Intermittent Irrigation Outweighs the Methanogenic Stimulation of Potassium Fertilization in Rice Paddies

The interplay between water-saving irrigation and potassium (K) management in regulating paddy methane (CH4) emissions remains poorly understood. It remains unclear whether carbon pool enhancement induced by K fertilization could counteract the oxidation effects under non-flooded irrigation. Here, we conducted a field-based trial over two years to explore how irrigation regimes (continuous flooding, IF; intermittent irrigation, II) and K application rates (K0, K75, K150 kg ha−1) modify soil redox status, carbon pools, crop growth, CH4 emissions and economic benefits. Intermittent irrigation markedly elevated soil redox potential (Eh), suppressed dissolved organic carbon (DOC), and substantially reduced two-year average CH4 emissions by 75.1–76.9%, regardless of K supply. Under IF, high-rate K fertilization (K150) stimulated cumulative CH4 emissions; nevertheless, such K-driven CH4 stimulation was completely offset under intermittent irrigation. Soil Eh, DOC and microbial biomass carbon (MBC) dominated CH4 variation, among which Eh exerted the primary control. Intermittent irrigation combined with K fertilization improved rice grain yield. A comprehensive TOPSIS-Entropy multi-criteria evaluation identified the IIK75 treatment as the optimal option balancing environmental benefits and economic returns. Collectively, intermittent irrigation can mitigate the methanogenic risk from high potassium input, providing a promising redox-regulated strategy for low-CH4 emission and high rice production.