Research progress on the effects of elevated atmospheric CO2 concentration on CH4 emission and related microbial processes in paddy fields.

Paddy fields are recognized as significant sources of methane (CH₄) emissions, playing a pivotal role in global climate change. Elevated atmospheric carbon dioxide (CO₂) concentrations (e[CO₂]) exert a profound influence on the carbon cycling of paddy fields. Understanding the effects of e[CO₂] on CH₄ emissions, as well as the underlying microbial processes, is crucial for enhancing carbon sequestration and reducing emissions in paddy fields. We reviewed the impacts of e[CO₂] on CH₄ emission in paddy fields, focusing on the activity, abundance, community structure, and diversity of carbon-cycling-related microbes. We also delineated the roles of various microbial processes in mitigating CH₄ emissions under e[CO₂], as well as the primary environmental determinants. Overall, the type of e[CO₂] experimental platforms, duration of fumigation, concentration gradients, and the methods of CO₂ enrichment all influence CH₄ emissions from paddy fields. e[CO₂] initially stimulates CH₄ emissions, which may decrease over time, indicating an adaptability of the methane-emitting microbial community to e[CO₂]. This response exhibits a trend of initial attenuation followed by an intensification of the positive effects on CH₄ emissions. Experiments with abrupt increase of CO₂ concentration might overestimate CH₄ emissions. The impact of e[CO₂] on microbial processes is predominantly characterized by enhanced activities and abundance of methanogens, aerobic and anaerobic methanotrophs. It significantly alters the community composition and diversity of methanotrophs, with minimal effects on methanogens and anaerobic methanotrophic communities. Finally, we outlined future research directions: 1) Integrated investigations into the effects of e[CO₂] on CH₄ emissions, methanogenesis, and both aerobic and anaerobic methanotrophs in paddy fields could elucidate the mechanisms underlying the impacts of climate change on CH₄ emissions; 2) Long-term studies are essential to understand the mechanisms of e[CO₂] on CH₄ emissions and associated microbial processes more accurately and realistically; 3) Multi-scale (temporal and spatial), multi-factorial (CO₂ concentration, temperature, atmospheric nitrogen deposition, and water management practices), and multi-methodological (observational, data, and model integration) research is necessary to effectively reduce the uncertainties in assessing the response of CH₄ emissions in paddy fields and related microbial processes to e[CO₂] under future climate change scenarios.