Effects of heavy metal pollution on the soil microbial activity

The effects of heavy metals on soil microbial processes were investigated over a period of six weeks. Analytical grade (Sigma) sulphate salts of copper, zinc and nickel were added individually and in combinations to soil samples and incubated in different plastic pots. Samples were taken from the pots forthnightly and the rates of microbial carbon and nitrogen mineralization, microbial biomass carbon and respiration were measured. The results showed the effect of metals on the measured parameters were significant (P<0.05.). By the 6(th) week postreatment, the rates of carbon accumulated were high in the copper (6.03%) and copper:zinc (5.80%) treatments but low in the nickel and zinc (4.93% and 5.02% respectively). The rates of nitrogen mineralization were 0.41 and 0.44% in samples treated with copper and copper:zinc compared to 0.22%-0.24% obtained at the beginning of the experiments. Soil microbial biomass carbon declined from average value of 183.7-185.6 mu g/g before treatment to as low as 100.8 and 124.6 mu g/g in samples treated with copper:zinc and copper respectively. The rate of respiration of the soil microbial populations was equally inhibited by the metals. From an average rate of 2.51-2.56 mu g of C/g respiration of the soil microbes declined to 0.98, 1.08 and 1.61 mu g of C/g in the copper:zinc, copper and zinc treated soils by the end of the experiment. The results suggest additive or synergistic effects of the metals.