LONG-TERM CHANGES IN FOREST SOIL ACIDITY IN PENNSYLVANIA, USA

Forest soil acidification has been reported to cause reduced forest pr oductivity and decline of some tree species. Soil acidification may ca use increased bioavailability of Al and Mn, which are potentially phyt otoxic. In an attempt to measure whether or not Pennsylvania forest so ils have become more acidic, soil samples were collected at 11 undistu rbed forested sites in Pennsylvania in 1993. Limited soil chemistry da ta obtained through 4 previous studies conducted between 1957 and 1979 were available for each site. Soil pH and concentrations of exchangea ble Ca and Mg were measured; results were compared to those obtained i n 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 compa rable 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 h orizon and increased in the A horizon. Given the high sulfate and nitr ate deposition loads in Pennsylvania, it seems likely that the observe d changes are at least in part a consequence of acidic deposition.