Livestock grazing in pastures has been identified as a possible factor controlling sediment and phosphorus (P) delivery to Rathbun Lake and associated water bodies in the watershed. The risk of P loss from soils in this watershed was estimated by Mehlich-3 (M3) extractable P, P sorption indices (S_max and k), degree of P saturation (DPS) and P desorption indices (equilibrium phosphorus concentration (EPC) and phosphorus equilibrium buffering capacity (PEBC)) for seven representative soil samples. Most of selected soils had low risk of P loss as indicated by P_M3 and DPS. However, EPC values indicated that some soils could potentially behave as sources by releasing P to the water once they become suspended sediments. Selected soil physicochemical properties, including pH, particle size, total C, total N, total P, Fe_M3, Al_M3, Ca_M3, Mg_M3, oxalate-Fe and Al (Fe_ox and Al_ox), dithionite-Fe and Al (Fe_d and Al_d), were correlated with k, EPC, and PEBC to better understand P sorption-desorption of selected soils. We found that k was positively correlated with Fe_d (r² = 0.96, p < 0.001). PEBC was not correlated with any selected soil physicochemical properties. EPC was positively correlated to Fe_M3 (r² = 0.72, p < 0.05) and Fe_ox (r² = 0.62, p < 0.05) suggesting that P desorption was controlled by Fe.

Stream sediments often play an important role in regulating phosphorus (P) concentrations in stream water. Equilibrium P concentration (EPC) is the concentration of P in the solution phase (stream water), at which P is neither sorbed nor desorbed by the solid phase (sediments). In this study, EPC was used as a desorption index to determine whether sediments behave as sinks or sources of P to the stream water in the Rathbun Lake watershed. Eight stream sediments (banks and beds) were collected and EPC determined. Sediment physicochemical properties were correlated with EPC to better understand P desorption of sediments. The EPC of the sediments ranged from 0.02 to 0.12 mg L^-1. After comparing EPC of sediments with mean dissolved P concentration in stream (0.08 mg L^-1), we found that some sediments could behave as a source of P to water once they became suspended. EPC of sediment increased with sediment pH (r = 0.92, p < 0.01) and sand content (r = 0.78, p < 0.05), but decreased with Mehlich-3 extractable Fe (Fe_M3) (r = -0.93, p < 0.001). Mehlich-3 P and the degree of P saturation (DPS) predicted a low risk of P loss from sediment. For the Rathbun Lake watershed, these results suggest P desorption increases with sediment pH and sand content while P sorption increases with sediment Fe_M3 content.