Objective The aims were to clarify the accumulation characteristics of nitrate nitrogen (N) in the soil profile, and determine the control mechanism of its migration to deeper layers, so as to provide a basis for improving N utilization efficiency and controlling environmental pollution caused by excessive N.

Method Typical farmland in Jinzhong Basin was selected as the research object, and the sampling sites were determined by grid distribution. Soil samples were collected from seven layers at each sampling site: 0 - 20, 20 - 40, 40 - 60, 60 - 80, 80 - 100, 100 - 150, and 150 - 200 cm. Nitrate nitrogen, ammonium nitrogen, organic matter, and soil water content were measured in the laboratory. The vertical spatial distribution characteristics of nitrate N content were systematically analyzed. At the same time, the spatial differentiation pattern of nitrate N concentration in different soil layers was characterized by geostatistical methods, and the nitrate N concentration in 0 - 100 cm and 100 - 200 cm soil layers were used as response variables to construct structural equation models (SEM) in different layers. The direct and indirect effects of fertilization intensity, soil moisture, ammonium N and organic matter on nitrate N accumulation and migration were analyzed quantitatively.

Result ① In the vertical distribution of the soil, the nitrate N concentration showed a significant decrease with depth, meanwhile, the highest concentration was in the surface layer, fluctuating and decreasing in the middle layer, and remaining at a low value in the deep layer. ② The results of geostatistics indicated that the surface nitrate N showed strong spatial heterogeneity. The spatial distribution of nitrate N in the soil layer of 40 - 150 cm was relatively flat, but there was still accumulation risk in some areas of the depth of 150 - 200 cm, and the accumulation risk mainly occurred in the orchard area. ③ Stratified SEM results showed that nitrate N accumulation in shallow layer was mainly controlled by fertilization intensity and ammonium transformation process, while nitrate N migration in deep layer was more directly affected by ammonium transformation process and 0 - 100 cm nitrate N migration.

Conclusion The distribution characteristics and migration mechanisms of nitrate N in farmland in the Jinzhong Basin were different in different soil layers. The surface soil was the main accumulation area of nitrate N, while although there was nitrate N migration in the deep soil, the concentration was relatively low and showed a gentle distribution. The spatial distribution of nitrate N was highly correlated with the type of crop. The nitrate N in the upper soil layer (0 - 100 cm) was mainly affected by the intensity of fertilization and the transformation of ammonium N. The accumulation of nitrate N in the lower soil layer (100 - 200 cm) was mainly the combined result of the transformation of ammonium N and the vertical transport process of nitrate N. The research results could provide a scientific basis for the rational management of N in farmland and the prevention and control of groundwater pollution in the Jinzhong Basin.