The Role of Compost in Alleviating Cadmium Effects on Microbial Respiration and Biomass, and Phosphatase Activity in Soil

Cadmium is one of the heavy metals with a considerable importance for its potential toxic effects on soil microbial activities and composition. Although, the toxic effect of cadmium on soil microbial activities is somewhat well-known, but the extent to which Cd affects soil biota depends largely on soil properties and conditions, particularly soil organic matter contents. Thus, the aim of this research was to study the effect of increasing cadmium levels on soil microbial biomass and activities, and to examine the role of compost materials in the alleviation of Cd effects. A 2×5 factorial experiment consisting of two levels of compost (0 and 2.5 t ha-1) and five levels of cadmium (0, 50, 100, 150 and 200 mg Cd kg-1) arranged in a completely randomized design with three replicates was carried out under laboratory conditions. The responses of soil microbiological properties consisting of C mineralization, microbial biomass C; and acid and alkaline phosphatase activities to cadmium and compost additions were evaluated during 10 weeks of laboratory soil incubation. Results show that cadmium additions had a significant, negative effect on all the measured microbiological properties. The accumulated C mineralization reduced with increasing cadmium concentrations. Nonetheless, compost addition lowered the detrimental and inhibiting effect of cadmium on soil microbial activities. Results demonstrate that in spite of a significant correlation (r=0.96) between the two Cd fractions, the added Cd at the beginning of soil incubation was not reflected totally in the total Cd concentrations determined at the end of soil incubation. Data also indicated that the fraction of available Cd had a stronger negative (cor)relation with soil microbial activities than the other fractions had. In summary, the results of the current study illustrate that even a lower concentration of Cd (50 mg kg-1) may inhibit soil microbial activities due to it toxicity in the studied soil, and that the addition of organic materials could be effective in reducing the toxicity of cadmium via lessening its bioavailability to soil biota.