combined with warming‐caused decreases in CH4 solubility led to slightly lower dissolved CH4 concentrations through the soil profiles. Elevated CO2 enhanced ecosystem productivity and SOM mineralization, resulting in higher DOC and acetate concentrations. Higher DOC and acetate concentrations increased acetoclastic and hydrogenotrophic methanogenesis and led to higher dissolved CH4 concentrations and CH4 emissions. Both warming and elevated CO2 had minor impacts on CH4 oxidation. A meta‐analysis of warming and elevated CO2 impacts on carbon cycling in wetlands agreed well with a majority of the modeled mechanisms. This mechanistic understanding of the stimulating impacts of warming and elevated CO2 on peatland CH4 emissions enhances our predictability on the climate‐ecosystem feedback. Plain Language Summary: Peatlands are one of the largest natural sources for a potential greenhouse gas—methane. In this study, we took use of a number of field observational data to parameterize a microbial model before applying the model to understand the methane processes in a northern temperate peatland under a gradient of warming and doubled atmospheric carbon dioxide concentration. We found that warming and elevated carbon dioxide stimulated methane emissions through different mechanisms. Warming initially stimulated but then suppressed vegetative productivity while stimulating soil organic matter mineralization and dissolved organic carbon fermentation, which led to higher acetate production and enhanced methane production. Elevated carbon dioxide enhanced ecosystem productivity and soil organic carbon decomposition, resulting in higher dissolved organic carbon and acetate concentrations, which stimulate methane production. Both warming and elevated carbon dioxide had small impacts on methane oxidation. The modeling results are consistent with a global data synthesis. This mechanistic understanding of the stimulating impacts of warming and elevated carbon dioxide on peatland methane emissions enhances our ability to predict the interactions between the climate system and the terrestrial ecosystems. Key Points: Warming and elevated CO2 stimulate peatland CH4 emissions through different mechanismsThe stimulating impact of warming is primarily through stimulation of microbial processesThe stimulating impact of elevated CO2 is primarily through enhanced substrate availability by increased photosynthesis [ABSTRACT FROM AUTHOR]

Wetlands are the major natural source of the greenhouse gas methane (CH4) and are also potentially an important source of nitrous oxide (N2O), though there is considerable variability among wetland types with some of the greatest uncertainty in freshwater mineral-soil wetlands. In particular, trace gas emissions from seasonal wetlands have been very poorly studied. We measured fluxes of CH4, N2O, and CO2 (carbon dioxide), soil nutrients, and net primary productivity over one year in natural, restored, and agricultural seasonal wetland prairies in the Willamette Valley, Oregon, USA. We found zero fluxes for CH4 and N2O, even during periods of extended waterlogging of the soil. To explore this lack of emissions, we performed a laboratory experiment to examine the controls over these gases. In a fully-factorial design, we amended anaerobic soils from all wetlands with nitrate, glucose, and NaOH (to neutralize pH) and measured production potentials of N2, N2O, CH4, and CO2. We found that denitrification and N2O production were co-limited by nitrate and carbon, with little difference between the three wetland types. This co-limitation suggests that low soil carbon availability will continue to constrainN2Oemissions and denitrification in these systems even when receiving relatively high levels of nitrogen inputs. Contrary to the results for N2O, the amended wetland soils never produced significant amounts of CH4 under any treatment. We hypothesize that high concentrations of alternative electron acceptors exist in these soils so that methanogens are noncompetitive with other microbial groups. As a result, these wetlands do not appear to be a significant source or sink of soil carbon and thus have a near zero climate forcing effect. Future research should focus on determining if this is a generalizable result in other seasonal wetlands. [ABSTRACT FROM AUTHOR]

Unraveling the mechanistic controls of methane (CH 4) cycling in northern peatland ecosystems is crucial for understanding peatland-climate feedbacks. Growing evidence indicates that the microbial reduction of organic matter as a terminal electron acceptor can be a key regulator of CH 4 production in peatlands, but the role of microbial organic matter reduction in different peatlands has not been well explored. Using an electron shuttling capacity assay, we investigated the relationship between the microbial reduction of organic matter and anaerobic CH 4 and carbon dioxide (CO 2) production in peatland soils in three experiments. In the first experiment, we surveyed the importance of microbial organic matter reduction in six soils representing the ombrotrophic-minerotrophic peatland gradient. In the second experiment, we further explored the reduction of solid-phase organic electron acceptors in a minerotrophic fen and compared these results to previously published values from an ombrotrophic bog (the end members of the initial gradient surveyed). Results from these experiments suggest that microbial organic matter reduction suppresses CH 4 production, especially in ombrotrophic peat soils, likely helping to explain low CH 4 production in bog-like soils. In contrast, the pool of oxidized organic matter was quickly reduced in minerotrophic peat soils which subsequently exhibited higher rates of CH 4 production. In the final experiment, we investigated the effect of temperature on microbial organic matter reduction in the same ombrotrophic bog soil, demonstrating that warmer temperatures resulted in both a faster reduction of solid-phase organic matter and an apparent increase in the size of the organic electron acceptor pool that can be reduced by microbes. Future work should explore the drivers of the observed differences in microbial organic matter reduction in different peatland soils to provide a stronger mechanistic explanation for how this process will regulate peatland greenhouse gas dynamics in the face of global change, including increases in temperature. • Microbial reduction of organic matter regulates greenhouse gas production in peatlands. • In ombrotrophic soils, organic matter reduction inhibits methane production. • Organic matter reduction and methane production are faster at warmer temperatures. [ABSTRACT FROM AUTHOR]

Peatlands are important ecosystems in the global carbon cycle, serving as both the largest terrestrial soil carbon pool and a significant source of the greenhouse gas methane (CH4). In Sphagnum moss-dominated wetlands, anaerobic decomposition, and in particular the production of CH4, is highly variable and controlling factors are poorly understood. The main objective of this study was to determine if leachates of Sphagnum can explain differences in anaerobic decomposition and CH4 production from three Sphagnum-dominated peatlands. Soils from each peatland were incubated anaerobically for 40 days with Sphagnum-derived organic matter (S-DOM) extracted using distilled water at 25 or 60 °C in a fully-crossed experimental design. S-DOM extracted at 25 °C had a minimal effect on decomposition, but S-DOM extracted at 60 °C increased CO2 production in all soils. The magnitude of the increased CO2 production in response to S-DOM depended on the source site of the S-DOM. The response of CH4 production to additions of S-DOM extracted at 60 °C was more complex. Soils from one peatland produced no CH4 during the incubation, regardless of S-DOM source. The same S-DOM additions led to an increase in CH4 production in a second soil, but a decrease in CH4 production in the third soil. Stable isotopic evidence suggests that these patterns were driven by the selective inhibition or stimulation of acetoclastic methanogenesis. Taken together, these data suggest S-DOM alone does not explain differences in anaerobic decomposition in peatlands, but may play a role in regulating CO2 and CH4 production. [ABSTRACT FROM AUTHOR]

Abstract: Peatlands, including fens, are important ecosystems in the context of the global carbon cycle. Future climate change and other anthropogenic activities are likely to increase nutrient loading in many peatland ecosystems and a better understanding of the effects of these nutrients on peatland carbon cycling is necessary. We investigated the effects of six years of nitrogen and phosphorus fertilization, along with liming, on carbon mineralization dynamics in an intermediate fen in northern Minnesota. Specifically, we measured CO2 and CH4 emission from intact peat cores, as well as CH4 production and CH4 consumption at multiple depths in short-term laboratory incubations. Despite increased nitrogen and phosphorus availability in the upper 5cm of peat, increased pH, and clear shifts in the vegetation community, fertilization and liming had limited effects on microbial carbon cycling in this fen. Liming reduced the net flux of CO2 approximately 3-fold compared to the control treatment, but liming had no effect on CH4 emissions from intact cores. There were no nutrient effects on CO2 or CH4 emissions from intact cores. In all treatments, rates of CH4 production increased with depth and rates of CH4 consumption were highest near the in situ water-table level. However, nutrient and liming had no effect on rates of CH4 production or CH4 consumption at any depth. Our results suggest that over at least the intermediate term, the microbial communities responsible for soil carbon cycling in this peatland are tolerant to wide ranges of nutrient concentrations and pH levels and may be relatively insensitive to future anthropogenic nutrient stress. [Copyright &y& Elsevier]

Climate change will directly affect carbon and nitrogen mineralization through changes in temperature and soil moisture, but it may also indirectly affect mineralization rates through changes in soil quality. We used an experimental mesocosm system to examine the effects of 6-year manipulations of infrared loading (warming) and water-table level on the potential anaerobic nitrogen and carbon (as carbon dioxide (CO2) and methane (CH4) production) mineralization potentials of bog and fen peat over 11 weeks under uniform anaerobic conditions. To investigate the response of the dominant methanogenic pathways, we also analyzed the stable isotope composition of CH4 produced in the samples. Bog peat from the highest water-table treatment produced more CO2 than bog peat from drier mesocosms. Fen peat from the highest water-table treatment produced the most CH4. Cumulative nitrogen mineralization was lowest in bog peat from the warmest treatment and lowest in the fen peat from the highest water-table treatment. As all samples were incubated under constant conditions, observed differences in mineralization patterns reflect changes in soil quality in response to climate treatments. The largest treatment effects on carbon mineralization as CO2 occurred early in the incubations and were ameliorated over time, suggesting that the climate treatments changed the size and/or quality of a small labile carbon pool. CH4 from the fen peat appeared to be predominately from the acetoclastic pathway, while in the bog peat a strong CH4 oxidation signal was present despite the anaerobic conditions of our incubations. There was no evidence that changes in soil quality have lead to differences in the dominant methanogenic pathways in these systems. Overall, our results suggest that even relatively short-term changes in climate can alter the quality of peat in bogs and fens, which could alter the response of peatland carbon and nitrogen mineralization to future climate change. [ABSTRACT FROM AUTHOR]

Limited previous research indicates that anaerobic oxidation of methane (AOM) can be an important control over methane (CH 4) emissions in freshwater wetlands. We examined the importance of AOM and its response to warming and elevated atmospheric carbon dioxide (CO 2) in a whole-ecosystem climate change experiment in an ombrotrophic bog in northern Minnesota, USA. In the first experiment, we sampled plots seasonally and measured rates of net CH 4 production or consumption with and without added porewater through the depth profile at in situ temperatures. Net CH 4 production occurred in most samples with porewater addition, but net CH 4 consumption occurred in most samples without porewater addition, despite being incubated under strictly anaerobic conditions. We hypothesized that these porewater-dependent results were due to the low solubility and initially low headspace concentrations of CH 4 , which together inhibited rates of AOM when porewater was added. In a second experiment, we directly measured rates of AOM and net CH 4 production in the same plots and calculated gross CH 4 production during a single sampling event. AOM and gross CH 4 production rates were much higher in surface peat, and AOM responded more strongly to warming than CH 4 production. Elevated CO 2 had minimal effects on any measured process. AOM consumed over 50% of the CH 4 produced in some samples, indicating that it is an important process in this bog. Surface soil samples with added porewater also had lower rates of AOM, corroborating the results of the first experiment. Overall, we show that AOM can be an important control over CH 4 emissions and their response to climate change in peatlands. Also, laboratory incubations of net CH 4 production represent the dual process of gross CH 4 production and AOM, and incubation conditions that differentially affect these two processes can provide widely disparate results. • Anaerobic oxidation of CH 4 (AOM) has been rarely measured in freshwater ecosystems. • AOM consumed large amounts of CH 4 production in surface peat in a northern bog. • AOM responded more strongly to temperature than gross CH 4 production. • Slurried, N 2 -flushed incubations typically underestimate AOM rates. [ABSTRACT FROM AUTHOR]

Peatlands contain up to half of terrestrial soil organic carbon (C) while simultaneously emitting the potent greenhouse gas methane (CH 4). Global change will alter C biogeochemistry in peatlands, but is hard to predict without a mechanistic understanding of the processes that control anaerobic C cycling. One of the least known anaerobic C cycling pathways in wetland systems is homoacetogenesis, the reduction of carbon dioxide (CO 2) with dihydrogen (H 2) to acetate. We developed a new technique to infer rates of putative homoacetogenesis by measuring the incorporation of 14C-labeled dissolved inorganic C into dissolved organic matter. We determined rates of homoacetogenesis and total and hydrogenotrophic CH 4 production throughout the peat profile on two sampling dates in a whole-ecosystem warming and elevated CO 2 experiment in a northern Minnesota bog, USA. Homoacetogenesis and hydrogenotrophic CH 4 production were greatest in surface soils and at warmer temperatures. However, homoacetogens were stronger competitors than hydrogenotrophic methanogens for H 2 in deeper soil depths and at lower temperatures. A higher ratio of acetoclastic to hydrogenotrophic methanogenesis is often associated with greater total CH 4 production, indicating that homoacetogenesis potentially can indirectly increase total CH 4 production. Additionally, failure to account for homoacetogenesis may lead to erroneous conclusions in stable isotope models of anaerobic C cycling and CH 4 production. This work shows that homoacetogenesis responds to warming and may impact CH 4 production, and thus this process should be included in future modeling of climate change effects in wetlands. • Homoacetogenesis is rarely studied but potentially important in natural ecosystems. • We developed a new method to quantify putative homoacetogenesis. • Homoacetogenesis competed with hydrogenotrophic methanogenesis for H 2 in a wetland. • Temperature and peat depth affected the degree of this competition. [ABSTRACT FROM AUTHOR]

Methane (CH4) production varies greatly among different types of peatlands along an ombrotrophic-minerotrophic hydrogeomorphic gradient. pH is thought to be a dominant control over observed differences in CH4 production across sites, and previous pH manipulation experiments have verified the inhibitory effect of low pH on CH4 production. In this experiment, we asked (i) if the major effect of low pH is direct inhibition of one or both pathways of methanogenesis and/or inhibition of 'upstream' fermentation that provides substrates for methanogens, and (ii) to what extent is pH sufficient to explain differences in CH4 production relative to other factors that co-vary across the gradient. To address these questions, we adjusted the pH of peat slurries from 6 peatlands to 4 levels (3.5, 4.5, 5.5, and 6.5) that reflected their range of native pH, maintained these pH levels over a 43-day anaerobic laboratory incubation, and measured a suite of responses within the anaerobic carbon cycle. Higher pH caused a significant increase in C02 production in all sites. Regardless of site, time, and pH level, the reduction of inorganic electron acceptors contributed to <12% of total C02 production. Higher pH caused acetate pooling by Day 7, but this effect was greater in the more ombrotrophic sites and lasted throughout the incubation, whereas acetate was almost completely consumed as a substrate for acetoclastic methano-genesis by Day 43 in the minerotrophic sites. Higher pH also enhanced CH4 production and this process was up to 436% more sensitive to changes in pH than C02 production. However, across all sites and pH levels, CH4 production accounted for <25% of the total gaseous C production. Fermentation appeared to be the main pathway for anaerobic C mineralization. Our results indicate that low pH inhibits CH4 production through direct inhibition of both methanogenesis pathways and indirectly through its effects on fermentation, but the direct effects are stronger. The inability of acetoclastic methanogenesis to fully compensate for acetate pooling in ombrotrophic peats at higher pH suggests that CH4 production is inhibited by some factor(s) in addition to pH in these sites. We examine a variety of other potential inhibitory mechanisms and postulate that humic substances may provide an important inhibitory effect over CH4 production in ombrotrophic peatlands. [ABSTRACT FROM AUTHOR]

• The ELM-SPRUCE was applied to understand the CH 4 processes under warming and elevated CO 2. • Warming and elevated CO 2 enhanced CH 4 emission and altered hydrology in peatland. • Warming-induced hydrological feedback mitigated the warming's effects on CH 4 emission. Peatland carbon cycling is critical for the land–atmosphere exchange of greenhouse gases, particularly under changing environments. Warming and elevated atmospheric carbon dioxide (eCO 2) concentrations directly enhance peatland methane (CH 4) emission, and indirectly affect CH 4 processes by altering hydrological conditions. An ecosystem model ELM-SPRUCE, the land model of the E3SM model, was used to understand the hydrological feedback mechanisms on CH 4 emission in a temperate peatland under a warming gradient and eCO 2 treatments. We found that the water table level was a critical regulator of hydrological feedbacks that affect peatland CH 4 dynamics; the simulated water table levels dropped as warming intensified but slightly increased under eCO 2. Evaporation and vegetation transpiration determined the water table level in peatland ecosystems. Although warming significantly stimulated CH 4 emission, the hydrological feedbacks leading to a reduced water table mitigated the stimulating effects of warming on CH 4 emission. The hydrological feedback for eCO 2 effects was weak. The comparison between modeled results with data from a field experiment and a global synthesis of observations supports the model simulation of hydrological feedbacks in projecting CH 4 flux under warming and eCO 2. The ELM-SPRUCE model showed relatively small parameter-induced uncertainties on hydrological variables and their impacts on CH 4 fluxes. A sensitivity analysis confirmed a strong hydrological feedback in the first three years and the feedback diminished after four years of warming. Hydrology-moderated warming impacts on CH 4 cycling suggest that the indirect effect of warming on hydrological feedbacks is fundamental for accurately projecting peatland CH 4 flux under climate warming. [ABSTRACT FROM AUTHOR]

• Microbial reduction of solid-phase OM regulates CH 4 dynamics in a northern peatland. • Warming enhanced microbial OM reduction and increased acetate availability for CH 4 production. • Ecosystem warming did not affect rates of microbial OM reduction through changes in soil quality. • Lower water tables increased the volume of oxidized OM with the potential to impact CH 4 production. Peatlands store approximately one-half of terrestrial soil organic carbon (C) and the future of this C in the face of ongoing global change remains a key question in global biogeochemistry. Particularly pressing is the need to understand if this C will remain in peatland soils or be returned to the atmosphere as the potent greenhouse gas methane (CH 4) in response to warming. Past work has demonstrated that the microbial reduction of organic matter (OM) as an alternative terminal electron acceptor (TEA) under anaerobic conditions can suppress CH 4 production and is a key mechanistic control of peatland CH 4 dynamics. Here we show that warming directly enhances rates of potential OM reduction in peatland soils, enhancing acetate availability and allowing for a faster onset of CH 4 production. Using soils collected from the ecosystem-scale Spruce and Peatland Responses Under Changing Environments (SPRUCE) experiment, we also show that while there were no indirect effects of 2 years of soil warming on potential OM reduction through changes in soil quality, warming-mediated lowering of the water table increased the volume of oxidized OM which will suppress CH 4 production following rewetting events. [ABSTRACT FROM AUTHOR]