Optimizing nitrogen reduction for improved grain yield and nitrogen-use-efficiency in two rice genotypes grown in...

The hilly region of Sichuan is a core rice-producing area in Southwest China.Long-term excessive fertilization has resulted in low nitrogen-use-efficiency and aggravated agricultural non-point source pollution, which seriously restricts the green and high-yield development of rice production.

To determine the optimal nitrogen reduction rate for nitrogen-efficient rice varieties in this region and to reveal the regulatory effects of nitrogen reduction on rice yield, nitrogen utilization, and physiological metabolism, a two-year field experiment was conducted using two rice cultivars, Byou 3611 and Yixiangyou 2115. Four nitrogen treatments were applied: 180 kg·hm^-2 (N3), 150 kg·hm^-2 (N2), 120 kg·hm^-2 (N1), and 0 kg·hm^-2 (N0, control). Yield, dry matter accumulation and translocation, nitrogen-use-efficiency,sugar content, and key enzyme activities involved in sucrose metabolism were systematically measured.

The results showed that 150 kg·hm^-2 nitrogen (N2) was the optimal treatment, with the highest grain yield in both years. The yields of Byou 3611 and Yixiangyou 2115 reached 12101.05 kg·hm^-2 and 11941.19 kg·hm^-2, respectively, which were significantly higher than those under conventional nitrogen application (N3). Excessive nitrogen failed to increase yield but reduced the seed-setting rate, whereas large reduction in nitrogen supply (N1) significantly decreased the number of effective panicles and grains per panicle. Dry matter accumulation and translocation were the best under N2, with crop growth rates of 13.15 and 12.47 g·(m^-2·d)⁻¹ for the two cultivars, respectively. Nitrogen-use-efficiency first increased and then decreased with increasing nitrogen applicatiopn rates. Under N2, nitrogen recovery efficiency exceeded 24%, and nitrogen agronomic efficiency of Byou 3611 reached 32.02 kg·kg⁻¹. Soluble sugar and sucrose contents, as well as the activities of sucrose phosphate synthase and sucrose synthase in grains, were highest under N2. Excessive nitrogen supply inhibited enzyme activity and sugar accumulation in grains. Significant genotypic differences were observed between the two cultivars: Byou 3611 performed better in terms of effective panicles, seed-setting rate, and dry matter translocation than Yixiangyou 2115.

This study indicates that Byou 3611 is superior to Yixiangyou 2115 in terms of grain yield and nitrogen-use-efficiency. The application rate of 150 kg·hm^-2 nitrogen can simultaneously optimize yield components, dry matter translocation, nitrogen utilization and sugar metabolism. These findings provide a theoretical basis and technical support for nitrogen saving and high-efficiency rice cultivation in the hilly regions of Sichuan.