Effects of repeated (NH4)(2)SO4 application on sulfur pools in soil, soil microbial biomass, and ground vegetation of two watersheds in the Black Forest/Germany

The effect of repeated (NH4)(2)SO4 applications (3 x 700 kg ha(-1) in 1988, 1991, and 1994, respectively) on S pools in soil, soil microbial biomass, and ground vegetation was studied at two Norway spruce (Picea abies L. [Kar st.]) sites in the Black Forest/Germany. In both eco-systems, most of the t otal S pool was located in the soil. The soil also was the predominant comp artment for retention of applied SO42--S. The fractions of organic and inor ganic S forms in the initial soil S content, and the retention of experimen tally applied S was different for both sites. In the podzol Schluchsee, org anic S accounted for 92% of total S. In the cambisol Villingen, the S pool consisted of 33% organic S and 67% inorganic S. The retention of applied S in various compartments of both ecosystems reflected these proportions. Onl y minor amounts of fertilized S (<1%) was retained in the spruce trees, gro und vegetation, and soil microbial biomass. However, between 51% (Villingen ) and 72% (Schluchsee) of the applied S was retained in the soil. In the Sc hluchsee podzol, 75% of retained fertilizer S was accumulated as ester sulf ate, whereas SO42-adsorption and precipitation of Al hydroxy sulfates were restricted by dissolved organic matter in the soil solution. In the Villing en cambisol, SO42- adsorption was the dominant process of S retention, alth ough 20% of the fertilized S again was retained as ester sulfate. The signi ficant relevance of organic S forms in the retention of fertilizer S in bot h soils emphasizes the need for models which include the formation and re-m ineralization of organic S compounds, especially of ester sulfates, for cor rectly simulating and predicting the retention and remobilization of S in a cid forest soils subject to changing atmospheric N and S deposition.