Research papersVadose zone processes delay groundwater nitrate reduction response to BMP implementation as observed in paired cultivated vs. uncultivated potato rotation fields
Introduction
Nitrate is one of the most frequently-detected contaminants in groundwater in agricultural areas (Grizzetti et al., 2011, Howden et al., 2011, Puckett et al., 2011). Exposure to water with a high nitrate concentration can have negative health implications (de la Monte et al., 2009). Nitrate-enriched groundwater discharges to receiving surface waters, contributing to eutrophication (Mitsch et al., 1999, National Research Council (NRC), 2000, Bowen et al., 2007). In many cases, elevated groundwater nitrate levels are correlated with greater farming intensity and excessive nitrate leaching from the local crop production systems (Benson et al., 2006, Exner et al., 2010, Wallis et al., 2011, Jiang et al., 2012). Nitrate is normally conservative in the aerobic vadose zone and can migrate downward with excessive moisture to the water table via porous matrix (or uniform) flow (Baran et al., 2007, Rudolph et al., 2015) or preferential flow (Harter et al., 2005) or both of these two pathways (McMahon et al., 2006); it can also travel to surface water via interflow, depending on the lithology and topography. A significant amount of nitrate originating from previous farming activities can accumulate in the thick vadose zone underlying the crop root zone (McMahon et al., 2006, van Meter et al., 2016). Nitrate is generally mobile in groundwater, but can be lost via denitrification under anaerobic conditions, diffusion from fractures into the porous matrix of fractured bedrock, plant uptake in the riparian zone and groundwater extractions. Any remaining nitrate mass will be transported to different flow paths, and will eventually be discharged into receiving surface waters (Bachman et al., 1998, Bowen et al., 2007) and/or impact private or municipal water supply wells (Jiang and Somers, 2009).
Nitrogen (N) is the nutrient that is required in the largest quantities for crop growth and almost all non-leguminous crops need N inputs from mineral or organic fertilizer or through biological N fixation to enhance production (Foth, 1984). Lowering fertilizer N input can not only reduce nitrate leaching but also compromise crop yield if optimal crop N fertilization is not ensured (Errebhi et al., 1998, Zebarth et al., 2012). Thus, developing and implementing beneficial management practices (BMPs) that can balance the economic and environmental objectives of crop production has been promoted as a key strategy for mitigating groundwater nitrate contamination in intensive crop production areas (Zebarth and Rosen, 2007, Rudolph et al., 2015, Woli et al., 2016). While various levels of reduction in nitrate leaching achieved by optimizing management practices have been documented through soil, lysimeter and tile-drain sampling at a plot/field scale (e.g., Vos and Vanderputten, 2004, Gheysari et al., 2009, Wilson et al., 2010, Bero et al., 2014), there are few studies that document the improvements in groundwater quality resulting from BMPs, despite the fact that agriculture-related nitrate contamination of groundwater is a problem that has been recognized for decades. Several authors (Wassenaar et al., 2006, Gallagher and Gergel, 2014, Zebarth et al., 2015b) have reported that long-term monitoring in the Abbotsford-Sumas Aquifer between British Columbia, Canada, and Washington State, USA failed to detect any groundwater quality improvements after BMPs have been implemented for decades. However, no consensus has been reached on the processes controlling the lack of a positive response in groundwater quality, a situation that highlights the challenges of linking BMPs to groundwater quality. The difference in specialization between agricultural sciences and hydrogeology and the requirements related to long term monitoring and management of large areas of land are some of the key barriers to establishing these links (Meals et al., 2010, Hansen et al., 2011, Rudolph et al., 2015).
Nitrate contamination of groundwater associated with potato growing is of particular interest because potatoes play a significant role in global food and nutrition security, and intensive potato production has commonly been linked to groundwater nitrate hotspots (Levallois et al., 1998, Peralta and Stockle, 2001, Wilson et al., 2010, Zebarth et al., 2015a). Potatoes are grown in over 100 countries because of their high nutritional value, ability to adapt to a wide range of soil and weather conditions and ease of cultivation. Potato is the fourth most important crop after rice, wheat, and maize in terms of global food security (Defauw et al., 2012). World potato production is expected to increase by about 30% from 2015 to 2050 as global population and food needs grow (Porter and Faulkner, 2015). Generally, potatoes receive high fertilizer N inputs in order to meet industry tuber yield, quality and size requirements. Potato plants have a shallow root system with 85% of root length within the upper 0.3 m soil layer (Opena and Porter, 1999) and their apparent N recovery (ANR = whole plant N accumulation minus soil N divided by fertilizer N input) is as low as 40–60% (Zebarth and Rosen, 2007, Vos, 2009). As a result, potato production systems are prone to high nitrate leaching and therefore pose a high risk of groundwater contamination (Shrestha et al., 2010, Jiang et al., 2011, Bero et al., 2014). Prince Edward Island (PEI) is the smallest province (5750 km2) in Canada, yet it is the largest producer of potatoes in the country, accounting for 25% of Canadian potato crops (Statistics Canada, 2012). Intensive potato production (mainly rain-fed) has been carried out on sandy soils underlain by a fractured sandstone aquifer, which provides all of the community’s drinking water supplies (Jiang et al., 2012). High nitrate leaching from these production systems has been linked to the contamination of drinking water in private and municipal supply wells. This is evidenced by the fact that about 15–20% of the wells in intensively farmed areas have nitrate levels exceeding the Canadian drinking water guideline (10 mg N/L) (Jiang and Somers, 2009). Nitrate-enriched groundwater discharges to local streams and associated estuaries, leading to reoccurring anoxic events in many estuaries (Bugden et al., 2014, Jiang et al., 2015b). PEI represents a microcosm of the challenges affecting intensive crop production and water quality protection. Improving knowledge of the links between intensive potato production and water quality will help experts conceptualize flow and nitrate transport processes within the soil-vadose zone-bedrock aquifer continuum and support BMP development and water quality protection in PEI, and provide new evidence related to nitrate fate and transport in similar subsurface environments of agricultural landscapes worldwide.
A study was conducted in a 7-ha potato-barley-forages rotation (since 1990) field in the eastern part of Prince Edward Island (PEI), Canada from 2011 to 2016 (Fig. 1) to establish the links between potato rotations and groundwater quality. The specific objectives were to determine the nitrate sources resulting from potato production, gain a better understanding of the controls of soil, weather, geology and management practices on nitrate fate and transport in the subsurface and obtain information for BMP development. In this study, field treatments included removing one field zone (D) from production in 2011 and continuing the standard potato rotation in the remaining zones (A, B and C). The treatment that involved removing potato cropping land out of production represented an extreme scenario of nitrate reduction as a BMP and was intended to permit assessment of the controls of soil, weather, and geology on nitrate fate and transport in the subsurface without complicating the field condition with field management practices.
Section snippets
Study site and field treatments
PEI is located on the eastern coast of Canada (Fig. 1). The climate is characterized by January and July mean temperatures of −7 and 18.7 °C respectively, and annual mean precipitation of 1100 mm (25% as snow). The precipitation is fairly uniformly distributed throughout the year (Jiang et al., 2012). The frost-free period varies between 100 and 160 days. The Island is entirely underlain by a terrestrial sandstone formation with a thickness of 1200–1600 m, (which includes a sequence of “red
Physical hydrogeology
Geologic logging during drilling indicated that the field is underlain by a layer of 7.6 to 8.7 m of glacial till (including soil) which overlies an unconfined fractured sandstone aquifer, which is consistent with the regional geology described in Section 2.1. Borehole geophysical profiling mapped out at least 11 fracture zones in each of P1D and P3D, and coring in P2I and P3I also mapped out 4 to 6 fracture zones in the shallow portion of the aquifer (Fig. 2). Hydraulic head measurements in
Conclusions
Several important conclusions can be drawn from this joint agronomy and hydrogeology study.
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A significant amount of nitrate was leached from below the soil profile after potato plant kill at the study site. The nitrate leaching concentrations exceeded the Canadian drinking water guideline (10 mg N/L), and the related nitrate concentrations in shallow groundwater in the bedrock aquifer could also exceed the guideline. Nitrate leaching concentrations during the rotation crop years were higher than
Acknowledgments
This work was funded by Agriculture and Agri-Food Canada (AAFC) under several projects: Project 252 “Evaluation of Effects of Nitrate Reduction BMPs on Groundwater Quality in Prince Edward Island” and Project 132 “Development and Transfer of a Forage Plowing BMP for Improving Water Quality and Economic Resiliency of Potato Production” led by Dr. Yefang Jiang, and AAFC project “Watershed Evaluation of Beneficial Management Practices (WEBs)”. It also received assistance under the Canadian Water
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