Response of soil mineral nitrogen accumulation and enzyme activities to nitrogen application in spring maize

【Objectives】Nitrogen fertilization (N) is ubiquitously over-applied in the spring maize production in Northeast China, inducing decreased N use efficiency and serious environmental N pollution. Little information about the effects of N fertilization on soil mineral N (Nmin) accumulation characteristics has been reported in this area. The behaviors of soil Nmin accumulation characteristics and the activities of some key enzymes regulating N metabolism are affected by N fertilization. A reasonable N rate is needed for spring maize production.【Methods】Through a two-year field experiment and laboratory analysis, the behaviors of soil Nmin accumulation and the activities of key enzymes regulating N metabolism were studied under different N fertilizer rates. Based on the survey of fertilization levels in test area, six levels of N application rates (N0, 60, 120, 180, 240 and 300 kg/ha) were designed. Soil samples were collected in 0-120 cm layer every 15 days since sowing stage, with a total of 10 samplings, and maize yield were investigated at maturity.【Results】The yields of spring maize increased significantly with increasing of N application rate, once the rate exceed N 240 kg/ha, the yields begin to decline. The dynamics of soil NO-3-N is influenced by both the N rates and growing stages. Because of topdressing of N fertilizer, the NH+4-N accumulation in 60 cm soil layer at the jointing to the big trumpet and tasseling stage is significantly higher than at seeding and maturity stage. The NO-3-N accumulation in 0-60 cm soil layer increases significantly from seeding to tasseling stage, and fluctuates below 60 cm soil layer. Except for maturity, NO-3-N accumulation is significantly increased with the increased N application rate, and decreased with increasing of soil depth. There is no accumulation peak appeared through the soil layer under lower N application rate, they move from 20-60 cm at jointing stage to 80-100 cm soil layer at tasseling stage under N 240 and N 300 treatments, indicating the high NO-3-N leaching risks caused by excessive N fertilization. Both the activities of soil urease and nitrate reductase are increased with the increase of N application rate, and the highest level are at the jointing to tasseling stage, synchronized with the accumulation trend of soil ammonium N and nitrate N. 【Conclusions】 Considering comprehensively, the reasonable N application rate should be in the range of N 179-209 kg/ha under the experimental conditions. With this rate, the grain yield increase, N use efficiency and environmental safety can be synchronized realized.