- Soluble reactive phosphorus (SRP)concentrations measured in runoff fromabandoned agricultural land now in forestsuccession in the northeastern United Stateswere significantly higher than expected fromundisturbed forest land. This finding differsfrom P uptake in hardwood forest successionfollowing natural disturbance. Fieldmonitoring of a 16.6 ha old-field regrowthforest stand in the Catskills Mountains, NewYork, USA demonstrated runoff SRP trendsincluding an early summer flush that could notbe explained by simple dilution. An assay ofoutflow sediment and biomass, flowpath sedimentand biomass, forest floor leaf litter andbiomass, and Bh horizon mineral soil indicatedthat surface litter from the regrowth forestprovided the most significant contribution tothe elevated SRP in runoff. It is posited thatmicrobial mineralization of residual organic Pin surface litter coupled with the transientprocess of SRP mobilization at the soil surfaceresulting from a rising saturated layerfollowed by dissolution in surface runoff mayelevate SRP to the range observed. MeasuredSRP concentrations remain lower than reportedvalues for crop or pastureland. The resultsreported represent an important deviation fromthe prevailing view that forest land does notcontribute to eutrophication (based on enhancedP uptake in forest succession); this is aconsequence of residual P from landabandonment – a widespread practice throughoutthe northeastern US and other regions.