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TECHNICAL REPORT
Bioremediation and Biodegradation

In Situ Bioremediation through Mulching of Soil Polluted by a Copper–Nickel Smelter

^a Vantaa Research Center, Finnish Forest Research Institute, P.O. Box 18, FIN-01301 Vantaa, Finland
^b Rovaniemi Research Station, Finnish Forest Research Institute, P.O. Box 16, FIN-96301 Rovaniemi, Finland
^c Dep. of Limnology and Environmental Protection, P.O. Box 62, FIN-00014 Univ. of Helsinki, Finland

* Corresponding author (Oili.Kiikkila{at}metla.fi)

Received for publication September 8, 2000. Bioremediation of a heavy metal–polluted soil was investigated in a 3-yr field experiment by adding mulch to a polluted forest floor. The mulch consisted of a mixture of compost and woodchips. The remediation treatment decreased the toxicity of the soil solution to bacteria as determined by the [³H]-thymidine incorporation technique, that is, by measuring the growth rate of soil bacteria extracted from unpolluted humus after exposing them to soil solution containing heavy metals from the experimental plots. Canonical correlation analysis was performed in order to identify the chemical and microbiological changes in the soil. The pH of the mulched organic layer increased by one unit. The concentration of complexed Cu increased and that of free Cu²⁺ decreased in the soil solution from the mulch treatment. According to basal respiration and litter decomposition, microbial activity increased during the 3 yr following the remediation treatment. The [³H]-thymidine incorporation technique was also used to study the growth rate and tolerance of bacteria to Cu. The bacterial growth rate increased and the Cu tolerance decreased on the treated plots. The structure of the microbial community, as determined by phospholipid fatty acid (PLFA) analysis, remained unchanged. The results indicate that remediation of the polluted soil had occurred, and that adding a mulch to the forest floor is a suitable method for remediating heavy metal–polluted soil.

Abbreviations: AO, number of bacterial cells • BIOL, canonical biological variable • CCA, canonical correlation analysis • CHEM, canonical chemical variable • Cu_comp, complexed copper in soil solution • Cu_exc, exchangeable copper concentration in soil • DM, dry matter weight • IC₅₀, inhibition concentration (i.e., bacterial copper tolerance) • MDS, multidimensional scaling • Ni_exc, exchangeable nickel concentration in soil • OM, organic matter weight • PLFA, phospholipid fatty acid • PLFA_bact, an indicator of bacterial biomass • PLFA_fung, an indicator of fungal biomass • T, treated • TdR, [³H]-thymidine incorporation rate (i.e., bacterial growth rate) • TdR/AO, specific bacterial growth rate, U, untreated