Cadmium solubility and phytoavailability in sludge-treated soil: Effects of soil organic carbon

Slow mineralization of organic matter in sewage sludge-treated soil could release metals into more labile forms, which would then be more available for plant absorption. We examined the effect of soil organic matter decomposition on the phytoavailability of Cd in soils which, prior 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 experiment, one-half of the experimental pints continued to receive sewage sludge application at the prescribed rates. Sewage sludge application in the other half of the plots was terminated. All treated soils were cropped to Swiss chard (Beta vulgaris) twice each year for 10 yr and the soil was sampled after each crop harvest. When sewage sludge applications 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 applications, the soluble Cd concentration of sludge-treated soils increased with each incremental addition of sludge for the same 10 yr. In soils receiving continuous sewage sludge inputs, the organic C content continued to rise with sludge inputs. After the termination of sludge application, organic C content of the sludge-treated soils decreased by approximately 10% over the 10-yr study. There was no indication that the soluble Cd concentration or the phytoavailability of Cd in the sludge-treated soils increased as the organic C in these soils declined over the 10 yr following termination of sewage sludge application.