Residual phosphorus in runoff from successional forest on abandoned agricultural land: 1. Biogeochemical and hydrological processes

Soluble reactive phosphorus (SRP) concentrations measured in runoff from abandoned agricultural land now in forest succession in the northeastern United States were significantly higher than expected from undisturbed forest land. This finding differs from P uptake in hardwood forest succession following natural disturbance. Field monitoring of a 16.6 ha old-field regrowth forest stand in the Catskills Mountains, New York, USA demonstrated runoff SRP trends including an early summer flush that could not be explained by simple dilution. An assay of outflow sediment and biomass, flowpath sediment and biomass, forest floor leaf litter and biomass, and Bh horizon mineral soil indicated that surface litter from the regrowth forest provided the most significant contribution to the elevated SRP in runoff. It is posited that microbial mineralization of residual organic P in surface litter coupled with the transient process of SRP mobilization at the soil surface resulting from a rising saturated layer followed by dissolution in surface runoff may elevate SRP to the range observed. Measured SRP concentrations remain lower than reported values for crop or pastureland. The results reported represent an important deviation from the prevailing view that forest land does not contribute to eutrophication (based on enhanced P uptake in forest succession); this is a consequence of residual P from land abandonment - a widespread practice throughout the northeastern US and other regions.