Intra- and inter-annual variability of nitrogen and irrigation management effects on nitrate leaching and maize yield in the Bazile Groundwater Management Area, Nebraska

Increasing groundwater nitrate (NO₃-N) contamination has raised significant environmental and health concerns in irrigated sandy soils of Nebraska. This study evaluated the effects of suboptimum nitrogen (N) and deficit irrigation rates on NO₃-N leaching, crop yield, and economic returns to nitrogen, both with (RTN_Env) and without (RTN) accounting for environmental costs. The two-year on-farm study utilized a two-factor factorial design with three N rates (optimum, suboptimum, and low) and three irrigation rates (farmer's full irrigation [FIT], 80 % of FIT, and 60 % of FIT) in continuous maize grown on irrigated sandy soils in the Bazile Groundwater Management Area, Nebraska. The results indicated that nitrogen rates had a greater impact on seasonal NO₃-N leaching than irrigation rates. Compared to the optimum (270 kg N ha⁻¹), the suboptimum (202 kg N ha⁻¹) and low N rates (135 kg N ha⁻¹) reduced NO₃-N leaching by 24 % (7 kg NO₃-N ha⁻¹) and 51 % (15 kg NO₃-N ha⁻¹), respectively. Maize yield decreased by 8 % (14.5 Mg ha⁻¹) and 11 % (14.0 Mg ha⁻¹), while RTN dropped by $215 ha⁻¹ and $298 ha⁻¹, respectively. Notably, reduced N rates did not affect RTN_Env. The 80 % FIT treatment produced significantly higher grain yield and RTN but did not affect NO₃-N leaching compared to both the 60 % FIT and FIT treatments. Additionally, inter-annual variability had a more pronounced effect on nitrate leaching than the treatments themselves. In 2021, NO₃-N leaching was 64 % lower due to 38 % less irrigation, 37 % higher grain N uptake, and 74 % lower residual N—resulting in $214 more in RTN and $587 more in RTN_Env compared to 2022. In conclusion, findings suggest that reducing nitrogen rates is more effective in minimizing nitrate leaching than within-season irrigation reductions under current sprinkler irrigation practices. Additionally, inter-annual variability in NO₃-N leaching should be considered when developing strategies to improve nitrogen management in groundwater contaminated areas.