Determining optimal nitrogen input rate on the base of fallow season precipitation to achieve higher crop water productivity and yield

Abstract Precipitation is the major cause of crop yield variation in rainfed agriculture production on the Loess Plateau. As overfertilization is economically and environmentally undesirable, and crop yield and the resulting returns for N input are uncertain when rainfall variability is high, optimizing N management according to the precipitation of fallow season is vital for efficient crop water use and high yield in dryland/rainfed farming systems. Thus, this study established a reference range describing wheat cultivation year types based on fallow season precipitation over 37 years. We conducted an 8-year field experiment using seven different N input rates, to focus on soil water content and its vertical distribution in the 0–300 cm soil layers and to assess the relationship between soil water utilization and yield formation. The final objective was to evaluate how the optimal N input affects crop water consumption, with the aim of maximizing yields. Eight experimental years (2009–2017) were classified as dry and normal years (four of each), with optimum N application rates of 150 and 180 kg N ha−1, respectively. Compared with that used by farmers (210 kg N ha−1), optimum N input saved 26.8% and 12.3% of N fertilizer, increased grain yield by 5.0% and 5.2%, and improved crop water productivity (CWP) by 6.1% and 8.5% in dry and normal years, respectively. Optimized N inputs reduced soil water consumption before the jointing stage and increased the soil water available for grain yield production. Soil water depletion occurred from deeper soil layers with crop development, and soil water storage (SWS) at 80–240 and 200–300 cm depth played an important role in increasing spike number and grain filling, respectively. Furthermore, soil water conservation treatment (increasing rainwater storage via deep plowing at start of the summer fallow season) was unable to eliminate the variability in soil water storage at sowing, and as “drought at sowing” effects on yield were more adverse than “drought in growing season” effects in dryland systems, our results indicate that adjusting N fertilizer inputs based on summer rainfall variation could enhance wheat yield and CWP in the rainfed farming system.