Nitrogen Runoff and Leaching Losses During Rice-Wheat Rotations in Taihu Lake Region,China

Although nitrogen (N) loss through runoff and leaching from croplands is suspected to contribute to the deterioration of surrounding water systems,there is no conclusive evidence for paddy soils to prove this hypothesis.In this study,field plot experiments were conducted to investigate N losses through runoff and leaching for two consecutive years with 3 N fertilization rates in rice (Oryza sativa L.)-wheat (Triticum aestivum L.) rotations in the Taihu Lake region,China.A water collection system was designed to collect runoff and leachates for both the rice and wheat seasons. Results showed that dissolved N (DN),rather than particulate N (PN),was the main form of N loss by runoff. The NO~--N concentration in runoff was between 0.1 and 43.7 mg L-1,whereas the NH~+-N concentration ranged from below detection limit to 8.5 mg L.Total N (TN) loads by runoffwere 1.0-17.9 and 5.2-38.6 kg haduring rice and wheat seasons,respectively,and the main loss occurred at the early growing stage of the crops.Nitrogen concentrations in leachates during the rice seasons were below 1.0 mg Land independent of the N application rate,whereas those during the wheat season increased to 8.2 mg Land were affected by the fertilizer rate.Annual losses of TN through runoff and leaching were 13.7-48.1 kg hafrom the rice-wheat cropping system,accounting for 5.6%-8.3% of the total applied N.It was concluded that reduction in the N fertilization rate,especially when the crop was small in biomass,could lower the N pollution potential for water systems.