Spatio-Temporal Modeling of Nitrate Leaching from Agricultural Lands Under Varying Tillage Practices and Fertilizer Regimes in the Mississippi River Basin

Agricultural nonpoint source nitrogen pollution remains the primary driver of recurring seasonal hypoxia in the Northern Gulf of Mexico, with the Mississippi River Basin (MRB) contributing the vast majority of riverine nitrogen loads. This study developed a high-resolution, spatio-temporally explicit agro-hydrological modeling framework combining the Soil and Water Assessment Tool (SWAT) with field-calibrated biogeochemical algorithms to evaluate nitrate-nitrogen (NO3-N) leaching dynamics across distinct sub-basins of the MRB from 2000 to 2022. We simulated combinations of three tillage management practices (conventional tillage, reduced tillage, and no-till) across four nitrogen fertilizer regimes (baseline synthetic rates, 20% rate reduction, split-application timing, and enhanced-efficiency fertilizers). Model predictions exhibited robust performance against multi-gauge streamflow and groundwater nitrate observation networks, achieving Nash-Sutcliffe efficiencies greater than 0.76. Results demonstrated that the combination of split-fertilizer application and no-till management reduced annual root-zone NO3-N leaching by 34.2% across the Midwestern Corn Belt sub-basins without compromising baseline crop yields. Spatio-temporal analysis revealed critical seasonal vulnerability windows during spring recharge events (March–May), which accounted for over 58% of cumulative annual leaching. These findings underscore that uniform regional management policies are insufficient and that targeted, climate-adaptive conservation practices synchronized with regional hydrological recharge regimes are imperative for achieving downstream water quality objectives.