SOLUTE FLUXES AND SULFUR CYCLING IN FORESTED CATCHMENTS IN SW GERMANYAS INFLUENCED BY EXPERIMENTAL (NH4)(2)SO4 TREATMENTS

Results are presented from the research project ARINUS which investiga tes biogeochemical cycling in Norway spruce (Picea abies KARST,) ecosy stems in the Black Forest (SW Germany) and effects of experimental (NH 4)(2)SO4 additions. The interaction of the terrestrial and aquatic sys tem is assessed using an integrated approach which combines flux measu rements in representative plots on the stand level with input-output b udgets of small catchments. The approach, field installations and expe rimental manipulations are described, Results from element flux measur ements in the untreated systems are presented and processes controllin g N and S transformations are discussed for two catchments representin g contrasting site conditions. Even though the S budget is negative fo r 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. S ulfate retention is not only a function of soil properties, but also o f water fluxes and pathways. The uptake of added SO42- was highly cont rolled by the counter-cation. Microbial N retention in the soil was hi ghly influenced by the site management history. The extent of streamwa ter acidification was highly dependent on the transformations and mobi lity of N and S in the soils which in turn controlled cation leaching and alkalinity.