LONG-TERM CHANGES IN FOREST SOIL ACIDITY IN PENNSYLVANIA, U.S.A.

Forest soil acidification has been reported to cause reduced forest productivity and decline of some tree species. Soil acidification may cause increased bioavailability of Al and Mn, which are potentially phytotoxic. In an attempt to measure whether or not Pennsylvania forest soils have become more acidic, soil samples were collected at 11 undisturbed forested sites in Pennsylvania in 1993. Limited soil chemistry data obtained through 4 previous studies conducted between 1957 and 1979 were available for each site. Soil pH and concentrations of exchangeable Ca and Mg were measured; results were compared to those obtained in earlier studies. Soil exchangeable Al concentrations were determined for 6 sites for which previous data were available. Chemical analysis methods were evaluated to ensure that methods used in 1993 were comparable to those of the original investigators. Mean pH and exchangeable Mg declined most consistently over the study period in the O horizon and the uppermost A horizon. Mean exchangeable Al decreased in the O horizon and increased in the A horizon. Given the high sulfate and nitrate deposition loads in Pennsylvania, it seems likely that the observed changes are at least in part a consequence of acidic deposition.