Biologically Labile and Refractory Phosphorus Loads from the Agriculturally-Managed Upper Eau Galle River Watershed, Wisconsin

Abstract Fractionation techniques were used to quantify various biologically labile (i.e., directly available for biological uptake or subject to recycling pathways) and refractory (i.e., biologically unavailable and subject to burial) particulate and soluble phosphorus (P) forms along the longitudinal axis of the agriculturally-managed Upper Eau Galle River watershed in west-central Wisconsin. P loading increased as a function of increasing distance from the river's headwaters. However, areal P export rates were similar longitudinally, indicating a relatively homogeneous land-use mosaic throughout the watershed. P loads were composed of predominantly biologically labile constituents (i.e., 79%), with soluble P forms (i.e., soluble reactive and unreactive P) accounting for 49% and labile particulate P forms (i.e., loosely-bound PP, iron-bound PP, and labile organic/polyphosphate PP) accounting for 30% of the P load. Soluble P forms are either directly available for biological uptake or can be converted to...