Mechanism of improving water and nitrogen use efficiency and reducing soil nitrate leaching by suitable irrigation during the anthesis stage of wheat

Objectives This study investigated the effects of soil water content during the flowering period of wheat on N accumulation and transfer and soil NO₃⁻-N leaching to provide a theoretical basis for water conservation to promote high wheat yield and efficient N use.

Methods Field experiments were conducted during 2018–2019 and 2019–2020 wheat-growing seasons using Jimai 22 as the test cultivar. Three water treatments were set up during the anthesis stage: no watering (W0), watering 0–40 cm soil depth to a relative moisture content of 70% (W1) and 85% (W2). The wheat N accumulation and translocation at anthesis and maturing stage were determined; wheat yield and N fertilizer efficiency were investigated at the maturing stage, and soil nitrate-nitrogen content in 0–200 cm soil depth was analyzed.

Results After anthesis, the average N transfer in the vegetative organ of W1 at maturity was 11.6% and 7.3% higher than W0 and W2, and the N transfer rate in W1 was 9.5% and 6.1% higher than W0 and W2. At the maturity stage, the grain N distribution in W1 was 22.5% and 12.9% higher than W0 and W2, but the N distribution in the leaf and spike axis and glume in W1 was (P < 0.05) lower than in W0 and W2, thus increasing N harvest index. Compared with W0 and W2, W1 treatment reduced NO₃⁻-N content in 60–120 cm soil depth, increased wheat N uptake by 11.4% and 6.5%. The apparent excess soil N in W1 treatment was 51.0% and 40.9% lower than W0 and W2, reducing the risk of NO₃⁻-N leaching into the deeper soil layer. W1 reduced the residual inorganic N in 0–200 cm soil layer and the apparent excess soil N, which benefited absorption and utilization by wheat roots. Compared with W0 and W2, a thousand-grain weight of W1 treatment was 11.0% and 5.4% higher, grain yield was 25.9% and 11.8% higher, and the water use efficiency was 17.0% and 12.7% higher in the two growing seasons. Similarly, N use efficiency was 13.0% and 4.9% higher in W1 than W0 and W2, and the N uptake efficiency was 11.4% and 6.5% higher on average.

Conclusions Irrigating 0–40 cm soil layer to a moisture content of 70% during the flowering period benefits N transfer from vegetative organs to grains in the middle and late grain filling stages and at maturity, thereby promoting grain N accumulation, yield, N harvest index, and water use efficiency. Irrigating to 70% of the soil water capacity at the flowering stage reduces the NO₃⁻-N content in 60–120 cm soil depth, thereby decreasing the risk of NO₃⁻-N leaching, which improves wheat N use efficiency and uptake for production. Excessive irrigation leads to excessive downward movement of NO₃⁻-N, which affects root absorption. Insufficient water, on the other hand, decreases the transport of N to the grains.