Optimal water and nitrogen rate combination for winter wheat yield and water-nitrogen efficiency in Guanzhong area of Shaanxi

Objectives We explored the effects of water and nitrogen rate on regulating winter wheat yield, and soil nutrient availability, to provide an efficient water and nitrogen coupling management scheme for wheat production in the Guanzhong region.

Methods A field split plot experiment on winter wheat was conducted in Fufeng County, Shaanxi Province. Four sprinkling irrigation levels of 30 mm (W1), 60 mm (W2), 90 mm (W3), 120 mm (W4) were used as the main zone, and four nitrogen (N) fertilization levels of 150.0 kg/hm² (N1), 187.5 kg/hm² (N2), 225.0 kg/hm² (N3), 262.5 kg/hm² (N4) were used as the subzones. After harvest, 0−20 cm soil samples were collected to determine soil physicochemical properties and microbial biomass of carbon (MBC), nitrogen (MBN), and phosphorus (MBP) contents. Plant samples were collected to determine dry matter, N content, yield and yield components.

Results The wheat yield increased first and then decreased with the increase of irrigation and N application level. The highest (9053 kg/hm²) was recorded under W3N2 treatment, which was 1973 kg/hm² higher than W1N1 treatment. And the maximum yield fitted by the Curved surface model was 8848 kg/hm², corresponding to irrigation amount 98 mm and N application rate 212 kg/hm². Increasing irrigation amount benefited wheat dry matter and N accumulation, with the highest average dry matter accumulation (20306 kg/hm²) and the highest average N accumulation (221 kg/hm²) under W3 treatments. The highest dry matter and N accumulation were recorded in the combination of W3 and N2, which was 21252 and 237 kg/hm², respectively. Increasing N application was not conducive to N use by wheat, the N partial productivity and N use efficiency of N4 treatments were 34.5%−40.4% and 2.5%−6.9% lower than those of N1 treatment, respectively. The water use efficiency in all the treatments were in range of 15.1−21.6 kg/(hm²·mm), and controlling N application rate within 217−228 kg/hm² could maintain water use efficiency at a high level. Under W3 irrigation treatments, soil physical and chemical properties were optimal, and W4 led to a synchronous decrease in soil NO₃⁻–N, MBC and MBN content. According to the path analysis, the total effects on yield by dry matter and N accumulation, water and N input, soil nutrient content, and yield components were 0.716, 0.642, 0.522, and 0.400, in turn, the dry matter and N accumulation exhibited the highest effect on yield. In addition, the dry matter and N accumulation and yield components regulated wheat yield mainly through direct path, with direct effect size of 0.555 and 0.400, respectively, water and N input and soil nutrients regulated wheat yield mainly through indirect path, with indirect effect size of 0.554 and 0.544, respectively.

Conclusions The dry matter and N accumulation shows strongest direct effect, and irrigation and N application rate show strongest indirect effect on wheat yield. They are important factors in achieving wheat yield increase. Excessive irrigation and N application will deteriorate soil physicochemical properties, resulting in lower yield and water and N efficiencies. In winter wheat production under sprinkling irrigation in the Guanzhong region, irrigation amount 80−100 mm and N rate of 210−230 kg/hm² show synergistically effect on wheat yield and water and N efficiency.