Watershed sulfur biogeochemistry: shift from atmospheric deposition dominance to climatic regulation.

North American atmospheric S emissions peaked in the early 1970s followed by a dramatic decrease that resulted in marked declines in sulfate (SO₄²⁻)) concentrations in precipitation and many surface waters. These changes in S biogeochemistry have important implications with respect to the mobilization of toxic (Al(n⁺), H⁺) and nutrient (Ca²⁺, Mg²⁺, K⁺) cations and the acidification of watersheds. We used the continuous long-term record for watersheds 1, 3, 5, and 6 (37-44 years from 1965 through 2008) of SO₄²⁻ concentrations and fluxes at Hubbard Brook Experimental Forest in New Hampshire (U.S.) for evaluating S budgets. Analysis revealed that the annual discrepancies in the watershed S budgets (SO₄²⁻ flux in drainage waters minus total atmospheric S deposition) have become significantly (p < 0.001) more negative, indicating the increasing importance of the release of S from internal sources with time. Watershed wetness, as a function of log₁₀ annual water flux, was highly significant (p < 0.001) and explained 57% (n = 157) of the annual variation for the combined results from watersheds 1, 3, 5, and 6. The biogeochemical control of annual SO₄²⁻ export in streamwater of forested watersheds has shifted from atmospheric S deposition to climatic factors by affecting soil moisture.