Microbial Biomass and Phosphomonoesterase Activity of the Willow (Salix sp.) Rhizosphere in a Heavy Metal Polluted Soil

The applicability of the chloroform fumigation extraction method was tested for detecting soil microbial biomass and p-nitrophenyl phosphate (pNP) for acid phosphatase activity to study their response to heavy metal pollution in the rhizosphere soil of planted willow (Salix sp.). The experimental site was located in the Toka River Valley (North-East Hungary) along the riverbank that had been severely polluted by flooding. The river had transported heavy metal and arsenic ions from several heaps deposited imprudently near a historic lead and zinc mining site. A phytoremediation experiment was set up by planting willow trees with the aim of extracting toxic elements from the soil. A strong significant difference between the control and the metal-contaminated rhizosphere soils resulted much lower microbial biomass values in the polluted soils, which suggests disturbance in the organic matter transformation dynamics. A significant increase in acid phosphomonoesterase activity was determined in the soil due to the pollution. The phosphatase enzyme production of living organisms may be stimulated by the measured higher moisture content and significantly lower LE-soluble phosphorus content of the polluted soil samples. The correlation established between soil water content and phosphatase activity was positive (r = +0.85), while that between LE-P content and phosphatase activity was negative (r = -0.69). The most important stimulating effect was attributable to the lower available phosphorus content, resulting from the heavy metal (Pb, Zn) content of polluted soil. Both measured biological parameters therefore were suitable for indicating soil pollution, but the change was adverse, the biomass decreased, while phosphatase activity increased. Microbial biomass and phosphatase activity were not correlated, indicating the different account of ecological factors that alter the biological properties of a soil.