Water, Air, and Soil Pollution 130: 75–86, 2001. © 2001 Kluwer Academic Publishers. Printed in the Netherlands. USE OF STABLE ISOTOPE RATIOS FOR EVALUATING SULFUR SOURCES AND LOSSES AT THE HUBBARD BROOK EXPERIMENTAL FOREST

Abstract. Anthropogenic S emissions have been declining in eastern North America since the early 1970s. Declines in atmospheric S deposition have resulted in decreases in concentrations and fluxes of so}- in precipitation and drainage waters. Recent S mass balance studies have shown that the outflow of soi- in drainage waters greatly exceeds current s inputs from atmos-pheric deposition. Identifying the S source(s) which contribute(s) to the discrepancy in watershed S budgets is a major concern to scientists and policy makers because of the need to better under-stand the rate and spatial extent of recovery from acidic deposition. Results from S mass balances combined with model calculations and isotopic analyses of soi- in precipitation and drainage waters at the Hubbard Brook Experimental Forest (HBEF) suggest that this discrepancy cannot be explained by either underestimates of dry deposited s or desorption of previously stored so1-. Isotopic results suggest that the excess S may be at least partially derived from net mineralization of organic S as well as the weathering of S-bearing minerals