Determining optimal water and nitrogen management under different initial soil mineral nitrogen levels in northwest China based on a model approach

Determining the best management practices (BMPs) under different initial soil mineral N (ISMN) levels is helpful to improve water and N use efficiencies (WUE, NUE) and protect the environment. In this study, a two-year field experiment of spring maize was conducted from 2014 to 2015 in northwest China. Five treatments were designed, including one local farmer practice (I480-4N280, 280 kg N ha−1 fertilizer and irrigated 480 mm with four irrigation events) and four conservation treatments (I420-4N200, I420-5N200, I420-6N200 and I420-7N200, standing for 200 kg N ha−1 fertilizer and irrigated 420 mm with four to seven irrigation events, respectively). Measured yield, leaf area index, soil water content and soil nitrate concentration were used to evaluate a soil-crop model (WHCNS, soil Water Heat Carbon Nitrogen Simulator), which was then used to simulate the effects of water and N management practices on water consumption, N fates, and crop growth. Then, the integrated index considering the agronomic, economic, and environmental impacts was simulated using 35,100 combinations of irrigation and fertilizer N scenarios, which were designed to develop BMPs under different ISMN levels (ranging from 0 to 625 kg N ha-1 in a 1.8 m-soil profile) and seasons of weather data (from 2001 to 2015). Results showed I420-5N200 was the best with a high yield, WUE and NUE, and low N loss among five treatments. The optimal irrigation amount (ranging from 350−600 mm) was related to precipitation amount. The optimal N rate decreased linearly with the increase of ISMN when ISMN was <384 kg ha−1, and reached a plateau (0 kg N ha−1) when ISMN was >384 kg ha−1. In this study, we developed a simple linear model to determine the best N application rates under different ISMNs. This method is important to help farmers achieve multiple goals of high yield, resource conservation and reduced environmental impacts simultaneously.