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United States Department of Agriculture

Agricultural Research Service


item Turner, Benjamin
item Kay, Mary
item Westermann, Dale

Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/14/2004
Publication Date: 7/1/2004
Citation: Turner, B.L., Kay, M.A., Westermann, D.T. 2004. Colloidal phosphorus in surface runoff and water extracts from semi-arid soils of the western united states. Journal of Environmental Quality. 33:1464-1472.

Interpretive Summary: Phosphorus is transported in water as soluble and particulate or colloidal forms. Soluble forms are immediately available for water eutrophication processes while the availability from colloidal particles remains poorly understood. Current methodologies to estimate phosphorus in both fractions are not distinct and confound efforts to characterize phosphorus transfer and bioavailability studies from western, calcareous irrigated agricultural land. In this study we found that up to 50% of the apparent soluble phosphorus in surface runoff was associated with colloidal particles. This causes a considerable error in the estimation of free orthophosphate as determined by molybdate colorimetry. It also suggests that phosphorus may be more mobile than expected since these colloidal materials are not readily sorbed to soil and sediment particles, and that our assumptions about the bioavailability of soluble phosphorus in runoff may be misleading when based on current methodologies. Additional studies should consider the role of colloidal phosphorus in the transfer/transport process and in eutrophication.

Technical Abstract: Colloidal particles in runoff may have an important role in P transfer from soils to waterbodies, but remain poorly understood. We investigated colloidal molybdate-reactive P (MRP) in surface runoff and water extracts of calcareous arable soils from the semi-arid western USA. Colloidal MRP was determined by ultrafiltration and operationally defined as MRP associated with particles between 1 um and 1 nm diameter, although a smaller pore-size filter (0.3 nm) was used to define the lower size limit of colloids in water extracts. In surface runoff generated by simulated sprinkler irrigation three calcareous soils, colloidal MRP concentrations ranged between 0.16 and 3.07 um, constituting between 11 and 56% of the MRP in the < 1 um fraction. Concentrations were strongly correlated with agronomic and environmental soil-test P concentrations for individual soils. Water extracts of a range of similar soils contained two size fractions of colloidal MRP: a larger fraction (1.0 ' 0.2 um) probably associated with fine clays, and a finer fraction (3 ' 0.3 nm) probably associated with Ca-phosphate minerals. Colloidal MRP was solubilized in the acidic medium of the colorimetric detection procedure, suggesting that a fraction of the filterable MRP in runoff from calcareous soils may not be as readily bioavailable as orthophosphate in waterbodies. Our results suggest that colloidal MRP is an important but poorly understood component of P transfer in runoff from calcareous western US soils, which should be given greater consideration in mechanistic studies of the P transfer process.

Last Modified: 06/22/2017
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