Sulphur balances and dynamics in three forested catchments in Sweden

The deposition of sulphur (S) has decreased considerably during the last de cade 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 deposi tion. Catchment studies have shown a net S output, indicating a soil-derive d S source besides the atmospheric S input. This study was performed in thr ee 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 th e ecosystems in order to trace the possible sources of excess S in runoff. The sulphate concentrations in soil water indicate that the excess S origin ates from mineralization and oxidation of organically bound S in humus and peat, as well as from desorption of sulphate from Fe and Al oxohydroxides i n the B-horizon. The latter process will quickly respond to changes in S de position, while the former probably is independent of S deposition in a sho rt time perspective. This will have implications for the modelling of recov ery from soil and surface water acidification.