To achieve high crop yields, the use of chemical fertilizers and pesticides have been increasing worldwide over the past 40 years. The application of nitrogen (N) and phosphorus (P) fertilizers in paddy fields can potentially increase the risk of N and P runoff losses, while pesticides affect rice growth and soil N and P cycles.

The runoff loss of N and P from farmlands is one of the important sources of agricultural non-point source pollution, and contributes a lot to the water eutrophication. However, the coupling effects of chemical fertilizers and pesticides on the N and P runoff losses in paddy fields are poorly understood.

By designing the interaction treatments with different chemical fertilizers (0%, 50%, 70%, 100% and 150% of conventional dosage) and pesticides (0%, 50%, 70% and 100% of conventional dosage), researchers from the Institute of Subtropical Agriculture (ISA) of the Chinese Academy of Sciences (CAS) observed ponding water N and P concentrations and N and P runoff losses with high-frequency in three rice seasons in the double rice-cropping paddy fields in hilly areas in South China.

The researchers found that the ponding water N and P concentrations decreased exponentially with time after the application of fertilizers, and kept stable and relative low concentrations after 5 to 15 days from the fertilizer application.

The ponding water N and P concentrations in the early rice season were greater than those in the late rice season, even though the N fertilizer application rate was lower in the early rice season. Thus, the early rice season has higher N and P runoff loss risks than those in the late rice season.

"This indicates that special measures, such as avoiding high irrigation and fertilizer deep application, should be taken during the high-risk periods to reduce the N and P runoff losses in the double rice-cropping system, especially in the early rice season," said Dr. SHEN Jianlin from ISA.

The researchers also found that the ponding water N and P concentrations and runoff losses increased significantly with increased fertilizer application rates, while pesticide application rate did not significantly affect N and P losses.

There are 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. 

"This study deepened our understanding of N and P runoff losses under the coupling of chemical fertilizers and pesticides, and clarified the high-risk periods and the seasonal features of N and P runoff losses in the double rice-cropping system," said Dr. SHEN.

The study, published in Science of The Total Environment, was supported by the National Key Research and Development Program of China, the National Natural Science Foundation of China, and the Youth Innovation Promotion Association of the Chinese Academy of Sciences.