Global Response and Mechanism of Methane Cycling in Wetlands Under Elevated Atmospheric CO2 and Warming

Abstract

Wetland is one of the most significant sources of methane (CH₄). Although global warming and elevated atmospheric carbon dioxide concentrations (eCO₂) are expected to affect the CH₄ cycle, the response of CH₄ 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 CH₄ dynamics, leaving the underlying mechanisms across global studies unknown. Here, we conducted a meta-analysis to elucidate the effects of warming, eCO₂, and their co-effects on CH₄ cycling in wetland. Our results demonstrate that eCO₂ significantly increased CH₄ emission in paddy (18.57%) but had no significant effect on that in natural wetland, attributed to eCO₂-induced increase in belowground biomass and methane production potential in paddy. Conversely, warming promoted CH₄ emission in natural wetland (26.71%) but had no substantial impact on CH₄ 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 eCO₂ and warming on paddy CH₄ emission were markedly greater than their individual effects, with a synergistic increase of 44.63%. Furthermore, the impact of eCO₂ on CH₄ emitted by natural wetland was enhanced with time, likely due to different extent of plant-induced priming effect and progressive nitrogen limitation, while CH₄ 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 CH₄ emissions to climate changes.