The effects of water and fertilizer practices on nitrogen leaching in open-field vegetable soil in North China

Groundwater nitrate pollution is a concern for the government and scientific community. During the growth period of open-field vegetables in North China, water and nitrogen (N) are often excessively used, resulting in a lower efficiency rate, which threatens groundwater quality. A field experiment was conducted in cinnamon soil on cucumber and Chinese cabbage crop rotation farmland to evaluate the effects of water and fertilizer on crop yields, N leaching, and N balance. Four standard N treatments were used conventional N application in each vegetable season, 550 kg(N)·hm^-2·a^-1, N3; 20% less N, N2; 50% less N, N1; no nitrogen, CK, and leaching was monitored using a lysimeter. Five additional treatments were tested that combined a 20% N reduction with an alternative management practice: urease and nitrification inhibitors (N2I), biochar (N2B), straw incorporation (N2S), 15% irrigation reduction (N2W1), and 30% irrigation reduction (N2W2). The results showed that deep soil nitrate accumulation and root zone nitrogen leaching were primarily nitrogen loss. Using conventional N (N3), 10.0% of the applied N leached from the 80 cm soil layer, and the leached amount decreased by 23.8% and 45.6% by using 20% less N (N2) and 50% less N (N1), respectively, compared with that of N3. A 20% N reduction did not affect vegetable yield, but a 50% reduction decreased the cucumber yield by 19.6%. The combined practices (inhibitors, biochar, and straw) decreased the total leached N by 40.7% (N2I), 43.0% (N2B), and 34.3% (N2S) without affecting yields. Reducing irrigation decreased the total leached N by 43.1% (N2W1) and 50.5% (N2W2) compared with N3, but N2W2 decreased the cucumber yield by 13.9%. After three years (six continuous seasons), large amounts of nitrate accumulated and then moved to deeper soil. Nitrate accumulation in the 0-80 cm soil layer after conventional fertilization accounted for 38.2%-50.7% of the 0-200 cm soil layer, which was high compared to other management practices. Decreasing water and N fertilizer use combined with urease and nitrification inhibitors may reduce N leaching and cost. These results provide solutions for improving water and nitrogen management, thereby decreasing soil nitrate accumulation and deep soil leaching, reducing vegetable production and groundwater quality threats.