Labile and Refractory Forms of Phosphorus in Runoff of the Redwood River Basin, Minnesota

ABSTRACT

Fractionation procedures and phosphorus (P) adsorption-desorption assays were used to delineate labile forms (i.e., subject to transformations which make them available for uptake by biota) and refractory forms (subject to permanent burial) of P in the runoff of the Redwood River basin, an agriculturally-dominated tributary of the Minnesota River. Over several storm periods monitored in 1999, 75% of the P load originating from the watershed was in labile form while only 25% was in refractory form. Labile particulate P forms included P loosely bound to suspended sediment (19%), P bound to iron (11%), and labile particulate organic P (14%). After deposition to receiving waters, the former two forms of labile particulate P can be transformed to soluble forms that are available to biota for uptake via eH and pH reactions and kinetic processes, while the latter form can be mineralized via decomposition processes. Labile soluble P forms included ortho-P (i.e., estimated as soluble reactive P) and soluble organic P (i.e., estimated as soluble unreactive P). Our results indicate that there is a need to identify loading of various labile and refractory P pools for incorporation into watershed models to develop better predictive capabilities for examining P sources and fates, and impacts on the eutrophication of receiving waters, under different management and operational scenarios.