Multi-Objective Optimization Water–Nitrogen Coupling Zones of Maize under Mulched Drip Irrigation: A Case Study of West Liaohe Plain, China

The impact of different combinations of water–nitrogen coupling on maize yield and the environment needs investigation. Low, medium, and high levels of irrigation and N application gradients were studied through field experiments to elucidate the suitable water–nitrogen coupling zone for spring maize in the West Liaohe Plain during three hydrological year patterns under drip irrigation with plastic film. The effects of different water–nitrogen couplings on maize yield, water- and nitrogen-use efficiencies (WUE and NUE), and post-harvest soil alkali-hydrolyzable N residues were studied under integrated drip irrigation by varying the application rates of water and fertilizer. A multi-objective optimization of water–nitrogen coupling zones was performed by integrating maize yield, harvest index, WUE, and soil environmental effects. Results show that with an increase in irrigation and N application rate, the residual amount of alkali-hydrolyzable N increased slowly within a certain range. Upon exceeding a certain amount, residual N increased rapidly, and more N entered the soil environment. The NUE of moderate water–nitrogen coupling treatment was high, with lower environmental risk of residual alkali-hydrolyzable N. Moderate irrigation yielded the highest harvest index in the normal hydrological year. Irrigation rate had a higher impact on yield compared to nitrogen application, because of drip irrigation under plastic film. An appropriate irrigation amount results in a higher WUE and the application of N application must be adjusted according to the rainfall in a particular year. This study highlights the need for structuring water–nitrogen coupling zones specifically for different hydrological years.