Abstract
Nitrate (NO3−) leaching into groundwater is a growing global concern for health, environmental, and economic reasons, yet little is known about the effects of agricultural management practices on the magnitude of leaching, especially in dryland semiarid regions. Groundwater nitrate–nitrogen (nitrate–N) concentrations above the drinking water standard of 10 mg L−1 are common in the Judith River Watershed (JRW) of semiarid central Montana. A 2-year study conducted on commercial farms in the JRW compared nitrate leaching rates across three alternative management practices (AMP: pea, controlled release urea, split application of N) and three grower standard practices (GSP: summer fallow, conventional urea, single application of urea). Crop biomass and soil were collected at ten sampling locations on each side of a management interface separating each AMP from its corresponding GSP. A nitrogen (N) mass balance approach was used to estimate the amount of nitrate leached annually. In 2013, less nitrate leached the year after the pea AMP (18 ± 2.5 kg N ha−1) than the year after the fallow GSP (54 ± 3.6 kg N ha−1), whereas the two AMP fertilizer treatments had no effect on nitrate leaching compared to GSPs. In 2014, leaching rates did not differ between each AMP and its corresponding GSP. The results suggest that replacing fallow with pea has the greatest potential to reduce nitrate leaching. Future leaching research should likely focus on practices that decrease deep percolation, such as fallow replacement with annual or perennial crops, more than on N fertilizer practices.
Highlights
Groundwater nitrate contamination due to leaching is an extensive and growing concern, especially in agricultural regions with shallow groundwater (Power and Schepers 1989; Dubrovsky et al 2010; Puckett et al 2011). Puckett et al (2011) showed that average nitrate–N concentrations in 424 shallow unconfined groundwater wells across the United States increased from \2 mg L-1 in the early 1940s to 15 mg L-1 by 2003
This study provides empirical evidence that management practices that decrease both deep percolation and soil nitrate levels, such as fallow replacement, can increase the likelihood of reducing nitrate leaching, whereas fertilizer timing and source management practices may be less effective because they do not substantially decrease deep percolation
Fallow is still common in parts of the northern Great Plains (NGP), in part because replacing fallow is perceived to reduce economic returns, despite evidence that pea-wheat often produces higher net revenue than fallow-wheat (Zentner et al 2001: Miller et al 2015), consistent with our findings
Summary
Groundwater nitrate contamination due to leaching is an extensive and growing concern, especially in agricultural regions with shallow groundwater (Power and Schepers 1989; Dubrovsky et al 2010; Puckett et al 2011). Puckett et al (2011) showed that average nitrate–N concentrations in 424 shallow unconfined groundwater wells across the United States increased from \2 mg L-1 in the early 1940s to 15 mg L-1 by 2003. Groundwater nitrate contamination due to leaching is an extensive and growing concern, especially in agricultural regions with shallow groundwater (Power and Schepers 1989; Dubrovsky et al 2010; Puckett et al 2011). Nitrate–N concentrations above the Environmental Protection Agency’s (EPA’s) primary drinking water standard of 10 mg L-1 have been linked to adverse health effects such as methemoglobinemia (Knobeloch et al 2000; Fewtrell 2004) and cancer (De Roos et al 2003; Ward et al 2005). In the 720,000-ha semiarid Judith River Watershed (JRW) of central Montana, a large agricultural production region in the northern Great Plains (NGP), nitrate–N concentrations in shallow wells are commonly above the EPA drinking water standard. Concentrations in the only long-term monitoring well in the watershed approximately doubled since 1994, increasing from ca. 10 to ca. 20 mg L-1 in two decades (Schmidt and Mulder 2010; Miller 2013)
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