Nitrogen and phosphorus runoff losses were influenced by chemical fertilization but not by pesticide application in a double rice-cropping system in the subtropical hilly region of China.

As one of the important nitrogen (N) and phosphorus (P) pollution sources of waters, the paddy water N and P runoff losses are still poorly understood in the double rice cropping system under the interaction of chemical fertilizer and pesticide. In the subtropical hilly region of China, we conducted a 1.5-year continuous and high-frequency monitoring of paddy water N and P concentrations, runoff N and P losses, and grain yield in a double rice-cropping system with different chemical fertilizer and pesticide application rates. The results showed that the high-risk periods for N loss were in the first 5 days after the base fertilizer (BF) application and the first 10 days after the topdressing fertilizer application in both early and late rice seasons, while the high-risk periods for P loss were in the first 5 days after BF application in the early rice season and the first 15 days after BF application in the late rice season. The N and P runoff losses in the early rice season were greater than those in the late rice season, due to that the N and P fertilizers use efficiencies were lower, and thus paddy water N and P concentrations were higher in the early rice season. The paddy N and P concentrations and N and P runoff losses increased significantly with increased fertilizer application rates, while the pesticide application rate did not significantly affect N and P losses. Therefore, special effects (e.g., avoiding high irrigation, fertilizer deep application) should be taken during the high-risk periods of N and P losses to reduce the N and P runoff losses in the double rice cropping system, especially in the early rice season. There are also potentials to reduce fertilizer and pesticide input without reducing rice grain yield for the double rice cropping system in the subtropical hilly region of China.