Thirty years of change in forest soils of the Allegheny Plateau, Pennsylvania

Numerous studies have investigated the potential depletion of available base cation pools from forest soils in regions impacted by acid deposition. However, these studies mostly used indirect methods. Retrospective studies, providing direct evidence of chemical changes in forest soils, are relatively rare due to a lack of appropriate sampling, documentation, and archiving of samples over decadal or longer periods. We were provided an unusual opportunity to conduct such a retrospective study with relocation of four sites on the Allegheny Plateau in Pennsylvania. Detailed soil sampling and analyses were conducted in 1967. An original investigator was available to insure field sampling protocol consistency during resampling in 1997, and the original samples had been archived and were available for reanalysis. At all four sites there were significant decreases in exchangeable Ca and Mg concentrations and pH at all depths. Exchangeable A] concentrations increased at all depths at all sites, however increases were only significant in upper soil horizons. Short-term temporal changes, estimated by sampling the Oa/A horizon annually for 3 yr, were insignificant, suggesting that the differences between 1967 and 1997 are part of a long-term trend. At most of the sites losses of Ca and Mg on a pool basis were much larger than could be accounted for in biomass accumulation, suggesting leaching of nutrients off-site.