Effects of Nitrogen Application Rates on Translocation of Dry Matter and Nitrogen Utilization in Rice and Wheat

The objectives of this study were to disclose the mechanism of dry matter translocation and nitrogen (N) utilization under rice–wheat rotation and to propose optimum rate of N application. In a field experiment from 2007 to 2009 at the Changshu Agroecological Experiment Station, Chinese Academy of Sciences, Changshu, China, 4 levels of N fertilizer were designed during the growing period of rice and wheat. Among the N application treatments, N rates were 125 kg ha⁻¹ + 94 kg ha⁻¹ (rice season + wheat season) for N1, 225 kg ha⁻¹ + 169 kg ha⁻¹ for N2, 325 kg ha⁻¹ + 244 kg ha⁻¹ for N3, and zero N fertilizer applied (NO) served as the control. The results showed no significant increases in rice and wheat yield when the N application rates exceeded 225 kg ha⁻¹ and 169 kg ha⁻¹ in rice and wheat seasons, respectively. The dry matter accumulations at anthesis stages of rice and wheat were increased with the increment of N rate. However, the contributions of dry matter translocation from vegetative organs to grains before flowering were reduced in both crops when more N fertilizer was applied. The N agronomic efficiencies and physiological efficiencies in rice and wheat declined with the increase of N rate, but there was no significant difference between the N2 and N3 treatments. The N1 treatment had the largest average marginal productions for both rice (13.1 kg kg⁻¹) and wheat (17.4 kg kg⁻¹), followed by the N2 treatment (9.1 kg kg⁻¹ for rice and 16.7 kg kg⁻¹ for wheat), and the N3 treatment showed much smaller marginal productions than other treatments with only 3.1 kg kg⁻¹ for rice and 2.4 kg kg⁻¹ for wheat. Therefore, the N2 treatment could maintain relatively high translocation rates of dry matter, N use efficiencies and economic benefits in both rice and wheat.