Nitrogen balance in paddy fields under different rotation systems in the Taihu Lake Region

A field micro-plot (labeled ¹⁵N) fertilizer experiment was conducted to investigate the use efficiency of fertilizer N and its residual features under different rice-based cropping systems (Chinese milk vetch-rice rotation, fallow-rice rotation and wheat-rice rotation) in the Taihu Lake Region. Results showed that 20.9%?49.6% of N uptake of rice was derived from the applied fertilizer N. Fallow-rice rotation system largely depended on inorganic N fertilizer to form yield. N fertilizer use efficiency of rice was 25.0%?41.5% of the labeled fertilizer N at harvest. Residual fertilizer N rate in soils was 13.4%-24.6%, over 90% of which was in the 0-20 cm soil layer. The amount of fertilizer N residue in the soil profile decreased rapidly with increasing soil depth. Only 0.2%-0.7% of the fertilizer N was in the soil layer of 30-40 cm. Fertilizer N use efficiency and soil residual N rate were largest at N application rate of 240 kg·hm^-2 under Chinese milk vetch-rice rotation and fallow-rice rotation systems. This respectively exceeded by 55.6% and 66.0% over N fertilizer use efficiency under wheat-rice rotation. Fertilizer N use efficiency, soil residual rate as well as total N recovery rate were lowest while N loss rate highest under wheat-rice rotation system at N application rate of 240 kg·hm^-2. As for the Chinese milk vetch-rice rotation system, the loss rate of N fertilizer was lowest, which was less than those of fallow-rice rotation and wheat-rice rotation systems by 13.9% and 39.2%, respectively. Under different rotation systems, rice yield increased with increasing urea application rate. Under straw-return application of Chinese milk vetch of Chinese milk vetch-rice rotation system, rice yield was significantly higher compared to those of wheat-rice rotation and fallow-rice rotation at N application of 240 kg·hm^-2. Although rice yield under wheat-rice rotation was slightly higher than that under fallow-rice rotation, it was not significantly different. The study suggested that N application at 240 kg·hm^-2 in addition to straw-return application of Chinese milk vetch of Chinese milk vetch-rice rotation not only ensured N fertilizer use efficiency and high rice grain yield, but also reduced the loss of N fertilizer and environmental risks. Considering these factors, Chinese milk vetch-rice rotation system was recommended as a suitable cropping system worthy of promotion in the Taihu Lake Region.