Yuhua Tian

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

Abstract 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 NO3−-N concentration in runoff was between 0.1 and 43.7 mg L−, whereas the NH4+-N concentration ranged from below detection limit to 8.5 mg L−. Total N (TN) loads by runoff were 1.0–17.9 and 5.2–38.6 kg ha− during 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 L− and independent of the N application rate, whereas those during the wheat season increased to 8.2 mg L− and were affected by the fertilizer rate. Annual losses of TN through runoff and leaching were 13.7–48.1 kg ha-1 from 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.

Optimizing Nitrogen Fertilizer Application for Rice Production in the Taihu Lake Region, China

Abstract To determine the optimal amount of nitrogen (N) fertilizer for achieving a sustainable rice production at the Taihu Lake region of China, two-year on-farm field experiments were performed at four sites using various N application rates. The results showed that 22%–30% of the applied N was recovered in crop and 7%–31% in soils at the rates of 100–350 kg N ha−1. Nitrogen losses increased with N application rates, from 44% of the applied fertilizer N at the rate of 100 kg N ha−1 to 69% of the N applied at 350 kg N ha−1. Ammonia volatilization and apparent denitrification were the main pathways of N losses. The N application rate of 300 kg N ha−1, which is commonly used by local farmers in the study region, was found to lead to a significant reduction in economic and environmental efficiency. Considering the cost for mitigating environmental pollution and the maximum net economic income, an application rate of 100–150 kg N ha−1 would be recommended. This recommended N application rate could greatly reduce N loss from 199 kg N ha−1 occurring at the N application rate of 300 kg N ha−1 to 80–110 kg N ha−1, with the rice grain yield still reaching 7 300–8 300 kg DW ha−1 in the meantime.