Nitrate occurrence, alternative management practices, and modeled transport in the vadose zone over Nebraska's High Plains Aquifer

Groundwater contamination by nitrate (NO 3 ‐N) is a growing environmental and public health concern for the High Plains Aquifer underlying Nebraska, which is driven largely by intensive agricultural fertilizer applications. However, uncertainty remains in the variability of nitrate concentrations across deep vadose zones and the potential for alternative agricultural practices to ameliorate contaminated vadose zones. Therefore, NO 3 ‐N leaching dynamics were investigated in (1) deep vadose zones near Columbus, NE where mean groundwater NO 3 ‐N concentrations are >20 mg/L and (2) with a calibrated Hydrus‐1D model to simulate seven scenarios over a 60‐year period (1994–2053) to evaluate the potential for alternative crop and fertilizer management practices to ameliorate the contaminated vadose zones. Vadose zone cores from 16 shallow (6.1 m) and four deep (25.9 m) were obtained across farms with inorganic fertilizer (Zones 1 and 4) and with manure plus inorganic fertilizer (Zones 2 and 3). In the model scenarios, the largest decrease in NO 3 ‐N when compared to the traditional continuous corn with a single fertilizer application was observed for the corn–soybean rotation with a three‐split nitrogen application (40%–30%–30%). This scenario reduced cumulative NO 3 ‐N flux to the deep vadose zone by 10% (range of 7.6%–12.6%) and resulted in 45% lower vadose zone nitrate concentrations (range of 39.7%–51.8%) compared to traditional practices. Time for modeled NO 3 ‐N to reach half of the maximum concentration varied from 4.8 to 15.4 years, demonstrating the persistence after improved management practices. The findings indicate the partial reduction in NO 3 ‐N risk by implementing alternative crop rotation and fertilizer timing to protect groundwater in nitrate‐vulnerable regions.