A multi-compartment system was used to study the importance of microorganisms for Cd desorption from soil amended with sewage sludge and simultaneous resorption of the mobilized metal by soil constituents. Using this system made it possible to study the participation of microorganisms (Arthrobacter, Trichoderma), montmorillonite, humic acids, and iron oxides in resorption of the released Cd. A filter-sterilized water extract of root-free soil of pH 6.7 (RF) or RF supplemented with glucose (RFG) were used to mobilize Cd from soil at 14 degrees C in 48 h. Cadmium found in those extracts after 48-h incubation was recognized as bioavailable. Changes in pH values and enrichment of soil extracts with organic acids and siderophores resulted from microbial growth. RFG with lower pH and a higher content of ligands mobilized, on average, 40% of Cd introduced with sewage sludge amended soil, whereas RF mobilized only 20% of it. Sequential extractions of Cd at time 0 and Cd remaining in soil showed that RFG had mobilized Cd mostly from the fraction bound with Fe and Mn oxides. Microbial biomass accounted for only up to 3.4% (w/w) of the soil constituents used in the experiments but resorbed 25% of mobilized Cd. The chemical composition of mobilizing soil extracts and the solid-to-mobilizing-extracts volume ratio had a significant effect on the amount of bioavailable Cd. The results of the study suggest that microbial metabolites were involved in Cd mobilization, while the biomass of microorganisms was involved in Cd resorption as a biosorbent.