Optimizing nitrogen-fertilizer management by using RZWQM2 with consideration of precipitation can enhance nitrogen utilization on the Loess Plateau

Crop yields are related to N fertilizer management, and also depend on local precipitation. Varying precipitation levels with long-term meteorological data have not been considered to optimize nitrogen (N) strategies in previous studies on the Loess Plateau of China. In this study, Root Zone Water Quality Model 2 (RZWQM2) was calibrated and validated using data from multi-year experiments and used to assess and optimize N management strategies for winter wheat cultivation. Results showed that the basal dressing fertilizer with 120 kg N ha^-1 together with the topdressing of 67–77 kg N ha^-1 was recommended in regions with 443 mm average annual precipitation. For those with 364 mm and 290 mm average annual precipitation, the basal dressing fertilizer with 90 kg N ha^-1 together with the topdressing of 67–77 kg N ha^-1 and the basal dressing with 90 kg N ha^-1 together with the topdressing fertilizer of 13–23 kg N ha^-1 were recommended, respectively. Compared with farmers’ practice (i.e., the single basal dressing), although decreasing the total rate by 12–18 kg N ha^-1, the optimized N strategies (i.e., the basal fertilizer together with one-time topdressing) can effectively promote grain N uptake, nitrogen harvest index, and agronomic efficiency of N. It also maintained similar grain yield, evapotranspiration, and crop water productivity. The minimum precipitation threshold was around 300 mm, where the topdressing N fertilizer had little influence on grain yield, evapotranspiration, and grain N uptake. Additionally, the largest advantage of optimized N strategies was saving N fertilizer and reducing the environment footprint of wheat production. However, the crop production under the optimized N strategies was more sensitive to the precipitation variation than that under farmers’ practice. Thus, if climate continues to change following historical data, greater harvest fluctuations are expected under optimized N strategies. To cope with the evolving climate change, optimized N strategies should be integrated with other management measures for smallholder farming households on the Loess Plateau.