Accumulation Patterns and Numerical Simulation of Nitrate-N in Layered Soils of the Vadose Zone in Cotton Fields

Excessive nitrogen fertilizer in cotton cultivation boosts yields but causes groundwater pollution via nitrate-N (NO3−-N) accumulation. This study combined field experiments and HYDRUS-1D modeling to analyze water and NO3−-N dynamics in the vadose zone of cotton fields in Nanpi, Hebei Province, North China, under deep groundwater conditions. Monitoring during a 184-day growth period revealed that NO3−-N accumulation increased from 11.4 to 21.2 g m−3 under conventional flood irrigation and pre-sowing fertilization. Soil texture critically influenced peak NO3−-N accumulation depth, while rainfall, moisture, and crop uptake affected migration patterns. The HYDRUS-1D model was employed to numerically simulate the accumulation and migration of water and N in the cotton vadose zone. The HYDRUS-1D simulations closely matched the observed data, demonstrating effectiveness at modeling water–nitrogen transport patterns in the cotton vadose zone under deep groundwater conditions. Various factors, including rainfall, soil texture, soil moisture content, and crops, influenced the accumulation in the soil vadose zone. Notably, the location of the nitrate-N accumulation peak in the soil vadose zone was influenced by soil texture. This study highlights the environmental risks of current practices and provides insights for optimizing fertilizer management in arid agricultural zones.