Availability and speciation of cadmium added to a calcareous soil under various managements

Potential risks for human health and adverse effects on soil quality caused by accumulation of cadmium (Cd) in soil at concentrations around or exceeding current European Union (EU) permitted limits have long been recognized. We have assessed availability and partitioning of Cd in a Mediterranean calcareous soil under four management regimes. Cadmium was added as a single pulse of CdSO4 at the maximum Cd concentration established by the EU for sludge-amended agricultural soils and concentrations exceeding the mandatory limits. Soils were treated with 0, 3, 10 and 50 mg Cd kg(-1) soil, incubated moist and analysed at selected times up to 600 days. Cadmium availability and distribution in soil were studied by neutral electrolyte and sequential extractions. During the incubation, the availability of Cd was not strictly dependent on the amount of metal added as the exchangeable fractions were similar shortly after the additions of Cd regardless of its initial concentration. Sequential extractions showed that for concentrations of 3 and 10 mg Cd kg(-1) soil Cd was evenly distributed among the soil phases, and its mobility was reduced mainly by adsorption on carbonates. At Cd concentrations exceeding 10 mg Cd kg(-1) soil a residual fraction appeared, perhaps from precipitation of Cd. Most of the Cd was associated with carbonates; land management and organic matter content had no major effects on the Cd distribution among different soil phases. The extraction protocols were effective for studying the fate of Cd in this calcareous soil as almost all of the Cd added was recovered. However, the introduction of a preliminary step with buffered NH4NO3 improved the determination of the most labile pools. Availability of Cd in calcareous soils estimated with reference methods appeared to be very small even when its total concentration far exceeded the current EU limits.