Evaluating Sediment Phosphorus Exchange in Rural Ontario Headwaters by Paired Sequential Extraction and Sorption Isotherms

Release of dissolved legacy phosphorus (P) from stream sediments is an unquantified vector of non-point nutrient loading across rural Southern Ontario watersheds. Sorption equilibrium isotherms and sequential extraction methods of operationally defined P phases are commonly used to measure the buffering capacity and P-saturation of sediments collected from catchments of concern. However, these methods are commonly performed independently; thus, insights into P exchange in, out of, and between sediment fractions as a result of batch incubations are overlooked. Here, by performing sequential P extractions on sediments both before and after a batch sorption isotherm experiment, the most vulnerable fractions of P were identified from three sites of differing agricultural intensity (inorganic fertilizer-based, organic fertilizer-based, and a natural reference site). The fertilized stream segment sediments had higher concentrations of water column total P (TP), soluble reactive P (SRP), and total sediment P, and exhibited reduced P-buffering capacity compared to a natural forested segment as characterized by linear, Freundlich, and initial mass isotherms. Sequential P extractions pre and post batch incubation revealed that labile and reducible Fe–P were highly variable and served as both sinks and sources of P depending on site and initial isotherm SRP concentration. In addition, high ratios of TDP:SRP were measured through extractions of labile P, suggesting that large reservoirs of condensed organic/inorganic dissolved P can become immobilized across sites. Organic P decreased after nearly all incubation parameters, suggesting that this reservoir of P in bed sediments may be more susceptible to desorption than previously identified.