Abstract: Nitrate-dependent anaerobic oxidation of methane (AOM) is a new pathway to reduce methane emissions from paddy ecosystems. The elevated atmospheric CO₂ concentration can affect methane emissions from paddy ecosystems, but its impact on the process of nitrate-dependent AOM is poorly known. Based on the automatic CO₂ control platform with open top chambers and the ¹³CH₄ stable isotope experiments, the responses of the activity of nitrate-dependent AOM, abundance and community composition of Candidatus Methanoperedens nitroreducens (M. nitroreducens)-like archaea to the gradual increase of CO₂ concentration were investigated in paddy fields. We set up two CO₂ concentration treatments, including an ambient CO₂ and a gradual increase of CO₂(increase of 40 μL·L^-1 per year above ambient CO₂ concentration until 160 μL·L^-1). The results showed the nitrate-dependent AOM rate of 0.7-11.3 nmol CO₂·g^-1·d^-1 in the studied paddy fields, and quantitative PCR showed the abundance of M. nitroreducens-like archaeal mcrA genes of 2.2×10⁶-8.5×10⁶ copies·g^-1. Compared to the ambient CO₂ treatment, the slow elevated CO₂ treatment enhanced the nitrate-dependent AOM rate and stimulated the abundance of M. nitroreducens-like archaea, particularly in 5-10 cm soil layer. The gradual increased CO₂ concentration treatment did not change the community composition of M. nitroreducens-like archaea, but significantly decreased their diversity. The soil organic carbon content was an important factor influencing the nitrate-dependent AOM process. Overall, our results showed that the gradual increase of CO₂ concentration could promote the nitrate-dependent AOM, suggesting its positive role in mitigating methane emissions from paddy ecosystems under future climate change.

Key words: paddy field, nitrate-dependent anaerobic oxidation of methane, rising CO₂ concentration, community composition