Nitrous oxide (N
2O) is recognized as a potent greenhouse gas, and 60% of atmospheric N
2O 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 N
2O emissions. However, few studies have explored the effect of K fertilization rates on N
2O 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 N
2O 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 (K
0: 0 kg ha
−1, K
75: 75 kg ha
−1, K
150: 150 kg ha
−1) on N
2O emissions, grain yield, aboveground N accumulation, and soil properties, including soil redox potential (Eh), NH
4+, NO
3−, soil gene abundance of AOA, AOB,
nirK,
nirS,
nirK/
nirS, and nosZ, under continuous flooding irrigation (I
CF) and alternate wetting and drying irrigation (I
AWD). 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 N
2O emissions. We found that I
AWD significantly increased N
2O emissions by 38% compared to I
CF, and K fertilizer significantly reduced N
2O emissions by 15% relative to K
0. The effects of I
AWD and K fertilizer on N
2O emissions can be attributed to the combined impact of soil physicochemical properties and the abundance of functional genes governing N
2O emissions. Both irrigation regimes produced equivalent grain yield and aboveground N accumulation. Shifting from I
CF to I
AWD, the increase in N
2O emissions can be mitigated by K fertilization. Moreover, K
75 and K
150 had similar effects in reducing N
2O emissions and yield-scaled N
2O emissions, while K
75 had a lower K fertilizer cost and higher K partial factor productivity. Therefore, applying K fertilizer at 75 kg ha
−1 under I
AWD is identified as a potentially suitable rate to secure grain yield while effectively mitigating N
2O emissions.
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