Wetlands are one of the most important terrestrial ecosystems for land-atmosphere CH 4 exchange. A new process-based, biophysical model to quantify CH 4 emissions from natural wetlands was developed and integrated into a terrestrial ecosystem model (Integrated Biosphere Simulator). The new model represents a multisubstance system (CH 4 , O 2 , CO 2 , and H 2 ) and describes CH 4 production, oxidation, and three transport processes (diffusion, plant-mediated transport, and ebullition). The new model uses several critical microbial mechanisms to represent the interaction of anaerobic fermenters and homoacetogens, hydrogenotrophic, and acetoclastic methanogens, and methanotrophs in CH 4 production and oxidation. We applied the model to 24 different wetlands globally to compare the simulated CH 4 emissions to observations and conducted a sensitivity analysis. The results indicated that (1) for most sites, the model was able to capture the magnitude and variation of observed CH 4 emissions under varying environmental conditions; (2) the parameters that regulate dissolved organic carbon and acetate production, and acetoclastic methanogenesis had the significant impact on simulated CH 4 emissions; (3) the representation of the process components of CH 4 cycling showed that CH 4 oxidation was about half or more of CH 4 production, and plant-mediated transport was the dominant pathway at most sites; and (4) the seasonality of simulated CH 4 emissions can be controlled by soil temperature, water table position, or combinations thereof.Plain Language Summary CH 4 emission from wetlands is an important part of global carbon cycle. A new process-based model was developed to quantify the CH 4 emission from wetlands. The new model considered main microbial mechanisms and transport processes in wetland CH 4 cycling, and the modeled results matched the observed CH 4 emissions well at evaluation sites globally. A sensitivity analysis indicated the important role of parameters that controlled dissolved organic carbon and acetate production and acetoclastic methanogenesis. The assessment of process components of CH 4 cycling demonstrated the importance of CH 4 oxidation and plant-mediated transport in wetland CH 4 emission.
Nitrous oxide (N 2 O) is the third most important greenhouse gas after carbon dioxide (CO 2 ) and methane (CH 4 ), with a long lifetime of ∼120 years in the atmosphere (Fleming et al., 2011). The concentration of atmospheric N 2 O has increased from 270 ppb in the preindustrial period to 332.0 ppb in 2019 (WMO, 2020), which substantially contributes to global warming as its single-molecular global warming potential is about 298 times higher than that of CO 2 (Ramanathan et al., 1985). Anthropogenic activities, such as fertilizer use, animal manure generation, and land cultivation, have greatly increased the global input of reactive nitrogen (N) into natural forest and grassland ecosystems (Fowler et al., 2013), which consequently accelerated the nitrogen transformation processes, including N 2 O production, in these ecosystems. The N 2 O emission from natural forest and grassland ecosystems accounts for approximately 24%-75% of the total global N 2 O emission (H. Tian et al., 2013;R. Xu & Prentice, 2008;Zhuang et al., 2013). Therefore, it is crucial to estimate the magnitude and trend of N 2 O emission accurately from natural forest and grassland ecosystems, and understand its impacts on climate change.
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