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[Characteristics of soil nitrate accumulation and leaching under different long-term nitrogen application rates in winter wheat and summer maize rotation system.]

Winter wheat and summer maize were the main crops in the North China Plain. While intensive farming system could generally achieve high yield, the perennial large amounts of nitrogen (N) fertilization application cause environmental problems including NO3--N accumulation and leaching at deep soil layer. Here, the effects of different N application rates on soil NO3--N accumulation and leaching in winter wheat-summer maize cropping system were investigated from 2010 to 2016 at Qingyuan County, Hebei Province, China. There were five treatments with N application rates at 0 (N0), 100 (N100), 180 (N180), 255 (N255) and 330 (N330) kg·hm-2. Results showed that crop yield and soil N status significantly varied among treatments for both wheat and maize after each harvest, respectively. Soil NO3--N were accumulated during winter wheat growing season and leached to deeper soil during summer maize growing season. Moreover, the soil NO3--N accumulation amount in the 90 to 180 cm soil profile decreased with the decreases of N inputs (N330 > N255 > N180 > N100 > N0). Soil NO3--N could be leached to 990 cm soil depth. There were six NO3--N accumulation peaks in the soil profile, with the peaks presenting at deeper soil profile with higher N fertilization rate. The deepest peak appeared at 840 cm soil depth with the N application rate of 330 kg·hm-2. From the distribution of NO3--N accumulation in the soil profile, only around 10% of total NO3--N was accumulated between 0-90 cm soil depth, while the rest accumulated below 90 cm, which could not be largely absorbed by plants. Therefore, NO3--N leaching during summer maize growing season was serious and it was greater with higher N fertilization rate which might lead to increased risk of underground water contamination. In terms of balanced crop yield and soil NO3--N accumulation, the rate of 180 kg·hm-2 would be the optimum one in areas with similar cultivation and environmental conditions to the present study.