Fallow replacement and alternative fertilizer practices : effects on nitrate leaching, grain yield and protein, and net revenue in a semiarid region
John, Andrew Augustus
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High nitrate concentrations in groundwater have been observed in agricultural regions worldwide. In the Judith River Watershed of central Montana, groundwater nitrate concentrations have increased from 10 to 23 mg L-1 over the span of 20 years. Nitrate leaching from agricultural fields is a major concern for growers and stakeholders in the region. Little research has been conducted in dryland semiarid regions on the effects of agricultural practices on nitrate leaching. We conducted a 2-yr study comparing three alternative management practices (pea rotation, controlled-release urea, split nitrogen application) to grower standard practices (fallow, conventional urea, spring broadcast urea) on grain yield, grain protein, net revenue, and the amount of nitrate leached. Eight field treatment interfaces were established across three farms and each treatment was in duplicate per year. Ten soil and biomass sampling locations were designated on both sides of the interface. Net revenue was calculated by enterprise budgets constructed from local and state data. Nitrate leaching was calculated using a nitrogen mass balance equation. Replacing pea with fallow decreased winter wheat grain yield and protein yet had no effect on net revenue during the first year of the study (2013). In the second year, pea-winter wheat earned $83 ha -1 more (P<0.1) than fallow-winter wheat. Neither fertilizer alternative management practice had an effect on net revenue. In the 2013 treatment year, wheat after pea leached less nitrate (20 kg N ha -1) than wheat after fallow (56 kg N ha -1), indicating more deep percolation of nitrate with fallow practice. In the 2014 treatment year, a greater amount of nitrate leached (P<0.1) while using controlled-release urea than conventional urea, possibly in part because the controlled release urea was applied earlier than conventional urea. The results of our study revealed that replacing fallow with pea can decrease the amount of nitrate that leaches out of the root zone. Also, this practice either increased or had no effect on net revenue, revealing its ability to be economically feasible for a grower to implement. Based on our findings, future research should likely focus on practices that decrease rates of deep percolation.