Global Response and Mechanism of Methane Cycling in Wetlands Under Elevated Atmospheric CO 2 and Warming

Wetland is one of the most significant sources of methane (CH 4 ). Although global warming and elevated atmospheric carbon dioxide concentrations (eCO 2 ) are expected to affect the CH 4 cycle, the response of CH 4 emission in natural wetland and paddy has been observed to be inconsistent. This variation is likely due to the complex interactions among soil, plant, and microbial processes that regulate CH 4 dynamics, leaving the underlying mechanisms across global studies unknown. Here, we conducted a meta‐analysis to elucidate the effects of warming, eCO 2 , and their co‐effects on CH 4 cycling in wetland. Our results demonstrate that eCO 2 significantly increased CH 4 emission in paddy (18.57%) but had no significant effect on that in natural wetland, attributed to eCO 2 ‐induced increase in belowground biomass and methane production potential in paddy. Conversely, warming promoted CH 4 emission in natural wetland (26.71%) but had no substantial impact on CH 4 in paddy. This difference is due to the lower mean annual temperature in natural wetland compared with paddy, where warming promotes plant growth and methanogen activity. Notably, the combined effects of eCO 2 and warming on paddy CH 4 emission were markedly greater than their individual effects, with a synergistic increase of 44.63%. Furthermore, the impact of eCO 2 on CH 4 emitted by natural wetland was enhanced with time, likely due to different extent of plant‐induced priming effect and progressive nitrogen limitation, while CH 4 emissions from paddies declined greatly with time. Our findings emphasize the pivotal role of wetlands in the global methane cycle and highlight the complex responses of CH 4 emissions to climate changes.