Ammonia-oxidizing archaea rather than ammonia-oxidizing bacteria drives methane emissions in a wheat-paddy rice rotation system

Abstract

Methane (CH₄) emissions in nature environments are closely associated with its production by methanogens and oxidation by methanotrophs. Moreover, ammonia oxidation is crucial in the nitrogen cycle and significantly influences CH₄ oxidation. However, knowledge about the role of ammonia oxidizers in controlling CH₄ release from rice-planted soil is scarce. This study measured CH₄ fluxes and the gene abundances of methanogen (mcrA), methanotroph (pmoA), ammonia-oxidizing archaea (AOA-amoA) and ammonia-oxidizing bacteria (AOB-amoA) in both the rice-planted and unplanted plots. The results showed that rice planting enhanced seasonal cumulative CH₄ emissions by 56.1 % and 41.5 % in 2018 and 2019, respectively, compared to the rice-unplanted treatment. Rice planting exhibited no obvious impact on the gene abundances of pmoA. The gene abundance of AOA-amoA was significantly reduced in the rice-planted treatment compared to the unplanted treatment, while the opposite results were observed for mcrA and AOB-amoA gene abundance. Structural equation modeling revealed that the increased CH₄ emissions induced by rice planting primarily resulted from the enhanced mcrA gene abundance and the decreased AOA-amoA gene abundance. Additionally, the stoichiometric ratio of dissolved organic carbon to mineral nitrogen emerged as the primary determinant of the temporal dynamics of CH₄ fluxes. These findings suggest that AOA and carbon‑nitrogen stoichiometry are important in regulating CH₄ emissions in paddy rice fields.