Residual phosphorus in runoff from successional forest on abandoned agricultural land: 2. Hydrological and soluble reactive P budgets

Residual P from historical farm practices has been linked to elevated soluble reactive phosphorus (SRP) transport in runoff from a field study site in the Catskills Mountains, New York, U.S.A., with a P source assay indicating that successional forest floor biomass was the major contributor to runoff SRP. In this paper, we assemble hydrological and SRP budgets that indicate net SRP loss of 0.123 kg ha^-1 yr^-1 occurs from the site (composed of 0.044 kg ha^-1 yr^-1 precipitation input, with 0.143 kg ha^-1 yr^-1and 0.024 kg ha^-1 yr^-1 losses in runoff and groundwater, respectively). These findings contrast with conservative P cycling reported for undisturbed forests. Coupled hydrological and SRP data are analyzed suggesting that catchment ambient and equilibrium SRP concentrations corresponding to groundwater and long-term average runoff concentrations are in the range capable of contributing to eutrophication of receiving waters. A physically based variable source area hydrological model is tested to simulate SRP export using deterministic concentrations. The three-layer model (surface runoff, shallow lateral flow, and groundwater) is parameterized using spatially distributed data from additional P source assays and field hydrological monitoring for the site. Differences in simulated and observed outflow and SRP export are partially explained by forest evapotranspiration and frozen soil processes. The field data, SRP budgets and simulations show that sufficient residual P pools exist to prolong net SRP loss rates until ecosystem processes re-establish more conservative P cycling.