Study on nitrogen application rate tolerance based on rice yield and ecological security in hilly areas of South China

To study the nitrogen application rate tolerance in the planting process of rice in the hilly areas of South China, and ensure the rice yield and ecological security and improve the nitrogen use efficiency, the relationships between nitrogen application rate, rice yield, apparent utilization rate of nitrogen, apparent loss of nitrogen and total nitrogen of surface water in paddy soil were analyzed through four field experimental points in Fujian (FJ), Sichuan (SC), Yunnan (YN) and Jiangxi (JX) provinces. The results showed that with the increase of the nitrogen application rates, the rice yields of every experimental point had the changes of quadratic function. The rice yields were the highest, which were respective 5.47, 11.24, 9.9 and 4.42 t/hm2 when the nitrogen application rates of the experimental points of FJ、SC, YN and JX were 184.7, 185.98, 288.8 and 249.5 kg/hm2. The apparent utilization rates of nitrogen had Sigmoidal functions descending relation to nitrogen application rates. The utilization rates of nitrogen slowly decreased when the nitrogen application rates were less. When the nitrogen application rates were more than 135, 155, 225 and 185 kg/hm2 in experimental points of FJ、SC, YN and JX, the utilization rates of nitrogen rapidly decreased. There were exponential function growths between the total nitrogen of surface water in paddy soil and the nitrogen apparent loss of every experimental point. The total nitrogen of surface water in paddy soil slowly increased with the nitrogen apparent budget in at the beginning stage, and total nitrogen of surface water in paddy soil rapidly increased, which respectively increased by 50.9%, 53.3%, 90.6% and 93.4% when nitrogen apparent rates of FJ、SC, YN and JX were more than 180, 225.5, 270 and 236.3 kg/hm2. The nitrogen application rate tolerance based on the principles of high rice yield, high nitrogen utilization rate and environment safety were confirmed by the analysis of the relationships of nitrogen application rate, rice yield, apparent utilization rate of nitrogen, apparent loss of nitrogen and total nitrogen of surface water in paddy soil. When the nitrogen application rates tolerance were respectively 135-184.7, 155-185.98, 225-236.3 and 185-249.5 kg/hm2 in Fujian, Sichuan, Yunnan and Jiangxi provinces, the rice yields were accordingly 5.38-5.46, 11.19-11.24, 9.77-9.81, 4.36-4.42 t/hm2. The study could provide a reference for the reasonable application amount of nitrogenous fertilizer in the process of rice cultivation.