Effects of irrigation and nitrogen application on NO3--N distribution in soil, nitrogen absorption, utilization and translocation by winter wheat

Irrigation and nitrogen application can not only increase wheat yield, but also cause nitrogen loss and soil nitrate accumulation. This study aimed to assess the effects of irrigation amount and nitrogen application rate on the soil NO3--N distribution and nitrogen absorption and utilization of winter wheat (Triticum aestivum L.) during the 2015–2020 growth season in the North China Plain. The experiments were designed in split plot arrangement, with irrigation amount (W0, no irrigation; W1, 60 mm irrigation at jointing; and W2, 60 mm irrigation at jointing and anthesis) as the main plots and nitrogen application rate (N0, 0; N120, 120 kg ha−1; and N240, 240 kg ha−1) as the subplots. The plants under W2N120 and W2N240 showed no significant difference in yield throughout the experimental period. With the decrease in soil total nitrogen content, the yield difference between the plants under N0 and those under N120 and N240 gradually increased throughout the experimental period. Compared with that of the plants under W2N240, the mean NO3--N content of the plants under W2N120 in the 0–1 m and 1–2 m soil layers decreased by 17.4% and 19.2%, respectively, in 2018–2020. The NO3--N concentration decreased with increasing soil depth, and the NO3--N concentration in each soil layer increased with increasing nitrogen application rate. Irrigation and nitrogen application increased nitrogen accumulation in vegetative organs at anthesis and nitrogen transport to grains. Compared with those of the plants under W2N240, the mean nitrogen uptake efficiency, nitrogen use efficiency, nitrogen fertilizer productivity, and nitrogen harvest index of the plants under W2N120 increased by 91.3%, 2.6%, 95.5%, and 1.9%, respectively. These results suggest that applying 120 kg ha−1 nitrogen and irrigating 60 mm at jointing and anthesis are the optimal measures for winter wheat in the North China Plain.