Effects of Potassium Management on Yield Formation and Nutrient Utilization in Japonica Rice Cultivars with Contrasting Nitrogen Efficiency Under a Simplified Nitrogen Regime

Nitrogen (N) and potassium (K) co-management is critical for optimizing grain yield in rice. However, the interactive effects of N supply and K application timing on cultivars with contrasting N efficiencies remain poorly understood. Here, we conducted a two-year field experiment (2020 and 2021) using two japonica rice cultivars, Shennong 265 (SN265) and Meifengdao 61 (MFD61), under three N rates (180, 225, and 270 kg ha−1) and three K application ratios (basal: panicle = 3:7, 5:5, and 7:3). SN265 exhibited a 20.31% higher average grain yield than MFD61, primarily attributable to increased crop growth rates during the tillering–booting (14.08%) and grain-filling–maturity phases (31.88%). Under moderate N supply (N180 and N225), increasing the proportion of basal K application (K7:3) consistently improved dry matter accumulation, enzyme activity, and grain yield in both cultivars. However, under high-N conditions (N270), excessive early-season K application reduced grain yield in MFD61 by 7.69%. For SN265, further yield improvement required an enhanced net assimilation rate during the tillering–booting phase. Although this study was conducted at a single site with only two cultivars, it provides a physiological and agronomic framework for cultivar-specific N–K co-management strategies to improve grain yield and nutrient use efficiency.