Influence of soil microbial activities by long-term Hg contamination

The suspended matter, heavily contaminated by mine spoils, that has been carried down from the Idrija mercury mining site (Slovenija), by the trans-boundary Soča-Isonzo river system, for centuries, has been deposited on riverside banks soils. As a consequence Hg concentrations in these soils can reach up to 80 μg g-1. Mercury is well known as one of the most toxic elements, but there is a lack of data about its long-term effects on soil microbial biomass and soil biological activities. The aim of this work was to determine the Hg distribution and chemical speciation in soils near the Isonzo river banks, by applying the Bloom et al. (2003) five steps selective sequential extraction procedure, and measure the effects of this diffuse and long term Hg contamination on the soil biological activities in both natural and cultivated soils. The results showed that soil Hg is in large part in the form of cinnabar (HgS) (72.9÷94.1%), whereas the presence of the most bioavailable forms (water soluble, acid soluble and organo-chelated) is generally low. Soil microbial biomass, estimated by the fumigation-extraction method, was not affected by Hg contamination, as were many soil enzymatic activities such as: FDA hydrolysis, β-glucosidase and alkaline phosphatise. Only arylsulphatase and acid phosphatise activities showed significant negative trends with total mercury and its fractions. Soil respiration and specific soil microbial biomass respiration (QCO2) were in the normal range but did not display the expected increasing trend with soil organic carbon, yet neither showed any significant decrease at high levels of Hg contamination, confirming the absence of a generalized microbial stress in our soils. Methanogenesis was not affected by either long-term Hg contamination or new acute HgCl2 additions. On the contrary, methane-oxidation rates were strongly depressed both in long-term contaminated soils and after new acute additions of HgCl2 up to 100 μg Hg g-1. The contrasting effects on microbial communities involved in methane synthesis and degradation may alter the overall balance of CH4 fluxes between soil and atmosphere in Hg contaminated soils. The results of this work show that although some general soil microbial activities have not been altered by this long-term Hg contamination, other biological activities linked to specific microbial groups may be significantly depressed.