Processes regulating sulphate flux after whole-tree harvesting

Abstract

Terrestrial processes that regulate transfer of strong-acid anions (for example SO2−4, 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 chemistry2,3. Much controversy has arisen over the relative importance of acidic deposition, natural processes of soil acidification and the effects of changes in land use on the acidification of surface waters4,5,6. Forest clearcutting represents a useful experimental tool to evaluate the effects of changes in strong-acid loading on biogeochemical processes controlling SO2−4 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 and nitrification led to substantial NO−3 loss, acidification of soil solutions and increased soil adsorption of SO2−4. As a consequence, solution concentrations and streamwater efflux of SO2−4declined. Substantial increases in streamwater concentrations of H+ and potentially toxic inorganic Alπ+ after removal of biomass also occurred. A similar disruption of the soil N cycle observed in areas of forest decline7,8 suggests that decreased vegetation uptake of N may adversely affect surface water quality in acid-sensitive regions.