Soil calcium status and the response of stream chemistry to changing acidic deposition rates

Despite a decreasing trend in acidic deposition rates over the past two to three decades, acidified surface waters in the northeastern United States h ave shown minimal changes. Depletion of soil Ca pools has been suggested as a cause, although changes in soil Ca pools have not been directly related to long-term records of stream chemistry. To investigate this problem, a co mprehensive watershed study was conducted in the Neversink River Basin, in the Catskill Mountains of New York, during 1991-1996. Spatial variations of atmospheric deposition, soil chemistry, and stream chemistry were evaluate d over an elevation range of 817-1234 m to determine whether these factors exhibited elevational patterns, An increase in atmospheric deposition of SO 4 with increasing elevation corresponded with upslope decreases of exchange able soil base concentrations and acid-neutralizing capacity of stream wate r. Exchangeable base concentrations in homogeneous soil incubated within th e soil profile for one year also decreased with increasing elevation. An el evational gradient in precipitation was not observed, and effects of a temp erature gradient on soil properties were not detected, Laboratory leaching experiments with soils from this watershed showed that (1) concentrations o f Ca in leachate increased as the concentrations of acid anions in added so lution increased, and (2) the slope of this relationship was positively cor related with base saturation, Field and laboratory soil analyses are consis tent with the interpretation that decreasing trends in acid-neutralizing ca pacity in stream water in the Neversink Basin, dating back to 1984, are the result of decreases in soil base saturation caused by acidic deposition.