Effect of heavy metals on the microbial activity of soil under red clover

Abstract

Heavy metals reach the environment, primarily soils, mostly through human activities. They enter certain biological links in nutrient and energy cycles, thus becoming a source of many environmental problems and health issues in both people and animals. Microorganisms as an important link in the cycling process can serve as indicators of both soil pollution and potential toxicity to other biological systems. The objective of this study was to evaluate the effect of different concentrations of lead (60; 120; 250 mg kg-1 soil) and mercury (1,0; 2,0; 4,0 mg kg-1 soil) on total microbial count and Azotobacter in two soils, a Vertisol and an alluvium under red clover cultivation in three growing seasons. The experiment was conducted under greenhouse conditions at the Faculty of Agronomy, Čačak. Numbers of the microorganisms tested were determined by indirect counting methods involving plating out a soil suspension onto appropriate selective culture media. Depending on the type and concentration involved, heavy metals had a significant effect on soil microbial count in the alluvium and vertisol during the red clover growing season. Low concentrations of lead and mercury (60 mg kg-1 and 1 kg-1 soil, respectively) did not lead to significant changes in total microbial and Azotobacter counts. At 250 mg kg-1 soil, lead induced a decrease in total microbial and Azotobacter counts. Mercury had a markedly higher depressive effect on soil microorganisms, with concentrations of 2 and 4 mg kg-1 soil significantly reducing the total microbial and Azotobacter counts.