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

Wetland is one of the most significant sources of methane (CH). Although global warming and elevated atmospheric carbon dioxide concentrations (eCO) are expected to affect the CH cycle, the response of CH 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 dynamics, leaving the underlying mechanisms across global studies unknown. Here, we conducted a meta-analysis to elucidate the effects of warming, eCO, and their co-effects on CH cycling in wetland. Our results demonstrate that eCO significantly increased CH emission in paddy (18.57%) but had no significant effect on that in natural wetland, attributed to eCO-induced increase in belowground biomass and methane production potential in paddy. Conversely, warming promoted CH emission in natural wetland (26.71%) but had no substantial impact on CH 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 and warming on paddy CH emission were markedly greater than their individual effects, with a synergistic increase of 44.63%. Furthermore, the impact of eCO on CH emitted by natural wetland was enhanced with time, likely due to different extent of plant-induced priming effect and progressive nitrogen limitation, while CH 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 emissions to climate changes.