A few key nirK- and nosZ-denitrifier taxa play a dominant role in moisture-enhanced N2O emissions in acidic paddy soil

Debate persists regarding the underlying factors influencing nitrous oxide (N2O) emissions and their mechanisms of action in rice paddy soils. The aim of the present study was to delineate the contributions of nitrification and denitrification to N2O emissions in acidic paddy soils under adequate N fertilisation. To identify the key taxa associated with N2O emissions and explore the potential underlying mechanisms, we performed stable N isotope (15N) tracing, quantitative PCR, and Illumina MiSeq sequencing of amoA, nirK, nirS, and nosZ genes. Under sufficient NH4NO3 input, soil N2O emissions increased significantly with an increase in soil moisture, peaking at 125% water-filled pore space (WFPS). N2O flux was positively associated with potential denitrification rates but not potential nitrification rates. The relative contribution of denitrification to N2O emissions increased with an increase in soil moisture and reached more than 90% at 75%, 100%, and 125% WFPS during the 96 h of incubation. Based on relative abundance and connectivity degree of operational taxonomic units, soil moisture-enhanced N2O emissions were strongly correlated with only a few key taxa, including nirK-containing denitrifiers belonging to Bradyrhizobium, as well as nosZ-containing denitrifiers belonging to Tardiphaga. In conclusion, denitrification could play a more dominant role in N2O emissions than nitrification in acidic paddy soil under sufficient NH4NO3 inputs, and the effects of soil moisture on N2O emissions could be attributed to a few key nirK- and nosZ-denitrifier taxa, through shifts in community composition and synergistic interactions of network structures between them.