Soil nitrate distribution and removal in a soybean–wheat system in response to groundwater level variations

Nitrate (NO 3 – ) contamination of groundwater caused by agricultural production processes has received considerable attention. However, the mechanisms governing NO 3 ⁻ migration and attenuation under varying groundwater table conditions remain unclear. Lysimeter tests provide an ideal supplement for studying such processes under controlled conditions. This study presents a lysimeter experiment that was designed for the investigation of NO 3 – transport and removal in the unsaturated zone. Experiments were conducted in soybean–wheat rotation systems from July 2022 to June 2024. Three groundwater depths (30, 60, and 90 cm) and three NO 3 – -N concentrations (0, 5, and 15 mg L −1 ) were applied in the experiments. The results suggested that shallow water levels (30 cm) impeded NO 3 – -N accumulation in the soil profile. Under the different treatments, the upward NO 3 – -N transport flux (TF) decreased with increasing water table depth. Moreover, the NO 3 – -N removal capacity increased with increasing groundwater level, with the observed NO 3 – -N removal efficiency (RE) values ranging from 54.8 % to 87.4 % across the treatments. The optimal groundwater level for maximizing RE was 30 cm. The soil volumetric moisture content (VMC) played a crucial role in driving the generation of excess dinitrogen (N 2 ). When multiple related variables, including the VMC, were combined as predictive factors, the explanatory power of the regression model for denitrification increased. Our research bridges a key knowledge gap regarding the upward migration and transformation of groundwater NO 3 – -N in the unsaturated zone and provides a theoretical basis for improving soil and water pollution prevention and control measures. • A shallow water levels (30 cm) reduce nitrate accumulation in the soil profile. • NO 3 – -N removal efficiency (RE) increased with groundwater level, ranging from 54.8 % to 87.4 %. • The optimal groundwater level for maximizing RE was 30 cm. • VMC is crucial in denitrification prediction models.