Response of Nitrogen Use Efficiency and Nitrogen Metabolism Characteristics to Nitrogen Fertilizer in Different Yield-Type Rice Varieties

A two-year field experiment (2022–2023) was conducted in Yangzhou, Jiangsu, China, to quantify the response of two contrasting rice yield types to increasing nitrogen (N) supply. Three N rates were imposed: 225 kg ha⁻¹ (N1), 270 kg ha⁻¹ (N2), and 315 kg ha⁻¹ (N3). Across the N1–N3 gradient, supplementary N significantly increased grain yield, total N accumulation (NA), and N translocation. Activities of key N-metabolizing enzymes—nitrate reductase (NR), glutamate synthase (GOGAT), and ferredoxin-dependent GOGAT (Fd-GOGAT)—were also markedly enhanced, indicating improved N assimilation. Throughout grain filling, the high-yielding variety (HYV) maintained superior NR and GOGAT activities relative to the low-yielding variety (LYV). Fd-GOGAT activity in HYV was slightly lower than in LYV at heading (0 DAH) and mid-grain filling (22 DAH), but exceeded LYV levels at maturity (44 DAH). All enzyme activities declined progressively with advancing phenology. Yield was strongly positively correlated with total nitrogen accumulation (NA) (r = 0.958**, p < 0.01), soluble-protein content, GOGAT, and NR activities. Consequently, high NA, elevated soluble-protein concentrations, and sustained GOGAT/NR activities during grain filling constitute robust physiological indicators for identifying high-yielding rice genotypes.