Uptake and release of phosphorus from overland flow in a stream environment.
Phosphorus runoff from agricultural fields has been linked to fresh-water 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.
McDowell
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RW
AgResearch Ltd, Invermay Agricultural Research Centre, Private Bag 50034, Mosgiel, New Zealand. richard.mcdowell@agresearch.co.nz
Sharpley
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AN
D000383:Agriculture; D018533:Biomass; D004784:Environmental Monitoring; D019015:Geologic Sediments / Q000737:chemistry* / Q000382:microbiology; D007700:Kinetics; D008372:Manure; D010758:Phosphorus / Q000493:pharmacokinetics*; D014197:Trees; D014871:Water Microbiology; D014872:Water Movements; D014873:Water Pollutants / Q000032:analysis*
D008372:Manure; D014873:Water Pollutants; D010758:Phosphorus