Modeling nitrogen dynamics in agricultural soils: Pathways of leaching and groundwater denitrification potential.

Groundwater nitrate (NO3-) contamination remains a major environmental risk in intensively irrigated agricultural regions. Excessive nitrogen (N) inputs and inefficient management practices are major drivers because they accelerate NO3- leaching. Assessing both the sources of NO3- enrichment and its natural attenuation potential is critical to evaluating the long-term risks to groundwater ecosystems. In this study, NO3- inputs to groundwater were quantified using the soil Water Heat Carbon Nitrogen Simulator (WHCNS) model, while groundwater denitrification potential was evaluated by integrating model simulations with field measurements in the Ningxia Yellow River Irrigation Area. Field experiments were conducted across three representative cropping systems, including paddy, corn, and vegetable fields, under multiple N fertilization regimes. The WHCNS model, calibrated and validated with two years of field observations, reliably simulated N transport, transformation, and leaching dynamics. The results revealed that NO3- leaching was influenced not only by N application amount but also by irrigation intensity, crop type, and the temporal synchronization of water and N inputs. Paddy fields exhibited the highest cumulative NO3- leaching, ranging from 21.36 to 51.82 kg N·ha-1, due to prolonged flooding. In contrast, corn fields displayed episodic but intense leaching following irrigation, with a maximum daily leaching loss of 4.36 kg N·ha-1 and a maximum cumulative of 30.71 kg N·ha-1. Simulated NO3- leaching loads were combined with field-measured groundwater NO3--N and excess N2 in regression analysis to estimate denitrification potential. At 200 cm depth, the groundwater NO3- removal efficiencies exceeded 30% in paddy and corn fields, and over 20% in vegetable fields. At 300 cm depth, NO3- removal efficiencies exceeding 70% in both corn and vegetable fields. These findings highlight the need to optimize N inputs and irrigation practices according to crop type and soil depths, in order to enhance denitrification process and reduce NO3- leaching risks. The integrated modeling-measurement framework provides a scalable approach to estimating groundwater NO3- loading and denitrification potential, offering a scientific basis for precision N management and groundwater quality protection in arid irrigated regions.