Potassium Fertilization Partially Mitigates Elevated N2O Emissions Under Alternate Wetting and Drying in Paddy Fields

Nitrous oxide (N2O) is recognized as a potent greenhouse gas, and 60% of atmospheric N2O emissions come from cropland soils. Potassium (K) is an important fertilizer for rice paddy fields. K fertilizer decreased the abundance of functional genes mediating nitrification and denitrification processes, thereby mitigating N2O emissions. However, few studies have explored the effect of K fertilization rates on N2O emissions and grain yields, as well as the associated soil properties and aboveground N accumulation in paddy fields under different irrigation regimes. This study aimed to propose an optimum combination of K fertilization rate and irrigation regime to increase grain yield while reducing N2O emissions. Here, a 2-year field experiment using a split-plot design with three replicates was conducted to assess the effect of three K fertilization rates (K0: 0 kg ha−1, K75: 75 kg ha−1, K150: 150 kg ha−1) on N2O emissions, grain yield, aboveground N accumulation, and soil properties, including soil redox potential (Eh), NH4+, NO3−, soil gene abundance of AOA, AOB, nirK, nirS, nirK/nirS, and nosZ, under continuous flooding irrigation (ICF) and alternate wetting and drying irrigation (IAWD). The soil physicochemical properties, the gene abundance and the aboveground N accumulation were evaluated and used to explain how irrigation and K fertilization affect grain yield and N2O emissions. We found that IAWD significantly increased N2O emissions by 38% compared to ICF, and K fertilizer significantly reduced N2O emissions by 15% relative to K0. The effects of IAWD and K fertilizer on N2O emissions can be attributed to the combined impact of soil physicochemical properties and the abundance of functional genes governing N2O emissions. Both irrigation regimes produced equivalent grain yield and aboveground N accumulation. Shifting from ICF to IAWD, the increase in N2O emissions can be mitigated by K fertilization. Moreover, K75 and K150 had similar effects in reducing N2O emissions and yield-scaled N2O emissions, while K75 had a lower K fertilizer cost and higher K partial factor productivity. Therefore, applying K fertilizer at 75 kg ha−1 under IAWD is identified as a potentially suitable rate to secure grain yield while effectively mitigating N2O emissions.