Effects of Nitrogen Application Rates and Nitrogen Topdressing at Different Leaf Growth Stages on the Yield, Nitrogen Absorption, and Utilization of Nanjing 9108

The effects of nitrogen (N) application rates and N topdressing at different leaf growth stages on the yield, N absorption, and utilization of japonica rice cultivar Nanjing 9108 were studied to screen the optimal N management mode for high yield and high N use efficiency. A field experiment was conducted from 2023 to 2024, with nine N regulation treatments (94-351 kg ha^-1) established through dynamic allocation of basal, tillering, and topdressing fertilizers. The results showed that with the increase of N application rate, the yield and N use efficiency of Nanjing 9108 first increased and then decreased. At a total N application rate of 270 kg ha^-1, the N6 treatment (basal N + tiller N + topdressing at the 13th leaf stage) demonstrated optimal overall performance, achieving the highest yield and N use efficiency. Topdressing at the 13th leaf stage (coinciding with young panicle differentiation) promoted spikelet differentiation and large panicle formation, increasing grains per panicle by 2.36-2.20% compared to other treatments under the same N rate. The N6 treatment exhibited enhanced N uptake and utilization: N accumulation increased by 39.27-67.12% during the elongating to heading stage and by 7.14-62.24% during heading to maturity, while N apparent efficiency and agronomic efficiency rose by 3.51-14.68% and 29.22-58.25%, respectively. At heading, the proportion of high-effective leaf area in N6 was 1.52-7.05% higher than in N4, N5, and N7 treatments, accompanied by a slower leaf area decay rate. These traits provided sustained photosynthetic support for dry matter accumulation in mid-to-late growth stages. Consequently, dry matter accumulation in N6 increased by 5.85-33.44% (elongating to heading) and 0.42-26.98% (heading to maturity), leading to a yield advantage of 3.8-17.2% over other treatments. In summary, the N management strategy combining basal, tiller, and 13th-leaf topdressing at 270 kg ha^-1 is most effective for achieving both high yield and high N efficiency in Nanjing 9108.