Stimulation of nitrate-driven AOM potential in paddy soils under elevated atmospheric CO2 concentration.

Elevated atmospheric CO2 concentrations have profound effects on methane (CH4) emissions from rice paddies, yet their impact on anaerobic oxidation of methane (AOM) remains largely unexplored. This study aimed to explore how elevated CO2 affects the nitrate-driven AOM potential and the abundance and community composition of the associated anaerobic methanotrophs in paddy fields across three soil depths (0-5, 5-10, and 10-20 cm) and three rice growth stages (tillering, jointing, and flowering) through a controlled field experiment using an open-top chamber (OTC) platform. The results showed that the elevated CO2 (increase of 200 ppm above ambient concentration for 4 years) significantly enhanced nitrate-driven AOM potential, with an average increase of 52.5 % compared to ambient condition. This stimulation was closely associated with the increased dissolved organic carbon levels under CO2 enrichment, showing a 22.7 % increase relative to ambient CO2. Furthermore, Methanoperedens-like archaea exhibited an average increase in abundance of 32.1 % under elevated CO2, but their community composition remained stable across different CO2 treatments. In addition, nitrate-driven AOM potential varied significantly with soil depth and rice growth stages, with the highest potential being observed in surface soils (0-5 cm) and during the jointing stage. Overall, our findings demonstrated that elevated CO2 concentrations greatly enhanced nitrate-driven AOM potential, highlighting the role of this AOM process as a potential key microbial CH4 sink under future climate change scenarios.