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Nitrogen Losses in Surface Runoff from Agricultural Watersheds on Missouri Valley Loess¹

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.

Key Words: fertilizer • sediment • erosion

¹ Contribution from the Agricultural Research Service, USDA, in cooperation with the Nebraska and Iowa Agr. Exp. Sta. Published as Paper No. 3400, Journal Series, Nebraska Agr. Exp. Sta., Lincoln 68503.

² Soil Scientist, USDA, Lincoln, Nebr.; Soil Scientist, USDA, Council Bluffs, Iowa; Hydrologic Engineer, USDA, Columbia, Mo.; and Agricultural Engineer, USDA, Council Bluffs, Iowa, respectively.

Received for publication July 10, 1972.

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