Microbial transformation of cadmium in two soils differing in organic matter content and texture

Results of laboratory soil suspension experiments with two soils differing in organic C content (1.1 and 4.2%) and texture (14.6 – 61.0 – 24.4% and 67.5 – 20.0 -12.5% for sand – silt – clay, respectively) showed that intensive microbial growth after the addition of a sucrose nutrient medium (SNMed) caused partial transformation of the solid phase Cd to forms soluble in 0.1 M NaOH. The transformation of Cd was accompanied by a simultaneous decrease of the solid phase Cd extractability by a solvent consisting of 0.005 M DTPA + 0.01 M CaCl2 + 0.1 M triethanolamine (DTPA). These changes of the solid phase Cd extractability were probably caused by Cd binding to microbial biomass and microbial metabolism products. After the addition of SNMed, a concomitant phenomenon of an increase of Cd DTPA-extractability was observed, especially in the case of the course-textured soil with the higher organic C content. In a soil (not soil suspension) experiment, stimulation of microbial growth by SNMed addition caused different changes of Cd DTPA-extractability in the examined soils. In the fine-textured soil with the lower organic C content the process decreasing the solid phase Cd solubility in DTPA prevailed, but in the other soil, the opposite phenomenon predominated. The occurrence in nature of the phenomena observed under laboratory conditions is discussed.