Concurrent Elevation of CO2 and Temperature Stimulates N2O Emissions from Rice Paddies in a Rice–Wheat Cropping System

Paddy fields are a major agricultural hotspot for nitrous oxide (N2O), contributing approximately 11% of global agricultural emissions. While elevated CO2 and warming individually regulate N2O emissions by modulating soil carbon, nitrogen (N) availability and microbial activity, the effects of concurrent elevated CO2 and temperature (ECT) and the underlying microbial mechanisms under field conditions remain poorly understood. Here, we used a free-air CO2 enrichment and temperature increase (T-FACE) system in a rice–wheat cropping system to investigate the impacts of ECT on N2O emissions from rice paddies and identify the underlying biogeochemical and microbial mechanisms. Results showed that ECT increased area-scaled and yield-scaled N2O emissions by 15.3% and 17.6%, respectively. Mechanistically, during the peak emission period, ECT significantly increased soil NH4+–N content by 40.2% and the denitrification gene ratio [(nirK + nirS)/nosZ] by 27.5%. Furthermore, ECT increased the diversity of nitrifying communities but decreased that of denitrifying communities, while reshaping the composition of ammonia-oxidizing archaea and denitrifiers, thereby altering nitrification and denitrification. Overall, our field-based evidence suggests that ECT can stimulate N2O emissions primarily by increasing soil N substrate availability and shifting denitrifier communities in ways that may favor N2O accumulation. These findings offer mechanistic insights into climate-driven N2O emissions.