No abstract

Most of the processes occurring in soil are catalysed by enzymes. As a result of their sensitivity towards heavy metals, enzymes in contaminated soils are usually less active. The purpose of this paper was to assess the influence of bioavailable forms of Cd, Cu, Pb and Zn on the activity of dehydrogenases, urease, acid and alkaline phosphatase, and to compare the results obtained from naturally and artificially contaminated soils. A pot experiment was carried out on two loamy sand soils, naturally and artificially contaminated with Cd, Cu, Pb and Zn. The total content of heavy metals classified these soils as very heavily contaminated with Cu, heavily contaminated with Pb and contaminated with Cd and Zn, all according to the IUNG system (1995). One of the following organic materials: swine manure or triticale straw, was added to the soil batches. The experiment was carried out in three replications, in two pH ranges: slightly acid and acid. Soil samples for analyses were taken after 14, 28, 165 and 450 days of incubation. The results of the experiment showed that the activity of soil enzymes depended on the content of bioavailable heavy metals; the total concentration of trace elements and H + were less important. However, considerable differences were found in enzyme activity between naturally and artificially contaminated soils. This indicates that results obtained from other research conducted on freshly contaminated soils cannot be easily transferred to field conditions. The analysed enzymes responded differently to the concentration of bioavailable forms of heavy metals. Alkaline phosphatase was the least tolerant to bioavailable forms of heavy metals, unlike urease, which was the most tolerant soil enzyme. A similar pattern of sensitivity toward trace elements, which could be ordered as Zn > Cd > Cu > Pb, was noticed for dehydrogenases, acid and alkaline phosphatases. Urease was found to be more tolerant to Zn.

The light fraction of soil organic matter (LF) has a rapid turnover and may be potentially metal-enriched, but the interaction between this pool of organic matter and trace elements has not been well studied. The present study aimed to investigate changes in LF content and its effect on heavy metals distribution and extractability in long-term contaminated soil by smelting activity. An incubation experiment was conducted on a surface horizon of Brunic Arenosol sampled from the previously-existing sanitary zone of Głogów smelter, for 450 days. The contaminated soil was divided into three combinations: with the addition of either triticate straw (at the dose of 4.5 Mg ha -1 ) or pig manure (at the dose of 40 Mg ha -1 ) or without any “foreign” organic materials (nil). The LF (ρ > 1.7 g cm -3 ) occurred to be metal-enriched and despite its low content (5.49%—nil, 7.18%—straw and 7.29%—manure combination) in the bulk soil, it was observed that initially Cd, Cu, Pb and Zn stock reached 16.2%, 11.9%, 18.0% and 32.3%, respectively. Incubation conditions where mineralization processes dominate led to a decrease in the LF share by about 12.6% in nil and 31.4–39.8% in the combinations with organic amendments. In consequence, the DOC (dissolved organic carbon) concentration doubled and metal distribution had changed. The increase in water-soluble (F1) fraction was observed for all metals, additionally for Cu, Pb, Zn in exchangeable fraction (F2) and in carbonate bound (F3) fraction for Cd and Zn. These results support the view that changes in the LF content may play a key role in controlling trace metals mobility, especially in long-term contaminated soil.

The simultaneous incorporation of heavy metals into the soil is still a matter of great concern. Interaction (competitive sorption) between these metals and the soil solid phase may result in a deterioration of soil quality which relies basically on amounts of alkaline cations saturating soils sorptive complex. Results of this study indicate that Pb, Cu, Cd, and Zn have induced solution pH decreases which were more intensive at highest metal loading rates. Partition parameters (Kd)-based sequences showed that Pb and Cu were more competitive than Cd and Zn and the overall selectivity sequence followed: Pb > Cu > Cd > Zn.Metal loadings and their competitive sorption have led to a strengthened displacement of alkaline cations (Le. Ca 2 +, Mg2+, K+, Na+), especially of Ca 2 + as a factor "stabilizing" soil sorptive complex. Such metals impact jointly with soils acidification are of great environmental concern since tremendous amounts of alkaline cations (especially Ca 2 +) may be potentially leached out, irrespective of the degree of soil contamination, as evidenced in the current study. High and positive ..:1G values implied that the studied soils were characterized by generally low concentrations of exchangeable potassium which required high energy to get displaced (desorbed). Further studies on heavy metal uncontaminated or contaminated areas should be undertaken to provide with data which should be used for predictions on changes related to soil buffering capacity as impacted by heavy metal inputs.