Simulation of soil water and nitrogen flux in the northern alluvial fan of Hutuo River

Alluvial fan areas are the main areas of agricultural development and are sensitive to nitrogen leaching. The current understanding of the spatial and temporal distribution of soil water and nitrogen leaching fluxes in deep soil at the alluvial fan area scale is still limited. In this study, the farmland in the northern alluvial fan of Hutuo River was selected as the primary research area. Based on the improved HYDRUS regional-scale water and nitrogen flux simulation model, the soil water and nitrate nitrogen transport processes in regional farmland from 1975 to 2021 were simulated in parallel. The results showed that crop type was the main factor affecting soil nitrogen accumulation and leaching in the study area. The average annual nitrate nitrogen accumulation of main crop types followed a descending order: vegetable (2 432 kg·hm⁻²∙a⁻¹) > wheat-maize rotation (1 508 kg·hm⁻²∙a⁻¹) > cotton (1 504 kg·hm⁻²∙a⁻¹) > orchard (364 kg·hm⁻²∙a⁻¹). The average annual flux of soil recharge of main crop types followed a descending order: vegetable (351 mm∙a⁻¹) > wheat-maize rotation (98 mm∙a⁻¹) > orchard (87 mm∙a⁻¹) > cotton (54 mm∙a⁻¹). The average annual nitrogen leaching of the main crop types followed a descending order: vegetable (374 kg·hm⁻²∙a⁻¹) > wheat-maize rotation (68 kg·hm⁻²∙a⁻¹) > orchard (23 kg·hm hm⁻²∙a⁻¹) > cotton (21 kg·hm hm⁻²∙a⁻¹). Crop type affected the spatial distribution of soil water and nitrogen fluxes and the lag time of the response of nitrate nitrogen loss to surface nitrogen input. High irrigation amount and fertilization rate for vegetable production resulted in the shortest lag time for nitrate nitrogen leaching, followed by wheat-maize rotation, then orchard, with cotton having the longest lag time. Soil nitrate nitrogen accumulation and leaching from wheat-maize rotation were positively correlated with flux of soil recharge. The interannual and seasonal responses of soil nitrate nitrogen leaching to precipitation were clear. The peak value of soil nitrate nitrogen leaching occurred in the rainy season, and the significant changes in soil nitrate nitrogen leaching occurred along with heavy precipitation. This study provides scientific support for regional non-point source pollution prevention, control, and groundwater quality protection.