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Seasonal Runoff Losses of Nitrogen and Phosphorus from Missouri Valley Loess Watersheds¹

ABSTRACT

Seasonal losses of nitrogen and phosphorus in surface runoff were determined for a 7-year period from three corn-cropped watersheds in southwestern Iowa. Three seasonal periods were defined: fertilizer, seedbed, and establishment period from April through June (P1); reproduction and maturation period from July through November (P2); and residue period from December through March (P3).

Most of the average annual total N and P losses were associated with the sediment portion of runoff and occurred during P1. The extreme susceptibility of the loess soils to erosion during P1 must be taken into account when designing conservation practices to control plant nutrient losses. Seasonal discharges of runoff, sediment, and nutrients were much lower from a level-terraced watershed than from two contour-farmed watersheds, which demonstrates the benefit of terracing in resource conservation.

Average annual soluble N and P losses were quite low and never exceeded 1% of the annual fertilizer application. These losses were the highest during P3 from the contour-farmed watersheds.

Water and sediment weighted nutrient concentrations were the highest from the contour-farmed watersheds during P3, when residues covered the soil surface. Leaching of soluble nutrients from the residue and the greater selectivity of the soil erosion process for the finer fractions of the soil (i.e., clay and organic material) during snowmelt runoff are probably the factors responsible.

Key Words: water quality • nutrient loss • surface runoff • erosion

¹ Contribution from the North Central Region, USDA-ARS, in cooperation with the Nebraska Agric. Exp. Stn. and the Iowa Agric. & Home Econ. Exp. Stn. Published as Pap. no. 5231, J.Ser., Nebraska Agric. Exp. Stn.

² Soil Scientists, USDA-ARS, Lafayette, IN 47907 (formerly Council Bluffs, Iowa), Cheyenne, WY 82001, and Columbia, MO 65201, respectively.

Received for publication July 15, 1977.

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