Characteristics of nitrogen accumulation and utilization in indica hybrid rice under different planting methods and nitrogen rates

There has been remarkable transition in rice planting methods in recent years, including mechanical transplanting and direct seeding. Different planting methods inevitably have different effects on rice growth and development. However, rice growth, nutrient accumulation, heat and light utilization are different under different planting methods. Nitrogen application has been one of the most common field management practices in rice cultivation. Thus a field experiment was conducted to study the characteristics of nitrogen accumulation and utilization in indica hybrid rice under different planting methods and nitrogen rates. A two-factor split-plot designed field experiment was conducted using indica hybrid rice 'F-you-498', a widely planted cultivar in Sichuan Basin. The main plot had three planting methods, including mechanical transplanting (T₁), precision hill direct seeding (T₂) and manual planting (T₃), the subplot was nitrogen (N) fertilizer rate, including 0 kg·hm^-2 (N₀), 90 kg·hm^-2 (N₁), 135 kg·hm^-2 (N₂) and 180 kg·hm^-2 (N₃). Nitrogen accumulation and transfer, yield and yield components, and nitrogen use efficiency under different treatments of the indica hybrid rice were analyzed. The study showed that nitrogen accumulation, transfer, yield and yield components of rice were significantly affected by planting method and N rate. In different planting methods, total nitrogen accumulation at heading and maturity stages was in the order of T₃ > T₁ > T₂. Nitrogen accumulation rate under T₂ was significantly higher than that under T₁ and T₃ from sowing to jointing stage and from heading to maturity stage. However, nitrogen accumulation rate was in the order of T₃ > T₁ > T₂ at jointing to heading stage. Agronomic efficiency of nitrogen and nitrogen harvest index were in the order of T₃ > T₁ > T₂. The results showed that the order of nitrogen requirement for 100 kg grains was T₂ > T₃ > T₁. Grain yield under manual transplanting was significantly higher than that under precision hill direct seeding, but not significantly different from that of mechanical transplanting. Nitrogen accumulation of total plant and panicle increased significantly with increasing nitrogen fertilization rate. However, leaf nitrogen contribution to panicle de-creased with increasing nitrogen fertilization rate. Planting method and nitrogen application rate significantly influenced nitrogen use efficiency. Rice agronomic efficiency of nitrogen under mechanical transplanting increased with increasing nitrogen fertilization rate, although the increase was insignificant. However, agronomic efficiency of nitrogen under precision hill direct seeding and manual transplanting decreased with increasing nitrogen application. With increasing nitrogen fertilizer rate, the nitrogen apparent recovery efficiency under manual transplanting and mechanical transplanting first increasing then decreasing, while that under precision hill direct seeding gradually decreased. Nitrogen grain production efficiency and nitrogen harvest index decreased with increasing nitrogen fertilization rate, and there was no significant difference among nitrogen fertilizer treatments. In combination with grain yield and nitrogen use efficiency, nitrogen fertilizer rate of 135 kg·hm^-2 under precision hill direct seeding, and 135-180 kg·hm^-2 under mechanical transplanting and artificial transplanting not only achieved stable rice yield, but also had high nitrogen use efficiency.