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Fertilizer Nutrient Losses from Rangeland Watersheds in Central Oklahoma¹

ABSTRACT

Four native grassland watersheds were monitored for nitrogen and phosphorus nutrient losses in surface runoff. The watersheds, paired in surface hydrology and grazing management, were 8 to 11 ha in area with 3% slopes. One watershed of each pair was fertilized with 85 kg N/ha (as NH₄⁺-N) and 75 kg P/ha surface broadcast. Ten 3.66 by 5.49 m (20 m²) areas within each watershed were covered with plastic sheets during fertilization to provide unfertilized check plots. Fertilizer losses in surface runoff over the first year were 5% or less of the amounts applied. Soluble NH₄⁺-N concentrations in surface runoff increased significantly only during the first month after fertilizer application. Soluble P concentrations increased sharply after fertilization and remained relatively high even after 12 mo. Forage yields were increased 50 to 100% on fertilized areas. Increases in nutrient uptake of fertilized vegetation the first year equaled 17 to 25.5 kg/ha and 7.5 to 11 kg/ha of the added N and P, respectively.

Key Words: nitrogen • phosphorus • grazing management • water quality • forage yields • erosion • bluestem

¹ Contribution from the USDA-SEA-AR Southern Plains Watershed and Water Quality Lab., Durant, OK 74701, in cooperation with the Oklahoma Agric. Exp. Stn., Stillwater, OK 74074.

² Soil Scientist, USDA-SEA-AR, Morris, Minn., formerly Durant, Okla.; Agricultural Engineer, Soil Scientist, and Director, USDA-SEA-AR, Durant, Okla., respectively.

Received for publication January 30, 1979.

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