CADMIUM SOLUBILITY AND PHYTOAVAILABILITY IN SLUDGE-TREATED SOIL - EFFECTS OF SOIL ORGANIC-CARBON

Slow mineralization of organic matter in sewage sludge-treated soil co uld release metals into more labile forms, which would then be more av ailable for plant absorption. We examined the effect of soil organic m atter decomposition on the phytoavailability of Cd in soils which, pri or to this experiment, received 0 (control), 22.5, 45, 90, 180 Mg ha(- 1) yr(-1) of sewage sludge for 6 yr. At the continued event of the exp eriment, one-half of the experimental pints continued to receive sewag e sludge application at the prescribed rates. Sewage sludge applicatio n in the other half of tile plots was terminated. All treated soils we re cropped to Swiss chard (Beta vulgaris) twice each year for 10 yr an d the soil was sampled after each crop harvest. When sewage sludge app lications were terminated, the soluble Cd concentrations of the sludge -treated soil were higher than those in the control soils and did not decline significantly over the next 10 yr. With continuous sludge appl ications, the soluble Cd concentration of sludge-treated soils increas ed with each incremental addition of sludge for the same 10 yr. In soi ls receiving continuous sewage sludge inputs, the organic C content co ntinued to rise with sludge inputs. After the termination of sludge ap plication, organic C content of the sludge-treated soils decreased by approximately 10% over the 10-yr study. There was no indication that t he soluble Cd concentration or the phytoavailability of Cd in the slud ge-treated soils increased as the organic C in these soils declined ov er the 10 yr following termination of sewage sludge application.