Effect of Agricultural Practices On Nitrate Leaching and Nitrogen Fertilizer Recovery in a Semi-Arid Region.

In the Judith River Watershed of central Montana (Northern Great Plains), groundwater nitrate contamination has become a concern for both agricultural producers and watershed stakeholders. Nitrate leaching into groundwater from cultivated lands may contribute to the contamination issue. This study is evaluating the effectiveness of proposed management practices on nitrate leaching and nitrogen fertilizer recovery (harvested N/fertilizer N). Three alternative management practices are being tested on farms within the watershed and compared to the regional grower standard practice (GSP). The GSP, at least of our collaborators, consists of a winter grain – spring grain – fallow rotation fertilized with conventional urea in early spring. The alternative practices, selected in conversation with our Producer Research Advisory Group as having a high likelihood for adoption, are: 1) pea (vs. fallow), 2) split application (vs. single application), and 3) seed-applied controlled release urea (vs. spring broadcast urea). To assess these practices, a nitrogen mass balance at the scale of the rooting zone quantified the amount of nitrate leached for each practice and the GSP. Nitrogen mineralization, volatilization, and denitrification were estimated based on the relevant study results located in the region and through primary literature research.  Soil core samples were taken along each treatment boundary to measure the change in soil nitrate storage over time. Through grain yield sampling and interviews with growers concerning fertilizer application, nitrogen fertilizer recovery was calculated. Preliminary results reveal that peas in place of fallow substantially reduced the amount of nitrate and water in the soil profile, suggesting that this practice will reduce nitrate leaching. The 2013 field season will provide results on the other two proposed management practices, which are hypothesized to depend more on 2013 precipitation amounts and patterns. Through this study we will better understand the effect of management practices on nitrate leaching and nitrogen fertilizer recovery.