Nitrogen Losses in Surface Runoff from Agricultural Watersheds on Missouri Valley Loess

Abstract

Abstract Nitrogen losses from surface runoff from four field‐size (30 to 60.8 ha) watersheds in southwestern Iowa, near Treynor, were measured during 1969, 1970, and 1971. A contour‐planted corn watershed and a pasture watershed were fertilized at the recommended N rate (168 kg/ha). A level‐terraced and a contour‐planted corn watershed were fertilized at 2.5 times this rate. The conservation practice of level‐terraced corn or pasture was very effective in reducing water, sediment, and N yields when compared with the contour‐planted corn watersheds. Annual water‐soluble N losses were low from all watersheds. The 3‐year average annual solution N loss from the contour‐planted corn watershed, fertilized at 2.5 times the recommended rate, was 3.05 kg/ha; the comparable watershed, fertilized at the recommended rate, lost only 1.89 kg/ha. Nitrogen losses associated with sediment in the runoff accounted for 92% of the total loss for the 3‐year period from the contour planted corn watersheds. A large portion of the N loss for the terraced watershed was also associated with the sediment; however, N loss was only one‐tenth that of the contour‐planted watersheds. Sediment‐N concentrations were similar for watersheds receiving 168 kg/ha and 448 kg/ha annual N applications. Water‐soluble‐N and sediment‐N losses in runoff were usually highest at the beginning of the cropping season and decreased progressively throughout the year, reflecting a seasonal effect believed to be associated with nutrient removal by the crop, leaching, and N tie‐up in organic matter.