Solute fluxes and sulfur cycling in forested catchments in SW Germany as influenced by experimental (NH4)2SO4 treatments

Results are presented from the research project Arinus which investigates biogeochemical cycling in Norway spruce (Picea abies KARST.) ecosystems in the Black Forest (SW Germany) and effects of experimental (NH_{4 2}SO₄ additions. The interaction of the terrestrial and aquatic system 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 installations 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 organicvs. 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 ₄ ^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.