Objective Deep storage water irrigation is an efficient utilization model of rain water and flood resources, but there are potential problems such as nitrate leaching, reduced nitrogen (N) utilization efficiency, and water environment pollution caused by large irrigation water volume. By exploring the N absorption of plants, residual soil nitrate N, and apparent N balance under different levels of deep water storage irrigation, this study provides theoretical and technical support for optimizing the application of deep water storage models in the region in the future.
Methods A wheat field experiment was conducted in the Baojixia Irrigation Area of the Guanzhong Plain (Caoxinzhuang Farm, Yangling Demonstration Area). The experiment used "Xiaoyan 22" as the test variety and non stored water irrigation as the control (CK). Five water storage irrigation soil layers with depths of 80, 120, 140, 160, and 180 cm were set up, denoted as T0.8, T1.2, T1.4, T1.6, and T1.8, respectively, to regulate the nutrient organ biomass of winter wheat plants in different water storage and storage soil layers. The total N content and soil nitrate N content in the 0 ~ 200 cm soil layer of farmland were measured to calculate the N accumulation, N utilization efficiency, and N profit and loss of winter wheat. The law of N utilization and leaching in farmland under deep water storage mode was analyzed.
Result As the depth of the irrigated soil layer (irrigation amount) increases, the pre flowering N accumulation and grain N content in winter wheat plants significantly increased. However, increasing the depth of the irrigation soil layer on the basis of deep water storage and irrigation to 120 cm, i.e. continuing to increase the irrigation amount, will inhibit crop growth and N absorption, reduce pre flowering N transfer, and thus reduce grain N content. In addition, the N fertilizer use efficiency of the CK treatment was higher than that of the deep storage irrigation treatment. Under the condition of deep storage and irrigation, the N use efficiency first increased and then decreased with the increase of water storage depth, and the N use efficiency was the highest at the irrigation depth of 140 cm. Under this mode, the combination of rainfall and irrigation could lead to the leaching depth of nitrate N far exceeding the irrigation depth, resulting in a "low" nitrate N content in various soil layers during wheat maturity. When the depth of water storage irrigation was 120 cm, the accumulated residual nitrate N content in the surface soil (0 ~ 100 cm soil layer) was the highest, and the N loss in the deep soil (100 ~ 200 cm soil layer) was the smallest. Deep water storage irrigation significantly increased soil N apparent loss. When the water storage irrigation depth was 120 cm, the N apparent surplus was 122.95 kg hm−2, and the N surplus was 43.30%. If the amounts of irrigation continued to be increased on this basis, the more surplus N was carried away and transported deeper under the drive of water, polluting groundwater.
Conclusion The application of deep storage water irrigation mode in the northwest region can significantly improve wheat N absorption and grain N accumulation at a depth of 120 cm compared to irrigation at depths of 160 and 180 cm. The problem of N leaching loss caused by excessive irrigation is also reduced compared to other deep treatments, but it still needs to be given sufficient attention. Small and multiple applications of N fertilizer and other N fertilizer management measures should be taken in farmland to slow down nutrient leaching and improve N utilization efficiency, in order to protect the farmland environment and prevent pollution of groundwater environment.
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