Wetlands are the largest natural source of atmospheric methane. Here, we assess controls on methane flux using a database of approximately 19 000 instantaneous measurements from 71 wetland sites located across subtropical, temperate, and northern high latitude regions. Our analyses confirm general controls on wetland methane emissions from soil temperature, water table, and vegetation, but also show that these relationships are modified depending on wetland type (bog, fen, or swamp), region (subarctic to temperate), and disturbance. Fen methane flux was more sensitive to vegetation and less sensitive to temperature than bog or swamp fluxes. The optimal water table for methane flux was consistently below the peat surface in bogs, close to the peat surface in poor fens, and above the peat surface in rich fens. However, the largest flux in bogs occurred when dry 30-day averaged antecedent conditions were followed by wet conditions, while in fens and swamps, the largest flux occurred when both 30-day averaged antecedent and current conditions were wet. Drained wetlands exhibited distinct characteristics, e.g. the absence of large flux following wet and warm conditions, suggesting that the same functional relationships between methane flux and environmental conditions cannot be used across pristine and disturbed wetlands. Together, our results suggest that water table and temperature are dominant controls on methane flux in pristine bogs and swamps, while other processes, such as vascular transport in pristine fens, have the potential to partially override the effect of these controls in other wetland types. Because wetland types vary in methane emissions and have distinct controls, these ecosystems need to be considered separately to yield reliable estimates of global wetland methane release.

Abstract. We investigate temporal changes in methane emissions over a three-year period from two peatlands in Michigan. Mean daily fluxes ranged from 0.6-68.4 mg CH4 mm2 d-' in plant communities dominated by Chamaedaphnecalyculata, an ericaceous shrub, to 11 S-209 mg CH4 m-* d-' in areas dominated by plants with aerenchymatous tissues, such as Carex oligosperma and Scheuchzeria palustris. Correlations between methane flux and water table position were significant at all sites for one annual cycle when water table fluctuations ranged from 15 cm above to 50 cm below the peat surface. Correlations were not significant during the second and third annual periods with smaller water table fluctuations. Methane flux was strongly correlated with peat temperatures at -5 to -40 cm (rs = 0.82 to 0.98) for all three years at sites with flora acting as conduits for methane transport. At shrub sites, the correlations between methane flux and peat temperature were weak to not significant during the first two years, but were strong in the third year. Low rates of methane consumption (-0.2 to -1.5 mg CH4 mm2 d-l) were observed at shrub sites when the water table was below -20 cm, while sites with plants capable of methane transport always had positive net fluxes of methane. The methane oxidizing potential at both types of sites was confirmed by peat core experiments. The results of this study indicate that methane emissions occur at rates that cannot be explained by diffusion alone; plant communities play a significant role in altering methane flux from peatland ecosystems by directly transporting methane from anaerobic peat to the atmosphere.

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A process-basemd odelh as beend evelopedin order to calculate methane emissions from natural wetlands as a function of the hydrologic and thermal conditions in the soil. The considered processes in the model are methane production, methane consumption and transport of methane by diffusion, ebullition and through plants. The model has been tested against data from a three-year field study from a Michigan peatland. The interannual and seasonal variations of the modelled methane emissions and methane concentration profiles are in good agreement with the observations. During the growing season the main emission pathway proceeds through plants. Ebullition occurs whenever the water table is above the soil surface, while diffusion is only significant in the first 15 days after a drop of the water table below the peat surface

Abstract.Bubbling is an important pathway of methane emissions from wetland ecosystems.However the concentration-based threshold function approach in current biogeochemistry models of methane is not sufficient to represent the complex ebullition process. Here we revise an extant process-based biogeochemistry model, the Terrestrial Ecosystem Model into a multi-substance model (CH 4 , O 2 , CO 2 and N 2 ) to simulate methane production, oxidation, and transport (particularly ebullition) with different model complexities. When ebullition is modeled with a concentrationbased threshold function and if the inhibition effect of oxygen on methane production and the competition for oxygen between methanotrophy and heterotrophic respiration are retained, the model becomes a two-substance system. Ignoring the role of oxygen, while still modeling ebullition with a concentration-based threshold function, reduces the model to a one-substance system. These models were tested through a group of sensitivity analyses using data from two temperate peatland sites in Michigan. We demonstrate that only the four-substance model with a pressure-based ebullition algorithm is able to capture the episodic emissions induced by a sudden decrease in atmospheric pressure or by a sudden drop in water table. All models captured the retardation effect on methane efflux from an increase in surface standing water which results from the inhibition of diffusion and the increase in rhizospheric oxidation. We conclude that to Correspondence to: J. Tang (tang16@purdue.edu) more accurately account for the effects of atmospheric pressure dynamics and standing water on methane effluxes, the multi-substance model with a pressure-based ebullition algorithm should be used in the future to quantify global wetland CH 4 emissions. Further, to more accurately simulate the pore water gas concentrations and different pathways of methane transport, an exponential root distribution function should be used and the phase-related parameters should be treated as temperature dependent.