Effects of nitrogen application on nitrogen and phosphorus leaching in fluvo-aquic soil on a winter wheat-summer maize rotation farmland

Fluvo-aquic soil is predominant in the North China Plain, where a large amount of wheat and corn are grown in China. Understanding the relationship between nitrogen and phosphorus leaching and nitrogen application is important for preventing and controlling nonpoint source pollution in this area. The field seepage pool method was used to explore nitrate nitrogen and total phosphorus leaching in fluvo-aquic soil on a winter wheat and summer maize rotation farmland. Three fertilization treatments were tested: traditional nitrogen application (CON), optimized nitrogen application (OPT), and optimized nitrogen plus nitrogen reduction (OPTJ). The results showed that from 2016 to 2018, the annual leachate volume from the 90 cm soil layer was between 79.0 and 102.5 mm (all treatments). The leached nitrate nitrogen concentrations were 18.9-208.7 (average 72.7) mg·L^-1 (CON), 9.0-99.2 (average 33.8) mg·L^-1 (OPT), and 4.7-55.5 (average 15.4) mg·L^-1 (OPTJ), fluctuating among leaching events. The nitrogen leaching risk was higher, and the phosphorus leaching risk was lower in the fluvo-aquic soil area. The average leaching amount was 66.4 kg·hm^-2 and apparent leaching loss coefficient was 10.3% for nitrate nitrogen; these values for total phosphorus were 0.06 kg·hm^-2 and 0.04%, respectively. Reducing nitrogen fertilizer decreased nitrogen leaching by 56.3% (OPT) and 78.9% (OPTJ), and the apparent leaching coefficients were 10.3% (CON), 6.2% (OPT), and 4.9% (OPTJ), indicating that nitrate nitrogen leaching increased as nitrogen fertilizer increased. Nitrogen leaching had interannual variation; 2018 had high rainfall, and the leaching amount was 57.0% higher than in 2017, which had low rainfall. During the sampling years, the total phosphorus leached was 0.06 kg·hm^-2 (CON), 0.06 kg·hm^-2 (OPT), and 0.08 kg·hm^-2 (OPTJ). A moderate nitrogen fertilizer reduction increased crop yield; the OPT yield was 1.08 times higher than the CON yield. However, excessive fertilizer reduction decreased yield. OPTJ had 56% less nitrogen than CON, and the yield decreased by 2.0%-8.1%. The partial factor productivities were 25.3 kg·hm^-2 (CON), 35.7 kg·hm^-2 (OPT), and 57.4 kg·hm^-2 (OPTJ) for winter wheat and 28.5 kg·hm^-2 (CON), 44.8 kg·hm^-2 (OPT), and 62.7 kg·hm^-2 (OPTJ) for summer maize. The nitrogen fertilizer partial factor productivities of OPT and OPTJ were significantly higher than that of CON. These results showed that the nitrate nitrogen leaching potential was high in fluvo-aquic soil, and reducing nitrogen fertilizer could significantly reduce nitrogen loss without decreasing crop yield. Considering crop yield and nitrate nitrogen leaching risk together, the optimal nitrogen amount for winter wheat and summer maize farmland in the study area was 465 kg·hm^-2. When nitrogen decreased to 285 kg·hm^-2, nitrogen leaching sharply decreased, but the crop yield decreased slightly.