Irrigation and N application strategies for spring wheat fields in the Hetao irrigation district based on N balance

Inappropriate irrigation and nitrogen (N) application have caused the severe N losses and budget imbalance in the spring wheat field in the Hetao Irrigation District (41°09′N, 107°39′E), Northwest China. This study aims to determine the rational irrigation and N application strategy for the spring wheat in the study area. A series of field experiments were carried out in the 2019-2021 seasons. A systematic investigation was made to clarify the effects of irrigation and N application on the soil N budget, NO3--N leaching, NH3 volatilization, N2O emissions, crop N uptake, as well as soil N surplus and deficit. Three irrigation levels (i.e., 450, 315, and 180 mm (I1, I2 and I3)) and two N fertilization levels (i.e., 340 and 170 kg/hm2 (N1 and N3)) were considered in the experiments. Furthermore, the control group was added in the medium N application level (i.e., 250 kg/hm2 (N2)) under the condition of a medium irrigation level (i.e., 315 mm (I2)) in 2020 and 2021. The results showed that the fertilizer N was accounted for the largest proportion of soil N input, followed by the irrigation water, atmospheric deposition, and non-bio fixation. The wheat N uptake was accounted for the largest proportion of soil N output, followed by NO3--N leaching, NH3 volatilization, and N2O emissions. The total N input in the soil showed a decreasing trend with the decrease of irrigation depth and N application. It infers that the reduced irrigation depth and N application significantly decreased the soil total N losses. But there was the lower threshold in the irrigation depth and N application treatment for the less N uptake of wheat. Among them, the control treatment (i.e., I1N1) resulted in the highest total N input, and the highest soil N losses (NO3--N leaching, NH3 volatilization, and N2O emissions). Compared with the control treatment, the I2N2 and I3N3 treatments decreased the total N input by 25% and 48%, respectively, and the soil total N losses by 21%-29% and 16%-34%, respectively. The wheat N uptake in the I3N3 treatment was significantly lower than that in the control treatment. But there was no significant diffidence in the wheat N uptake between the I2N2 and control treatments. The N application presented a significant effect on the soil N surplus and deficit. The N1 treatment was resulted in the higher soil N surplus, while the N3 treatment with about 50% decrease of N application rate was resulted in the higher soil N deficit. However, a slight deficit was obtained in the N2 treatment under the I2 irrigation conditions, where the soil N deficit was only 28-50 kg/hm2. This N deficit value was also accounted for 11%-20% of the total N application amount. Once the 4-8 t/hm2 per season wheat straw was returned to the field after harvest, the N balance could be maintained in the wheat field. Thus, the soil N losses can be effectively mitigated from the spring wheat farmlands in the Hetao Irrigation District, Northwest China. The soil N balance can also be expected to improve the irrigation and N application management, particularly with the appropriate straw returning.