Optimizing Nitrogen Management: Assessing Nitrate Transport in the Deep Vadose Zone and Its Impact on Groundwater Quality in East-Central Nebraska

Groundwater contamination by nitrate (NO₃-N) is a growing environmental and public health concern driven largely by intensive agricultural fertilizer applications. The vadose zone plays an important role in NO₃-N transport, influencing groundwater quality and agricultural sustainability. This study investigated NO₃-N leaching dynamics in the deep vadose zone of Water Quality Sub-Area 30 (WQA30) within the Lower Loup Natural Resources District (LLNRD), northeast of Columbus, Nebraska (mean groundwater NO₃-N: 20.3 mg/L), using soil cores from 16 shallow (to 6.1 m) and four deep (down to 25.9 m) across four zones, two with inorganic fertilizer (Zones 1 and 4) and two with manure plus inorganic inputs (Zones 2 and 3). Laboratory analysis revealed variable NO₃-N levels (e.g., Zone 1: 25.1 ± 38.8 mg/L; Zone 4: 13.0 ± 12.8 mg/L), with peaks near the surface (e.g., Zone 1: 110 mg/L at 60 cm). Hydrus-1D modeling, calibrated with 1994–2023 data (high Index of Agreement, low RMSE), simulated seven 60-year scenarios (1994–2053) to assess crop and fertilizer impacts. Compared to continuous corn with a single fertilizer application, corn-soybean rotation with three split applications (4:3:3 ratio) reduced cumulative NO₃-N flux by 7.6% to 12.6% (mean ~10%) in kg/ha and deep vadose zone concentrations by 39.7% to 51.8% (mean ~45%), reflecting enhanced nitrogen efficiency. NO₃⁻N flushing times varied from 5.9 to 20.2 years, indicating prolonged contamination risks in some zones. These findings underscore the efficacy of integrating crop rotation and precise fertilizer timing to mitigate NO₃-N leaching, emphasizing site-specific strategies to protect groundwater in nitrate-vulnerable regions. Advisors: Aaron R. Mittelstet and Troy E. Gilmore