Irrigation Regime Optimization Plays a Critically Important Role in Plastic-Shed Vegetable Production to Mitigate Short-Term and Future N Leaching Pollution

In northern China, plastic-shed vegetable production significantly contributes to nitrogen (N)-induced groundwater eutrophication due to excessive fertilization and irrigation. However, the impact of optimized farming practices on N leaching has seldom been systematically examined. We conducted a four-season field study to evaluate the impacts of optimal farming measures on tomato yield, water percolation, N concentration in leachate, and total N (TN) leaching. The treatments included conventional fertilization and flood irrigation (CON), fertilization decreased by 20% and flood irrigation (OPT1) or drip fertigation (OPT2), fertilization decreased by 30% and drip fertigation (OPT3), and no fertilization with flood irrigation (CK). Compared with the CON treatment, the optimal treatments significantly reduced annual TN leaching by 9.92–50.7% without affecting tomato yield (57.1–98.2 t ha−1 for CON and 48.1–106 t ha−1 for three optimal treatments). Drip irrigation contributed 73.8–79.0% to the mitigation of TN leaching. The N originating from soil and irrigation water exhibited a similar contribution to TN leaching (45.4–58.6%) to that of fertilizer N. The daily TN leaching at the basal fertilization stage was much greater than that at the top-dressing stage, due to over-fertilization. Optimizing fertilization, particularly basal fertilization, in combination with drip irrigation could substantially reduce N leaching in plastic-shed vegetable production. Other optimal practices, such as decision support systems (DSSs) and fertilizer amendments, could also be investigated to further mitigate the N leaching.