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 ab andoned agricultural land now in forest succession in the northeastern Unit ed States were significantly higher than expected from undisturbed forest l and. This finding differs from P uptake in hardwood forest succession follo wing 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 s imple 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 mo st significant contribution to the elevated SRP in runoff. It is posited th at microbial mineralization of residual organic P in surface litter coupled with the transient process of SRP mobilization at the soil surface resulti ng from a rising saturated layer followed by dissolution in surface runoff may elevate SRP to the range observed. Measured SRP concentrations remain l ower than reported values for crop or pastureland. The results reported rep resent an important deviation from the prevailing view that forest land doe s not contribute to eutrophication (based on enhanced P uptake in forest su ccession); this is a consequence of residual P from land abandonment - a wi despread practice throughout the northeastern US and other regions.