Fate and consequence of nutrients at an abandoned feedlot, Glacial Ridge National Wildlife Refuge, Minnesota, USA

Old, abandoned feedlots may serve as a source of nutrients that can degrade groundwater and downstream water quality. We characterized the distribution and concentration of nutrients at the Crookston Cattle Company feedlot (northwest Minnesota, USA), 15 years after it ended operations in 1999. Groundwater nitrate concentration decreased from 55 mg/L (as nitrogen) in 2003 to less than 5 mg/L since 2007. Results from stable isotope analysis, with delta N-15 and delta O-18 in groundwater nitrate ranging up to +44 and +30%, respectively, suggest denitrification as the cause, rather than either nitrate transport from the site or dilution. Phosphorus, with soil B-horizon concentrations as much as 112 and averaging 24 mg/kg, is sequestered by carbonate-rich glacial sediments and, serendipitously, an iron-rich sand deposit formed millennia ago by wave action along the shore of glacial Lake Agassiz. Map analysis indicates roughly 20,000 kg of P in excess of background concentration remains in soil at the 15 ha site. Evidence suggests that the former feedlot has not affected water quality significantly in an agricultural ditch that drains the feedlot and its vicinity. Rather than originating from the feedlot, small increases of total phosphorus observed in the downstream ditch likely result from release of phosphorus from nearby recently restored wetlands. More consequential than elevated nutrient concentrations to the future reclamation of this and similar sites is the persistence of robust non-native species. Our results suggest that before development, feedlot sites should be evaluated for their phosphorus sequestration and denitrification potential, thus mitigating the potential for later off-site transport of nutrients.