The relationships among soil microbial biomass, pH and available of heavy metal fractions were evaluated in longterm contaminated soils during an incubation experiment with the amendment of zeolite (natural clinoptilolite) and the subsequent addition of glucose. The values of pH after the addition of glucose decreased during the first day of incubation approximately at about one unit and corresponded with the maximum increase of microbial biomass. The available heavy metal contents extracted by H<sub>2</sub>O, 1 mol/l NH<sub>4</sub>NO<sub>3</sub> and 0.005 mol/l DTPA increased during the first two days of incubation. Only a few significant relationships were found between the available metal contents and pH or microbial biomass. This fact could be ascribed to the different dynamics of the microbial biomass, pH and metal availability after glucose addition, when the highest metal contents during the incubation were usually reached one day later in respect to the greatest changes of pH and microbial activity. In comparison to soils without zeolite addition, the variants with natural clinoptilolite showed lower heavy metal contents in all used extractants with the exception of Cd which in H<sub>2</sub>O extracts tended to increase.
The availability of Cd, Pb and Zn and their relationships with soil pH and microbial biomass in soils amended by natural clinoptilolite
