Effects of Nitrogen Application Rates and Nitrogen Topdressing at Different Leaf Growth Stages on the Yield, Nitrogen...

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.