Abstract: Excessive nitrogen fertilizer application is widespread in sugarcane planting in China. It is important to investigate the effects of nitrogen application rates on sugarcane biomass and concentrations of soil inorganic nitrogen for formulating reasonable nitrogen application rates to reduce the risk of environment pollution caused by nitrogen leaching. In this study, a field experiment was carried out to explore the effects of four nitrogen application rates \[0 (N0), 300 (N300), 400 (N400), and 500 kg N·hm^-2 (N500, the conventional nitrogen application rate)\] on sugarcane biomass and soil inorganic nitrogen concentration in different soil depths (0-10, 10-20, 20-40 cm) on May 9, May 19, June 19, July 20, August 19, September 21, October 20 and November 19, 2021. The results showed that sugarcane height, stem biomass, and root biomass were significantly higher under the treatments of N400 and N500 than that under N0 from June to November. There were no significant differences in sugarcane height and stem biomass between the treatments of N400 and N500 in September, October, and November. Sugarcane height and biomass of leaf, stem and root varied across different months, while nitrogen application rates significantly affected sugarcane height and root biomass. The concentrations of soil nitrate and ammonium nitrogen decreased with soil depth. However, the accumulated amount of soil nitrate and ammonium nitrogen at 0-10 cm soil depth was generally lower and that at 20-40 cm soil depth was generally higher. Growth month and nitrogen application rates significantly affected the concentration and accumulation of soil ammonium and nitrate nitrogen. Across different growth months, the accumulation of soil ammonium and nitrate nitrogen in the N500 treatment was generally higher than that in other fertilizer treatments. Therefore, the risk of inorganic nitrogen leaching may increase with increasing nitrogen application rates. Compared with conventional nitrogen application rate, reduced nitrogen application did not affect sugarcane biomass but could reduce the risk of soil inorganic nitrogen leaching.

Key words: nitrogen application rate, sugarcane height, sugarcane biomass, nitrate nitrogen, ammonium nitrogen.