STREAM NUTRIENT RETENTION IN THREE NORTHEASTERN OKLAHOMA AGRICULTURAL CATCHMENTS

Stream nutrient retention was examined in three adjacent agricultural catchments (Cherokee Creek, Cloud Creek, and Dry Creek) in the Ozark Plateau. Retention efficiency was measured using short–term nutrient and tracer injections to estimate nutrient uptake length (Sw ) during summer 1999 and winter 2000. A one–dimensional transport model was used to estimate dispersion, transient storage size, and exchange. Soluble reactive P (SRP) and NO3 –N concentrations were least in the stream with the lowest proportion of pasture in the upland (Dry Creek), whereas concentrations and land use were similar in Cherokee Creek and Cloud Creek. Water column SRP concentrations were similar between seasons in all streams, but NO3 –N concentrations varied significantly. Injected NO3 –N was not significantly retained in these systems, probably because the streams were saturated by ambient NO3 –N concentrations (greater than 0.1 mg L –1 ). Phosphorus was retained during summer injections (Sw ranged from 200–900 m), but Sw regressions were not significant in winter. Variation in catchment land use was not a major determinant in P retention during summer, but stream hydrology, such as discharge and transient storage, was a regulating factor. Therefore, land use changes that alter stream hydrology may have a greater impact on P retention in these streams.