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Nutrient Runoff Losses as Predicted by Annual and Monthly Soil Sampling¹

ABSTRACT

The transport of soluble and particulate P and N in runoff from several cropped and grassed watersheds in Oklahoma and Texas was related to the nutrient content of surface soil (0–50 mm) sampled monthly and annually (March) over a 2-yr period in an effort to improve the prediction of nutrient transport. For both monthly and annual soil samplings, the prediction of soluble P was good for events > 0.75 mm runoff (r values ranged from 0.57 to 0.92), but smaller events were poorly predicted. The nitrate content of runoff was not closely related to surface soil content (r vlaues ranged from 0.17 to 0.58), precluding its accurate prediction in runoff. Prediction of both particulate P and total N concentrations of individual runoff events was improved using separate equations for grassed and cropped watersheds compared to a general equation. Even so, predicted mean annual flow-weighted concentrations and amounts of soluble P, particulate P, and total N using either monthly or annual soil nutrient data were significantly related to measured values at the 0.01 level and above (r values ranged from 0.57 to 0.97). Results indicate that seasonal variation in soil nutrient content had less effect on the predictive equations than rainfall and management characteristics. Consequently, these characteristics must be accounted for before the transport equations can be reliably used on an individual runoff event basis.

Key Words: eutrophication • nitrate • particulate N • particulate P • sediment • soluble P • watersheds

¹ Contribution of the Dep. of Agronomy, Oklahoma Agric. Exp. Stn., Oklahoma State Univ., Stillwater & Water Quality & Watershed Research Laboratory, USDA-ARS, Durant, OK. Published with the approval of the Director Okla. Agric. Exp. Stn. Journ. Ser. no. 4523.

² Cooperating soil scientist and soil scientist, USDA-ARS, Durant, OK; soil scientist. USDA-ARS, Woodward, OK; and hydraulic engineer, USDA-ARS, Temple, TX. The first author is under cooperative agreement with USDA-ARS and Oklahoma State Univ., Agreement no. 58-7830-8-22, Water Quality & Watershed Research Laboratory, USDA-ARS, Durant, OK 74702-1430.

Received for publication August 29, 1984.

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