Effect of variable annual precipitation and nutrient input on nitrogen and phosphorus transport from two Midwestern agricultural watersheds

Precipitation patterns and nutrient inputs affect transport of nitrate (NO₃-N) and phosphorus (TP) from Midwest watersheds. Nutrient concentrations and yields from two subsurface-drained watersheds, the Little Cobb River (LCR) in southern Minnesota and the South Fork Iowa River (SFIR) in northern Iowa, were evaluated during 1996–2007 to document relative differences in timings and amounts of nutrients transported. Both watersheds are located in the prairie pothole region, but the SFIR exhibits a longer growing season and more livestock production. The SFIR yielded significantly more NO₃-N than the LCR watershed (31.2 versus 21.3 kg NO₃-N ha^− 1 y^− 1). The SFIR watershed also yielded more TP than the LCR watershed (1.13 versus 0.51 kg TP ha^− 1 yr^− 1), despite greater TP concentrations in the LCR. About 65% of NO₃-N and 50% of TP loads were transported during April–June, and