Effects of Located Nitrogen Application and Nitrogen Application After Exhaustion on Rice Yield and Nitrogen Use Efficiency in the Taihu Lake Region,China

The results of located consecutive nitrogen application and nitrogen application at one year after soil nitrogen exhaustion were compared in the Taihu Lake Region, China where nitrogen fertilizer was excessly applied in rice-wheat cropping system. The effects of different nitrogen application rates on rice yield, dry matter accumulation, nitrogen uptake, nitrogen use efficiency and soil nitrogen content were studied. The rice yield decreased significantly after three cropping seasons without nitrogen application, with the rice yield of the third season being only 6673.42 kg/hm2. When nitrogen application rate rose, the rice yield increased significantly both in the treatments of located nitrogen application and nitrogen application after exhaustion, but the extent of increase of production reduced with increasing nitrogen application rate. At the nitrogen application rate of 300 kg/hm2, the treatments of located nitrogen application and nitrogen application after exhaustion produced the highest rice yields, 10 514.95 kg/hm2 and 10 472.36 kg/hm2 , respectively. Rice plants lodged and yield decreased when the nitrogen application rate went even higher than that point. In both treatments of nitrogen application, the dry matter accumulation and nitrogen uptake of rice plants, as well as the soil total nitrogen content all increased, whereas the nitrogen use efficiency decreased with the increase of nitrogen application rate. There were no significant differences between the production indexes from the treatments of located nitrogen application and nitrogen application after exhaustion, indicating that these indexes were related to the seasonal nitrogen application rather than the background nitrogen level in the soil after one year of exhaustion.