Tracy A. Campbell

To address current groundwater contamination, the impact of nitrogen (N) management practices on corn (Zea mays L.) yield and nitrate (NO3) leaching needs to be evaluated, with a specific emphasis ...

Maintaining yield goals while reducing nitrate‐nitrogen (NO ₃ ‐N) leaching to groundwater is a challenge for potato ( Solanum tuberosum ) production in the Wisconsin Central Sands as well as across the United States. The objectives of this study were to quantify the effect of conventional and enhanced efficiency nitrogen (N) fertilizers on NO ₃ ‐N leaching, crop yield, and N uptake in potatoes. We compared five N treatments, which include a 0 N control and 280 kg ha ⁻¹ as ammonium sulfate and ammonium nitrate (AS/AN), polymer‐coated urea (PCU), urea with a urease inhibitor (Urea+UI), or urea with a UI and a nitrification inhibitor (Urea+UI+NI). The study occurred on grower fields during the 2009, 2010, and 2011 growing seasons, and NO ₃ ‐N leaching was measured with equilibrium tension lysimeters. PCU resulted in a reduction in NO ₃ ‐N leaching and an increase in yield compared to AS/AN in a year with large early‐season rainfall; Urea+UI also reduced NO ₃ ‐N leaching in this year. In 2010, large plot‐to‐plot variation and 250 kg ha ⁻¹ of additional N applied by the grower masked our ability to see differences among fertilized treatments. In 2011, a year with less intense rainfall events, no differences among treatments were observed. Collectively, these results show a potential benefit to PCU, but these benefits are only realized under specific seasonal weather conditions. Overall, the percentage of applied N lost to leaching during the growing season and removed in biomass was relatively low, suggesting substantial amounts of NO ₃ ‐N leaching outside of the growing season.

The Wisconsin Central Sands is home to large scale vegetable production on sandy soils and managed with frequent irrigation, fertigation, and widespread nitrogen fertilizer application, all of which make the region highly susceptible to nitrate loss to groundwater. While the groundwater is used as the primary source of drinking water for many communities and rural residences across the region, it is also used for irrigation. Considering the high levels of nitrate found in the groundwater, it has been proposed that growers more accurately account for the nitrate in their irrigation water as part of nitrogen management plans. Our objectives were to 1) determine the magnitude of nitrate in irrigation water, 2) quantify the spatiotemporal variability of nitrate, and 3) determine key predictors of nitrate concentration in the region. We sampled irrigation water from 38 fields across six farms from 2018 to 2020. Across the 3 years of our study, nitrate concentration varied more across space than time. On average, our samples were tested at 19.0 mg L⁻¹ nitrate‐nitrogen, or nearly two times the U.S. Environmental Protection Agency (EPA) threshold for safe drinking water, equivalent to 48.1 kg ha⁻¹ of applied nitrate‐nitrogen with 25.4 cm (or 10 in.) of irrigation. To better understand the spatiotemporal variability in nitrate levels, week of sampling, year, well depth, well casing, and nitrogen application rate were analyzed for their role as predictor variables. Based on our linear mixed effects model, nitrogen application rate was the greatest predictor of the nitrate concentration of irrigation water (p < 0.05).