Long-term fertilization enhances the activity of anaerobic oxidation of methane coupled to nitrate reduction and associated microbial abundance in paddy soils

Anaerobic oxidation of methane coupled to nitrate reduction (nitrate-coupled AOM) is performed by Candidatus Methanoperedens nitroreducens (M. nitroreducens)-related archaea, and is recently recognized as a crucial component of the global carbon cycle. The input of fertilizers is an essential agricultural practice that greatly impacts methane (CH₄) production and emission. However, the significance of nitrate-coupled AOM in CH₄ cycling and its response to fertilization in rice fields remain unclear. In this study, the potential nitrate-coupled AOM rates and the communities of M. nitroreducens-related archaea in rice fields were examined at different soil layers (0–10, 10–20, and 30–40 cm) at tillering, elongation, flowering, and ripening stages under three long-term fertilization treatments (CK-without fertilizer, CF-chemical fertilization, or CFS-chemical fertilization with straw incorporation). The results indicated that both CF (1.07 nmol ¹³CO₂ g⁻¹ d⁻¹) and CFS (1.21 nmol ¹³CO₂ g⁻¹ d⁻¹) treatments significantly promoted the potential nitrate-coupled AOM rates compared to CK (0.53 nmol ¹³CO₂ g⁻¹ d⁻¹). A greater response of potential activity to fertilization was observed at plough layer (upper 20 cm) and during elongation stage. The abundance of M. nitroreducens-related archaea under CF (1.12 × 10⁷ copies g⁻¹) and CFS (1.62 × 10⁷ copies g⁻¹) treatments was significantly greater than that under CK (6.93 × 10⁶ copies g⁻¹). Conversely, the growth response of these archaeal to fertilization was stronger at deeper layer (30–40 cm). Moreover, no significant change was observed in the community composition of M. nitroreducens-related archaea among treatments. Correlation analysis suggested that the variations of soil organic carbon, NH₄⁺, and NO₃⁻ contents caused by fertilization were key factors influencing the potential nitrate-coupled AOM rates and M. nitroreducens-related archaeal abundance. Our findings provide the first evidence for positive response of nitrate-coupled AOM to long-term fertilization, demonstrating its potential to act as an important process for mitigating CH₄ emission in rice fields.