The Importance of Equilibrium Processes in Regulating Soluble Phosphorus Runoff and Loading in Tributary Systems.

Excess tributary phosphorus (P) loading to lakes can accelerate eutrophication and drive severe cyanobacterial blooms in aquatic systems. Soil management and fertilizer amendment in excess of crop uptake can indirectly regulate soluble P runoff via equilibrium or buffering reactions (adsorption-desorption) between exchangeable particulate P and soluble P. While particulate P erosion and runoff can be controlled, soluble P loading is much more difficult to mitigate due to equilibrium reactions. This research examined soil and suspended sediment exchangeable P pools, equilibrium P concentration (EPC, mg/L), and runoff in a small WI watershed to better understand drivers of soluble P loading. Langmuir-type isotherm assays were conducted using soils collected from ~ 40 parcels that varied from less managed woods and CRP to row crop and livestock production fields. The EPC increased, while k (P buffering coefficient, L/kg) decreased, as a function of increasing crop-available P in soil. This pattern was related to declining P sorption capacity as soils became increasingly saturated with P due to fertilization, Soils associated with crop and livestock production in the lower watershed exhibited the highest EPC while less managed soils in the upper watershed had the lowest EPC. Tributary flow-weighted summer soluble P reflected spatial differences in soil EPC, Mean tributary soluble P from the upper watershed was only 0.08 mg/L but increased to 0.30 mg/L in the lower watershed in conjunction with localized runoff from managed fields containing P saturated soils with higher EPC and poor buffering capacity. Soil P equilibrium had a regulating influence on soluble P loading and needs to be considered within the context of the land use mosaic and hydrological susceptibility to runoff.