SOLUTE FLUXES AND SULFUR CYCLING IN FORESTED CATCHMENTS IN SW GERMANY AS INFLUENCED BY EXPERIMENTAL (NH4)2SO4 TREATMENTS K. H. FEGER Institute of Soil Science and Forest Nutrition, Albert-Ludwig-University, BertoldstraJ3e 17, D-79085 Freiburg i.Br., Germany (Received July 29, 1994; accepted August 11, 1994) Abstract. Results are presented from the research project ARINUS which investigates biogeochem- ical cycling in Norway spruce (Picea abies KARST.) ecosystems in the Black Forest (SW Germany) and effects of experimental (NH4)2SO4 additions. The interaction of the terrestrial and aquatic sys- tem is assessed using an integrated approach which combines flux measurements in representative plots on the stand level with input-output budgets of small catchments. The approach, field installa- tions and experimental manipulations are described. Results from element flux measurements in the untreated systems are presented and processes controlling N and S transformations are discussed for two catchments representing contrasting site conditions. Even though the S budget is negative for both systems there is a distinct difference in the relation between organic vs. inorganic S fractions in the soil. Sulfate mineralization and desorption, respectively are discussed as controlling processes. Sulfate retention is not only a function of soil properties, but also of water fluxes and pathways. The uptake of added SO 2- was highly controlled by the counter-cation. Microbial N retention in the soil was highly influenced by the site management history. The extent of streamwater acidification was highly dependent on the transformations and mobility of N and S in the soils which in turn controlled cation leaching and alkalinity. 1. Catchments in Forest Ecosystem Research Forest ecosystems are characterized by a complex network of inputs and outputs of energy and matter. Because of the lack of information about input-output rela- tionships and internal control functions, it is often difficult to determine the impact of human activities. In humid regions biogeochemical fluxes and cycling are intimately linked to the water cycle. Thus, one cannot assess the input and output of elements without simultaneously quantifying water fluxes. Unfortunately, there is no direct measure of subsurface water fluxes. Only a catchment can provide directly measurable values for the water outflow, providing that certain requirements are met. Given an impermeable base, normally a water-tight bedrock, all the water and element output will turn up in the stream draining the catchment. Such a unit has natural boundaries which are relatively easy to define. Furthermore, a catchment is the smallest natural component of a landscape that combines linked terrestrial and aquatic systems. On the other hand, it is large enough to comprise all the interacting biotic and abiotic components of an ecosystem (Likens et al., 1977). Catchments are therefore 'ultimate testing-grounds' for ecological research on natural and man-induced changes (Hewlett, 1982). Water, Air and Soil Pollution 79: 109-130, 1995. (~) 1995 Kluwer Academic Publishers. Printed in the Netherlands.