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.

Automated summary

The release of dissolved legacy phosphorus from stream sediments is an unquantified vector of non-point nutrient loading in rural Southern Ontario watersheds. This study identified the most vulnerable fractions of phosphorus in sediments from sites of differing agricultural intensity, and revealed that labile and reducible Fe-P can serve as both sinks and sources of phosphorus depending on site conditions. Additionally, high ratios of TDP:SRP indicate that large reservoirs of dissolved P can become immobilized across sites.