Nitrogen runoff dominates water nitrogen pollution from rice-wheat rotation in the Taihu Lake region of China

Nitrogen (N) runoff and leaching are two direct pathways of N pollution from agricultural land to water systems. It is not yet conclusive whether intensive rice-wheat rotation system contributes to N pollution in the surrounding water systems in the Taihu Lake region (TLR). We conducted a three-year field experiments to monitor N runoff and leaching from rice-wheat paddy soil. Annual N runoff and leaching was in the range of 55.3–93.1 kg N ha−1, with a mean of ca. 69.2 kg N ha−1. Of the total N export to water systems, 82–93% was from N runoff and only 7–18% was from N leaching. For seasonal distribution, the wheat season contributed 57–85% of total N, while the rice season contributed 15–43%. Because of different water practices, N runoff and leaching exhibited completely different patterns in two crop seasons. Considerable N runoff in wheat season can mainly be attributed to moderately heavy rainfall and accelerated water flow in existing drainage ditches. Large variations in N runoff during rice season were predominantly due to variable natural precipitation and irregular artificial draining. N leaching appeared to be related to fertilization and precipitation in wheat season, while it was associated with fertilization and soil depth during rice season. These field data suggest that runoff dominated N export from rice-wheat rotation under conventional N and water management contributes potentially to water pollution in the TLR.