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TECHNICAL REPORTS
Landscape and Watershed Processes

Uptake and Release of Phosphorus from Overland Flow in a Stream Environment

USDA-ARS, Pasture Systems and Watershed Management Research Unit, Curtin Road, University Park, PA 16802-3702

^* Corresponding author: (richard.mcdowell{at}agresearch.co.nz)

Received for publication April 14, 2002. Phosphorus runoff from agricultural fields has been linked to freshwater eutrophication. However, edge-of-field P losses can be modified by benthic sediments during stream flow by physiochemical processes associated with Al, Fe, and Ca, and by biological assimilation. We investigated fluvial P when exposed to stream-bed sediments (top 3 cm) collected from seven sites representing forested and agricultural areas (pasture and cultivated), in a mixed-land-use watershed. Sediment was placed in a 10-m-long, 0.2-m-wide fluvarium to a 3-cm depth and water was recirculated over the sediment at 2 L s^-1 and 5% slope. When overland flow (4 mg dissolved reactive phosphorus [DRP] and 9 mg total phosphorus [TP] L^-1) from manured soils was first recirculated, P uptake was associated with Al and Fe hydrous oxides for sediments from forested areas (pH 5.2–5.4) and by Ca for sediments from agricultural areas (pH 6.5–7.2). A large increase (up to 200%) in readily available P NH₄Cl fraction was noted. After 24 h, DRP concentration in channel flow was related to sediment solution P concentration at which no net sorption or desorption of P occurs (EPC₀) (r² = 0.77), indicating quasi-equilibrium. When fresh water (approximately 0.005 mg P L^-1; mean base flow DRP at seven sites) was recirculated over the sediments for 24 h, P release kinetics followed an exponential function. Microbial biomass P accounted for 34 to 43% of sediment P uptake from manure-rich overland flow. Although abiotic sediment processes played a dominant role in determining P uptake, biotic process are clearly important and both should be considered along with the location and management of landscape inputs for remedial strategies to be effective.

Abbreviations: DRP, dissolved reactive phosphorus • EPC₀, solution P concentration at which no net sorption or desorption of P occurs • P_max, phosphorus sorption maximum derived from Langmuir isotherm • PP, particulate phosphorus • TP, total phosphorus

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