Impact of Irrigation Regimes and Nitrogen Fertilizer on Grain Yield, Water and Nitrogen Use Efficiencies of Winter Wheat Varieties Released Between 1950 and 2020

Modern agriculture faces serious challenges including depletion of soil nutrients and water resources. Improving the nitrogen-use efficiency (NUE) and water use efficiency (WUE) are major constraints on wheat production in many semiarid regions around the world. Resolving these issues in winter wheat can help to mitigate the problems of low soil fertility and water scarcity under changing climate. The study was conducted with eight winter wheat varieties popularized in the Loess Plateau of northwest China from the 1950s to the 2020s used as materials. Nine combinations of irrigation and nitrogen (N) treatments were established by employing a three-factor split-plot design to evaluate their effects on NUE, WUE and yield with its components. The results showed that the improvement in grain yield of modern wheat varieties was primarily reflected in an increase in harvest index (HI), from 30-35% of the old varieties to 40-52%. The structural equation model (SEM) results showed that irrigation and variety directly affected grain yield (R2: 0.254, 0.339), HI and spike number (SN) had an indirect effect on grain yield (R2: 0.385, 0.288). Moreover, irrigation increased winter wheat yield by boosting SN and leaf area index (LAI) and SN. The application of varieties and N affected HI, and improved yield. Also, SPAD chlorophyll content is an important factor affecting SN and grain yield. The modern winter wheat varieties showed the highest WUE and NUE, along with a significant increase in yield under different irrigation and N treatments. These results provide theoretical support for further improving wheat yield potential and breeding high-efficiency wheat varieties. Further research is needed to elucidate the relationship between yield components and irrigation and N management for optimizing crop yield.