Effect of Spirulina green algae and incubation time on cadmium chemical forms in a calcareous soil

Cadmium is one of the most important heavy metals in soil that it is important in terms of environmental and human health. The main purpose of this study was to evaluate the effect of Spirulina algae and incubation time on chemical forms of cadmium in a calcareous soil. The experiment was conducted in a factorial (2 × 3 × 3) arranged in a completely randomized design with three replications. Treatments consisted of three Spirulina levels (0, 500 and 1000 mg kg-1 soil), cadmium levels (5, 20 and 40 mg kg-1 soil as cadmium sulfate (CaSO4.6H2O) and incubation times (1 and 2 months). Results showed that addition of 500 and 1000 mg Spirulina kg-1 soil in all of the cadmium levels in the first month decreased exchangeable fraction and increased the organic and carbonate fraction and cadmium that bound to iron and manganese oxides. Over the incubation time (in the second month), due to decomposition of algae, the organic fraction of cadmium decreased and soluble fraction of cadmium increased. In general, at first application of Spirulina decrease the bioavailability of metals and increase the organic and less soluble fraction of them, but over the time, it can increase the bioavailability of metal and therefore might be increase the uptake of Cd for plants. Therefore before application of Spirulina as an organic fertilizer, in cadmium polluted soils, it should be applied with caution. Further investigation using Spirulina at more incubation time and on a larger number of soils is recommended.