Increased N2O emission due to paddy soil drainage is regulated by carbon and nitrogen availability

Conversion from paddy field to upland cultivation, driven by economic benefits, generally increases N2O emission. However, the underlying mechanisms regarding the changes in N2O emission during the transition period of paddy soil drainage and conversion into upland remain unclear. To address this knowledge gap, a microcosm experiment was conducted to mechanistically elucidate the N2O emissions in response to paddy soil drainage and its interaction with carbon (C, glucose) and nitrogen (N, NH4Cl) availability. Results showed that N2O emissions were significantly lower in the submerged paddy soil (0.51 +/- 0.03 mg N kg(-1)) compared to drained paddy soil (3.63 +/- 0.66 mg N kg(-1)). The increased N2O emission was positively associated with soil NH4+ depletion and NO3- accumulation, indicating increased nitrification and NO3- supply for denitrification following paddy soil drainage. Substrate C and N availability had little effect on N2O emission from submerged paddy soil, but strongly mediated the response of N2O emission to paddy soil drainage. High C availability largely mitigated the increased N2O emission by drainage, mainly resulting from increased N2O-reductase gene (nosZ) abundance. The increased N availability interacting with drainage strongly increased N2O emissions by 1.99-16.34 folds, primarily due to increased NO3- content and gene abundances of ammonia-oxidizing bacteria (AOB) and nitrite reductase (nirK and nirS). The N2O emissions were positively correlated with the abundances of ammonia-oxidizing archaea (AOA), AOB, nirK and nirS genes, and (nirK + nirS)/nosZ ratio across treatments. Collectively, our findings suggested that the increased N2O emission due to paddy soil drainage is regulated by C and N availability, attributed to changes in NO3- content and abundances of nitrifying and denitrifying genes.