Processes regulating sulphate flux after whole-tree harvesting

Terrestrial processes that regulate transfer of strong-acid anions (for example SO^2-₄, NO^-₃, Cl^-) play an important role in determining the acid-base status of surface waters¹.Anthropogenic perturbations of forested watersheds can alter these processes, resulting in changes of surface-water chemistry^2,3. Much controversy has arisen over the relative importance of acidic deposition, natural processes of soil acidification and theeffects of changes in land use on the acidification of surface waters^4,5,6. Forest clearcutting represents a useful experimental tool to evaluate the effects of changes in strong-acid loading on biogeochemical processes controllingSO^2- ₄ retention and release. Here we report that after the whole-tree harvesting of an experimental watershed at the Hubbard Brook Experimental Forest (HBEF) in the White Mountains of New Hampshire, USA, increased mineralization andnitrification led to substantial NO^-₃ loss, acidification of soil solutions and increased soil adsorption of SO ^2-₄. As a consequence, solution concentrations and streamwater efflux ofSO^2-₄declined. Substantial increases in streamwater concentrations of H⁺ and potentially toxic inorganic Al^π+ after removal of biomass also occurred. A similar disruption of the soil N cycle observed in areas offorest decline^7,8 suggests that decreased vegetation uptake of N may adversely affect surface water quality in acid-sensitive regions.