Increased Soil N2o Emission During Drainage is Mitigated by Inputs of Labile Carbon and Amplified by Nitrogen

Conversion from submerged paddy field to upland cultivation, driven by economic benefits, generally increases N2O emission. However, the underlying mechanisms regarding N2O emission and its response to substrate availability during the transition period of paddy soil from submerged to drainage conditions remain unclear. To address this, a microcosm experiment was conducted to mechanistically elucidate the differences in N2O emissions among collected paddy soil under submerged and drainage conditions, and upland soil (previously converted from rice paddy) at 70% soil water-holding capacity. The soils were then subjected to substrate addition, including (i) control, and amended with (ii) glucose (C), (iii) NH4Cl (N) and (iv) C+N. Results showed that cumulative N2O emissions from submerged paddy soil (0.51±0.03 mg N kg−1) were significantly lower than those from drained soil (3.63±0.66 mg N kg−1), but comparable to upland soil (0.86±0.18 mg N kg−1). The increased N2O emissions by drainage were closely associated with soil NH4+ depletion and NO3− accumulation. Substrate addition mediated the response of N2O emissions to paddy soil drainage. Labile C addition mitigated the increased N2O emission by drainage, mainly resulting from increased N2O-reductase gene (nosZ) abundance. N addition 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. Overall, these findings suggested that the increased N2O emission during paddy soil drainage could be mitigated by labile C inputs and amplified by N fertilization, attributed to changes in NO3− availability and abundances of nitrifying and denitrifying genes.