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

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. © 2020 Society of Chemical Industry.