Sulphur balances and dynamics in three forested catchments in Sweden

The deposition of sulphur (S) has decreased considerably during the last decade and is now at levels below those recorded in the middle of the 1950's in Sweden. However, the sulphate concentrations, in surface waters have not decreased to the extent that could be expected from the decreased S deposition. Catchment studies have shown a net S output, indicating a soil-derived S source besides the atmospheric S input. This study was performed in three small, forested catchments in southern and central Sweden, that are part of the national, integrated monitoring program for forests. The aim was to assess S levels and fluxes in and between the different compartments of the ecosystems in order to trace the possible sources of excess S in runoff. The sulphate concentrations in soil water indicate that the excess S originates from mineralization and oxidation of organically bound S in humus and peat, as well as from desorption of sulphate from Fe and Al oxohydroxides in the B-horizon. The latter process will quickly respond to changes in S deposition, while the former probably is independent of S deposition in a short time perspective. This will have implications for the modelling of recovery from soil and surface water acidification.