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Losses of Nitrogen in Surface Runoff in the Blackland Prairie of Texas¹

ABSTRACT

Our objective in this study was to determine NO₃^–-N and total N losses in surface runoff from Houston Black clay, a swelling clay soil with a relatively low infiltration rate. The study was carried out on duplicate 4-ha watersheds cropped to a rotation of grain sorghum (Sorghum bicolor [L.]), cotton (Gossypium hirsutum [L.]), and oats (Avena sativa [L.]), all fertilized at recommended rates of N application. The loss of NO₃^–-N varied considerably during the study, depending on events before each runoff-producing storm. Concentrations of NO₃^–-N were usually highest just after fertilizer application when the soil was near field capacity and lowest when large amounts of water infiltrated into dry soil immediately before runoff. During runoff-producing storms just after fertilizer application, the concentrations were lowest in the initial runoff and highest near the end of the runoff event. To compute NO₃^–-N losses with reasonable accuracy on these soils, the shape of the entire NO₃^–-N concentration curve needed to be well defined.

In general, the results of this study indicate that a small and probably insignificant amount of N is lost to surface waters when crops are fertilized at recommended N rates in the Texas Blackland Prairie. For the entire 5-year study, the mean concentration of NO₃^–-N in runoff was 2.9 and 2.3 ppm NO₃^–-N for the duplicate watersheds. The mean total loss of NO₃^–-N was 3.2 kg ha^–1 year^–1. Losses of sediment associated N were about 5 kg N ha^–1 year^–1.

Key Words: pollution • nitrate • watershed

¹ Contribution from Tex. Agric. Exp. Stn., Texas A&M Univ. Syst., in cooperation with the Southern Reg., Agric. Res. Serv., U. S. Dep. of Agric.

² Associate Professor, Tex. Agric. Exp. Stn., Agricultural Engineer and Soil Scientist, Southern Reg., ARS, USDA, Temple, TX 76501, respectively.

Received for publication September 10, 1975.

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