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A 38-day incubation experiment was carried out in order to evaluate the response of plant-available portions of heavy metals in long-term contaminated arable and grassland soils on addition of ethylenediaminetetraacetic acid (EDTA) and alfalfa (Medicago sativa L.) meal. Soils with different soil management (arable and grassland) from the vicinity of a lead smelter were used in the experiment. Readily available heavy metal fractions of Cd, Pb, Zn and Cu increased in the presence of EDTA at the beginning of experiment. The increase of heavy metal availability was higher in the arable soil with lower content of soil organic carbon than in the grassland soil. Addition of EDTA increased content of K 2 SO 4 -extractable carbon which remained higher throughout the overall time of experiment. During the first part of the experiment, the alfalfa meal addition decreased the available metal concentrations in the EDTA-treated grassland soil whereas no effect of alfalfa meal was observed in EDTA-treated arable soil.

A 40-day incubation experiment was carried out in order to evaluate the microbial activities and heavy metal availability in long-term contaminated arable and grassland soils after addition of EDTA (ethylenediaminetetraacetic acid) or EDDS ([S,S]-ethylenediaminedisuccinic acid). Soils with similar contamination of heavy metal from the vicinity of a lead smelter were used in the experiment. The soil microbial carbon (C mic ) decreased significantly after addition of EDTA in the arable soil (CM1); lesser effects were observed in the grassland soil (CM2). Addition of EDDS caused a decrease of C mic during the first 10 days of incubation. In the later phases of the experiment, C mic increased, and even exceeded the amounts found in the control soils. Respiratory activities and metabolic quotients ( q CO 2 ) increased after the addition of the chelating agents into the soils. Higher respiratory activities and q CO 2 were observed in the EDTA-treated soils. The readily available heavy metal fractions were extracted with NH 4 NO 3 solution. Readily mobilizable heavy metal fractions of Cd, Pb, Zn, and (in part) Cu increased during the first 3–10 days of incubation in the presence of EDTA. The addition of EDDS particularly increased concentrations of available Cu. Significant correlations between NH 4 NO 3 -extractable metals, soil respiratory activities, and q CO 2 were found in both soil treatments with EDTA and EDDS. This indicates that enhanced metal mobility seriously affects the microbial processes in experimental soils. In addition, the relationships between NH 4 NO 3 -extractable Cd, Cu, and the microbial biomass were found in the CM1 soil amended with EDTA.

The relationships among soil microbial biomass, pH and available of heavy metal fractions were evaluated in long- term contaminated soils during an incubation experiment with the amendment of zeolite (natural clinoptilolite) and the subsequent addition of glucose. The values of pH aer 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 2 O, 1 mol/l NH 4 NO 3 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 aer 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 additi- on, the variants with natural clinoptilolite showed lower heavy metal contents in all used extractants with the excep- tion of Cd which in H 2 O extracts tended to increase.

A laboratory incubation experiment with zeolite and glucose was performed to evaluate the effects of zeolite amendment in heavy metal contaminated soils from two smelter areas on some microbial characteristics [Kremikovtzi (K1, K2) in Bulgaria and Přibram (P1, P2) in the Czech Republic]. The content of microbial biomass showed a tendency to decrease in Kremikovtzi soils whereas in Přibram soils no significant effects were found after zeolite amendment. Respiratory activity and metabolic quotient (qCO 2 ) decreased on the second and third day in Kremikovtzi soils amended with zeolite, no effects were observed in Přibram soils. Heavy metals decreased the content of microbial biomass in Kremikovtzi soils whereas the contaminated soil from Přibram area had the highest microbial biomass compared to non-contaminated soil during incubation, probably due to lower mineralization of carbon. The respiratory activity did not show any significant effects of zeolites on the evolution of CO 2 and qCO 2 in heavy metal contaminated Přibram soil. The respiratory activity in non-contaminated Přibram soil remained during the experiment lower in comparison to contaminated one, however the addition of zeolite increased qCO 2 .

The laboratory incubation with glucose treatment was carried out in order to estimate the possible effects of increased microbial activity on heavy metal availability. The soils from vicinity of a lead smelter operating for more than 200 years were used for the experiment. The DTPA-extractable heavy metal contents increased after glucose addition and mostly reached the highest values the second day of the incubation. The comparative study, where the chloroform fumigation was used prior to the incubation in order to decrease the microbial activities, showed especially from second day of incubation significantly lower DTPA-extractable metal contents compared to non-fumigated treatments. The interactions among the maximum possible availability of DTPA-extractable heavy metal fractions and native soil microbial characteristics were studied in differently contaminated arable and grassland soils. Irrespective of different heavy metal contents in soils, significant correlations were found among the maximum percentage increase of DTPA-extractable Pb and Cd and the ratio B c /TOC and metabolic quotient (qCO 2 ) which may be a result of the important role of organic matter and microbial characteristics in soils on the heavy metal availability.