Soil nitrate leaching under grazed cool‐season grass pastures of the North Central US

Abstract

Nitrate leaching from agricultural cropping systems contributes to widespread and devastating eutrophication of water bodies globally. In the North Central USA, this problem is acute, with millions of dollars spent annually in efforts to clean up recreational and drinking water. The frequent soil disturbance and exogenous nitrogen (N) amendments applied in annual cropping systems make them major sources of ground-and surface-water nitrate pollution. Perennial grasslands under managed livestock grazing have been touted for their ability to retain soils and nutrients while simultaneously providing milk and meat to society. The present study provides an evaluation of the peer-reviewed literature addressing nitrate leaching loads beneath corn, pasture and prairie in temperate humid and sub-humid regions of the US, with a focus on cool-season grass pastures. Inputs of exogenous N to these agroecosystems comes from wet and dry deposition, livestock manure from imported feed, biological fixation and inorganic N fertilizer. Nitrate loads were highest beneath corn and lowest beneath restored prairie and switchgrass managed for bioenergy. Cool-season grass pastures had relatively low levels of nitrate leaching loads where little or no N was applied. However, where grazed perennial grasslands had inorganic N applied, nitrate leaching loads rivaled those of corn in some cases. When producing milk and meat from livestock, grazed perennial cool-season grass pastures should reduce nitrate leaching loads compared to growing corn that is used to feed livestock in confinement. However, cool-season grass pastures can lose significant nitrate to leaching with moderate-to high-levels of exogenous N inputs.

Grazed perennial grasslands can match current beef production while contributing to climate mitigation and adaptation

“…Grasslands grazed by livestock can do this if managed well, which is to say (a) pastures are grazed but leaving significant residual biomass so that plants are not constantly drawing on belowground reserves to regrow, (b) pastures are allowed significant periods of rest from defoliation to vigorously regrow, and (c) livestock are restricted to relatively small paddocks for short durations before they are rotated to new paddocks. These principles help producers achieve efficient use of available forage and sufficient resting of pastures from defoliation to maintain productivity, all of which minimizes nutrients from concentrating in certain locations and reduces opportunities for losses of soil, nutrients, and C to the atmosphere and aquatic ecosystems (Jackson et al., 2019; Jackson, 2020). This type of managed grazing also affords land managers opportunities for manipulating grassland structure in ways that can be beneficial to wildlife (Lyons et al., 2000a; Lyons et al., 2000b; Rook & Tallowin, 2003).…”

“…Grass‐finishing beef allows cattle to distribute their excreta in more diffuse and uniform ways across the landscape than manure from feedlot facilities spread in pulses in spring and fall (Dahal et al., 2020; Jackson, 2020), and nutrient cycles are less leaky because little to no fertilizer is needed to maintain productive grasslands if stocking rates are managed for optimum, rather than, maximum yields. Eliminating or minimizing synthetic nitrogen (N) additions is key to improving water quality and reducing the carbon (C) footprint of grazed pastures.…”

Surface‐soil carbon stocks greater under well‐managed grazed pasture than row crops

“…Soil C storage, a key function of soils, can help mitigate climate change and ameliorate its consequences globally (Wiesmeier et al., 2019; but see Amundson, 2022), while providing other environmental and agronomic benefits. Increasing soil organic C (SOC) has the potential to reduce eutrophication of surface and ground waters by reducing leaching, runoff, and erosion from agricultural land (Hussain et al., 2019; Jackson, 2020; Motew et al., 2019; Porter et al., 2015). Benefits from building soil organic matter also accrue directly to farmers, including improved water infiltration, water and nutrient retention, and increased productivity (Paustian et al., 2019), which represent important agricultural climate change adaptations.…”

Perennial grassland agriculture restores critical ecosystem functions in the U.S. Upper Midwest

“…Nitrate leaching was greatly reduced under grass vegetation compared to both continuous corn and corn-soy rotation because no manure or fertilizer was added to grass vegetation, which aligns with empirical field studies. Under most perennial grass bioenergy and grazed systems, nitrate leaching is much lower than corn-based cropping systems (Hussain et al, 2019;Jackson, 2020), differences that can lead to significant disparities in water quality for rural regions, where nitrate leaching contributes to impaired health and infant mortality (Knobeloch et al, 2013). Further, nitrate leaching from common cornbased systems contributes to eutrophication and consequently impaired rivers, lakes, and oceans (Orth et al, 2006;Liu et al, 2022).…”

“…Critical ecosystem functions of agricultural systems include plant and animal productivity, soil carbon storage (Rowntree et al, 2020;Guillaume et al, 2022;Rui et al, 2022), soil stabilization (Montgomery, 2007;Palm et al, 2014;Schulte et al, 2017), nutrient retention (Schulte et al, 2017;Hussain et al, 2019;Jackson, 2020), water infiltration and storage (Basche and DeLonge, 2019;Baker et al, 2022), and wildlife habitat (Kimoto et al, 2012;Tsiafouli et al, 2015;Schulte et al, 2017). While these factors range in their scale of influence (e.g., soil carbon storage influences greenhouse gas concentrations globally while habitat for soil arthropods is quite local), each of the functions have practical relevance for those living in the U.S. Upper Midwest where surface and groundwater pollution, flooding, soil erosion, and plummeting biodiversity undermine human welfare and well being (Werling et al, 2014;Hussain et al, 2019;Antolini et al, 2020;Bendorf et al, 2021;Borchardt et al, 2021;Burch et al, 2021;Raff and Meyer, 2022;Wisconsin Groundwater Coordinating Council Report to the Legislature, 2022).…”