Contrasts between subsurface microbial communities and their metabolic adaptation to polycyclic aromatic hydrocarbons at a forested and an urban coal-tar disposal site

The abundance and distribution of microorganisms and their potential for mineralizing polycyclic aromatic hydrocarbons (PAHs) were measured in subsurface sediment samples at two geographically separate buried coal-tar sites. At a relatively undisturbed forested site in the northeastern United States, metabolic adaptation to the PAHs was evident: Radiolabeled naphthalene and phenanthrene were converted to ¹⁴CO₂ in core material from inside but not outside a plume of groundwater contamination. However, at the urban site in the midwestern United States these PAHs were mineralized in sediments from both contaminated and uncontaminated boreholes. Thus, clear qualitative evidence showing an adaptational response by the subsurface microbial community was not obtained at the urban site. Instead, subtler clues suggesting metabolic adaptation by subsurface microorganisms from the urban site were discerned by comparing lag periods and extents of ¹⁴CO₂ production from radiolabeled PAHs added to samples from contaminated and uncontaminated boreholes. Despite slightly higher PAH mineralization activity in contaminated borehole samples, p-hydroxybenzoate was mineralized equally in all samples from the urban site regardless of location. No striking trends in the abundances of actinomycetes, fungi, and either viable or total bacteria were encountered. However, colonies of the soil bacterium, Bacillus mycoides, were detected on enumeration plates of several samples from unsaturated and saturated zones in both urban boreholes. Furthermore, other common soil bacteria, Myxococcus xanthus and Chromobacterium violaceum, were identified in samples from the uncontaminated urban borehole. The occurrence of bacteria usually restricted to surface soil, combined with the observation of fragments of building materials in many of the core samples, suggested that past excavation and backfilling operations may have caused mixing of surface soil with subsurface materials at the urban site. We speculate that this mixing, as well as non-coal-tar-derived sources of PAHs, contributed to the PAH-mineralizing activity present in the sediment samples from the uncontaminated urban borehole.     

Methanogenic transformation of aromatic hydrocarbons and phenols in groundwater aquifers

Fermentative and methanogenic bacteria have been found repeatedly as important members of microbial flora in anoxic zones of the subsurface—in pristine as well as in contaminated groundwater aquifers. These bacteria, which together with obligate proton reducers form complex methanogenic communities, are significant as decomposers of organic matter under conditions of exogenous electron acceptor depletion. Their metabolic activity has been demonstrated in laboratory microcosms derived from aquifer material, and also in the subsurface in situ. Methanogenic communities have been shown to transform numerous organic pollutants, or even to completely degrade these compounds with the production of carbon dioxide and methane. Depending on the chemical structure of the pollutant, such a compound can be used as an electron donor and a carbon/energy source for fermentative microorganisms (which is typically the case with highly reduced compounds); alternatively, a highly oxidized pollutant can be used as a potential electron acceptor or electron sink. This review addresses fermentative/methanogenic degradation of chlorinated and nonchlorinated aromatic hydrocarbons and phenols by subsurface microorganisms; for comparison, it briefly relates also other types of anaerobic transformations (under sulfate‐reducing, iron‐reducing, and denitrifying conditions). Furthermore, it outlines transformation pathways, those that are proposed as well as those that are already partially proved, for aromatic hydrocarbons and phenols under fermentative/methanogenic conditions; finally, it discusses the relevance of these processes to bioremediation of contaminated groundwater aquifers.     

Microbial biomass and activities associated with subsurface environments contaminated with chlorinated hydrocarbons

Soil microcosms and enrichment cultures from subsurface sediments and groundwaters contaminated with trichloroethylene (TCE) were examined. Total lipids, [I‐‘⁴C]acetate incorporation into lipids, and [Me‐³H]thymidine incorporation into DNA were determined in these subsurface environments. In heavily TCE‐contam‐inated zones (greater than 500 mg/L) radioisotopes were not incorporated into lipids or DNA. Radioisotope incorporation occurred in sediments both above and below the TCE plume. Phospholipid fatty acids (PLFA) were not detected, i.e., less than 0.5 pmol/L in heavily contaminated groundwater samples. In less contaminated waters, extracted PLFA concentrations were greater than 100 pmollL and microbial isolates were readily obtained. Degradation of 30–100 mg/L TCE was observed when sediments were amended with a variety of energy sources. Microorganisms in these subsurface sediments have adapted to degrade TCE at concentrations greater than 50 mg/L.     

Bioremediation of creosote-contaminated soil in South Africa by landfarming

AIMS: To determine the combined effects of biostimulation and bioaugmentation in the landfarming of a mispah form (lithosol; food and Agriculture Organisation (FAO)) soil contaminated with >310000 mg kg-1 creosote with a view to developing a bioremediation technology for soils heavily contaminated with creosote. METHODS AND RESULTS: The excavated soil was mixed with 2500 kg ha-1 dolomitic lime and 2000 kg ha-1 mono-ammonium phosphate (MAP) before spreading over a treatment bed of shale reinforced with clay. Sewage sludge (500 kg) was ploughed into 450 m3 of contaminated soil in the second and sixth months of treatment. A further 1000 kg ha-1 MAP was added to the soil at the end of the fifth month. Moisture was maintained at 70% field capacity. Total creosote was determined by the US Environmental Protection Agency (EPA) method 418.1 and concentrations of selected creosote components were determined by gas chromatography/flame ionisation detection (GC/FID). Total creosote was reduced by more than 90% by the 10th month of landfarming. The rate of reduction in creosote concentration was highest after the addition of sewage sludge. The three-ring PAHs were more slowly removed than naphthalene and the phenolic compounds. The four- and five-ring PAHs, although persist until the end of treatment, were reduced by 76-87% at the end of the experiment. CONCLUSIONS: A combination of biostimulation and bioaugmentation during landfarming could enhance the bioremediation of soils heavily contaminated with creosote. SIGNIFICANCE AND IMPACT OF THE STUDY: The study provides information on the management of a combination of biostimulation and bioaugmentation during landfarming, and contributes to the knowledge and database necessary for the development of a technology for bioremediating creosote-contaminated land.     

Biodegradation of creosote and pentachlorophenol in contaminated groundwater: chemical and biological assessment

Shake flask studies examined the rate and extent of biodegradation of pentachlorophenol (PCP) and 42 components of coal-tar creosote present in contaminated groundwater recovered from the American Creosote Works Superfund site, Pensacola, Fla. The ability of indigenous soil microorganisms to remove these contaminants from aqueous solutions was determined by gas chromatographic analysis of organic extracts of biotreated groundwater. Changes in potential environmental and human health hazards associated with the biodegradation of this material were determined at intervals by Microtox assays and fish toxicity and teratogenicity tests. After 14 days of incubation at 30 degrees C, indigenous microorganisms effectively removed 100, 99, 94, 88, and 87% of measured phenolic and lower-molecular-weight polycyclic aromatic hydrocarbons (PAHs) and S-heterocyclic, N-heterocyclic, and O-heterocyclic constituents of creosote, respectively. However, only 53% of the higher-molecular-weight PAHs were degraded; PCP was not removed. Despite the removal of a majority of the organic contaminants through biotreatment, only a slight decrease in the toxicity and teratogenicity of biotreated groundwater was observed. Data suggest that toxicity and teratogenicity are associated with compounds difficult to treat biologically and that one may not necessarily rely on indigenous microorganisms to effectively remove these compounds in a reasonable time span; to this end, alternative or supplemental approaches may be necessary. Similar measures of the toxicity and teratogenicity of treated material may offer a simple, yet important, guide to bioremediation effectiveness.     

Biodegradation of creosote and pentachlorophenol in contaminated groundwater: chemical and biological assessment.

Shake flask studies examined the rate and extent of biodegradation of pentachlorophenol (PCP) and 42 components of coal-tar creosote present in contaminated groundwater recovered from the American Creosote Works Superfund site, Pensacola, Fla. The ability of indigenous soil microorganisms to remove these contaminants from aqueous solutions was determined by gas chromatographic analysis of organic extracts of biotreated groundwater. Changes in potential environmental and human health hazards associated with the biodegradation of this material were determined at intervals by Microtox assays and fish toxicity and teratogenicity tests. After 14 days of incubation at 30 degrees C, indigenous microorganisms effectively removed 100, 99, 94, 88, and 87% of measured phenolic and lower-molecular-weight polycyclic aromatic hydrocarbons (PAHs) and S-heterocyclic, N-heterocyclic, and O-heterocyclic constituents of creosote, respectively. However, only 53% of the higher-molecular-weight PAHs were degraded; PCP was not removed. Despite the removal of a majority of the organic contaminants through biotreatment, only a slight decrease in the toxicity and teratogenicity of biotreated groundwater was observed. Data suggest that toxicity and teratogenicity are associated with compounds difficult to treat biologically and that one may not necessarily rely on indigenous microorganisms to effectively remove these compounds in a reasonable time span; to this end, alternative or supplemental approaches may be necessary. Similar measures of the toxicity and teratogenicity of treated material may offer a simple, yet important, guide to bioremediation effectiveness.     

Influence of soil organic matter decomposition on arbuscular mycorrhizal fungi in terms of asymbiotic hyphal growth and root colonization

Soil organic matter is known to influence arbuscular mycorrhizal (AM) fungi, but limited information is available on the chemical components in the organic matter causing these effects. We studied the influence of decomposing organic matter (pure cellulose and alfalfa shoot and root material) on AM fungi after 30, 100, and 300 days of decomposition in nonsterile soil with and without addition of mineral N and P. Decomposing organic matter affected maize root length colonized by the AM fungus Glomus claroideum in a similar manner as other plant growth parameters. Colonized root length was slightly increased by both nitrogen and phosphorus application and plant materials, but not by application of cellulose. In vitro hyphal growth of Glomus intraradices was increased by soil extracts from the treatments with all types of organic materials independently of mineral N and P application. Pyrolysis of soil samples from the different decomposition treatments revealed in total 266 recognizable organic compounds and in vitro hyphal growth of G. intraradices in soil extract positively correlated with 33 of these compounds. The strongest correlation was found with 3,4,5-trimethoxybenzoic acid methyl ester. This compound is a typical product of pyrolysis of phenolic compounds produced by angiosperm woody plants, but in our experiment, it was produced mainly from cellulose by some components of the soil microflora. In conclusion, our results indicate that mycelia of AM fungi are influenced by organic matter decomposition both via compounds released during the decomposition process and also by secondary metabolites produced by microorganisms involved in organic matter decomposition.     

Optimization of soil physical and chemical conditions for the bioremediation of creosote-contaminated soil

Mispah type soil (FAO : Lithosol) contaminated with >250 000 mg kg^-1 creosote was collected from the yard of a creosote treatment plant. The soils carbon, nitrogen and phosphorus contents were determined. Due to creosote contamination, thecarbon content of the soil was found to be 130,000 mg C kg^-1. This concentration was found to greatly affect the nitrogen content (0.08%). The phosphorus content was less affected (4.5%). It was estimated that a nutrient amendment to bring the soil to a C : N 10 : 1 would be adequate to stimulate microbial growth and creosote degradation. The soil was amended with a range of C : N ratios below and above the estimated ratio. In one of the treatments, the phosphorus content was amended. Sterile and natural controls were also set up. The soil was incubated at 30 °C on a rotaryshaker at 150 rpm in the dark for six weeks. Water content was maintained at 70% field capacity. The lowest nitrogen supplementation (C : N = 25 : 1) was more effective in enhancing microbial growth (3.12E + 05) and creosote removal (68.7%) from the soil. Additional phosphorus was not very effective in enhancing the growth of microorganisms and removal of creosote. The highest nitrogen supplementation(C : N = 5 : 1) did not enhance microbial growth and creosote removal.A relationship between mass loss and creosote removal was also observed. Phenolics and lower molecular mass polycyclic aromatic hydrocarbons (PAHs) were observed to be more susceptible to microbial degradation than higher molecular mass compounds. Nutrient concentration, moisture content and pH were thus observed to play very significant roles in the utilization of creosote in soil. These results are being used for the development of a bioremediation technology for the remediation of creosote contaminated soils in a treatment plant in South Africa.     

Bioaugmentation of Atrazine and Fenamiphos Impacted Groundwater: Laboratory Evaluation

After the failure of a three-month pump-and-treat exercise to clean up an aquifer contaminated with the pesticides atrazine and fenamiphos, microcosm experiments using ¹⁴C-labeled compounds were undertaken to determine under what conditions bioremediation would be most effective, and to investigate the prospects for the use of bioaugmentation. The calculated half-lives for atrazine and fenamiphos mineralization to carbon dioxide in unamended, anaerobic aquifer material were 730 and 1,000 years, respectively. Oxygenation, coupled with bioaugmentation with enrichments of atrazine-mineralizing bacteria obtained from the contaminated site or an imported, atrazine-mineralizing pure strain, Pseudomonas sp. strain ADP, decreased the half-life of atrazine mineralization, to >20 days. Although strain ADP does not use atrazine as a source of carbon and energy, amendment of the aquifer material with citrate, which strain ADP uses as a source of carbon and energy, did not appreciably stimulate the mineralization rate of atrazine in the microcosms, suggesting that the aquifer contains enough natural organic carbon for atrazine mineralization. Aerobic enrichments of fenamiphos-degrading bacteria were prepared; however, oxygenation and bioaugmentation of aquifer material with these strains did not enhance mineralization of fenamiphos within the time constraints of the experiments. The shortest calculated half-life of fenamiphos mineralization in the microcosms was 6.8 years, which is exceedingly long compared with the half-life of fenamiphos in most surface soils.     

Controls on microbial CO2 production: a comparison of surface and subsurface soil horizons

Although a significant amount of the organic C stored in soil resides in subsurface horizons, the dynamics of subsurface C stores are not well understood. The objective of this study was to determine if changes in soil moisture, temperature, and nutrient levels have similar effects on the mineralization of surface (0–25 cm) and subsurface (below 25 cm) C stores. Samples were collected from a 2 m deep unsaturated mollisol profile located near Santa Barbara, CA, USA. In a series of experiments, we measured the influence of nutrient additions (N and P), soil temperature (10–35°C), and soil water potential (?0.5 to ?10 MPa) on the microbial mineralization of native soil organic C. Surface and subsurface soils were slightly different with respect to the effects of water potential on microbial CO₂ production; C mineralization rates in surface soils were more affected by conditions of moderate drought than rates in subsurface soils. With respect to the effects of soil temperature and nutrient levels on C mineralization rates, subsurface horizons were significantly more sensitive to increases in temperature or nutrient availability than surface horizons. The mean Q₁₀ value for C mineralization rates was 3.0 in surface horizons and 3.9 in subsurface horizons. The addition of either N or P had negligible effects on microbial CO₂ production in surface soil layers; in the subsurface horizons, the addition of either N or P increased CO₂ production by up to 450% relative to the control. The results of these experiments suggest that alterations of the soil environment may have different effects on CO₂ production through the profile and that the mineralization of subsurface C stores may be particularly susceptible to increases in temperature or nutrient inputs to soil.     

A comparison between the effect of fresh and dried organic materials added to soil on carbon and nitrogen mineralization

Summary Incubation experiments were carried out at 29°C in which fresh chopped, dried chopped, or dried and ground material of wheat plants,Polygonum nodosum, Senecio congestus (R. Br. DC.) and lucerne was mixed with a heavy calcareous loam. The C/N ratios of these materials were 45.9, 32.0, 19.3, and 12.6, respectively. At intervals of one or two weeks the content of mineral nitrogen in the treated and untreated soils was determined. In this way the immobilised or released nitrogen could be calculated. Parallel to this experiment the production of CO₂ in the soils treated in the same way was determined.The experiments have clearly shown that the mineralization of the carbon and nitrogen of incorporated organic materials is influenced not only by the N-content, the C/N ratio of the materials and the amount of the materials, but also depends on whether the plant materials are in fresh chopped, or dried chopped, or dried and ground condition.In most cases a retardation of the rate of nitrogen mineralization was found after drying the organic materials due to an increase of the C/N ratio of the water-soluble organic fraction by drying. The depressing effect of drying on the rate of nitrogen mineralization was increased by a mechanical breaking-up of the dried materials, presumably due to an increased surface of the non-soluble carbonaceous compounds.     

Effect of starch and inorganic nitrogen on microbial transformations of organic compounds in soil

It was found in long-term incubation experiments (100 weeks) that in soil non amended with organic compounds, mineralization proceeded in a steady state after an initial reactive respiration and corresponded approximately to the rate of mineralization of soil organic matter under natural conditions. The addition of nitrogen influenced this mineralization process only very slightly. The addition of starch decreased the stability of organic compounds in the soil at first but the newly formed organic substances were gradually stabilized again. The addition of nitrogen to starch increased the extent and the rate of these alterations. The overall extent of mineralization of the substrate after the addition of starch alone exceeds the amount of the added substrate; a small accumulation of the substrate could be observed after the addition of starch together with nitrogen.     

Spatial Electron Acceptor Variability: Implications for Assessing Bioremediation Potential

An extensive network of multilevel samplers was established in a hydrocarbon-contaminated wetland aquifer. Results of groundwater sampling for benzene, toluene, ethylbenzene, and xylenes (BTEX), and electron acceptors show that both pristine and contaminated groundwater have spatially variable chemical signatures, owing primarily to microbially mediated oxidation-reduction reactions. Due to these spatial variations, estimates of the efficiency of intrinsic bioremediation can vary significantly depending on how geochemical data are collected. Use of data collected from monitoring wells with screens longer than the vertical extent of the plume will generally underestimate the potential for intrinsic bioremediation for the most chemically active horizon of the plume. A comparison of pristine and contaminated redox patterns demonstrates that, although BTEX exerts the highest demand for electron acceptors, oxidation of natural organic matter also contributes to electron acceptor utilization. If natural and other non-BTEX losses of electron acceptors are ignored, the assimilative capacity, defined as the amount of a contaminant that can potentially be degraded with known amounts of electron acceptors, will be overestimated. Many numerical and analytical models designed to simulate biodegradation are directly or indirectly based on assimilative capacity estimates. Proper estimation of assimilative capacity is crucial if models are to accurately quantify solute concentrations over time and space.     

Adaptation to and biodegradation of xenobiotic compounds by microbial communities from a pristine aquifer.

The ability of subsurface microbial communities to adapt to the biodegradation of xenobiotic compounds was examined in aquifer solids samples from a pristine aquifer. An increase in the rates of mineralization of radiolabeled substrates with exposure was used as an indication of adaptation. For some compounds, such as chlorobenzene and 1,2,4-trichlorobenzene, slight mineralization was observed but no adaptation was apparent during incubations of over 8 months. Other compounds demonstrated three patterns of response. For m-cresol, m-aminophenol, and aniline intermediate rates of biodegradation and a linear increase in the percent mineralized with time were observed. Phenol, p-chlorophenol, and ethylene dibromide were rapidly metabolized initially, with a nonlinear increase in the percent mineralized with time, indicating that the community was already adapted to the biodegradation of these compounds. Only p-nitrophenol demonstrated a typical adaptation response. In different samples of soil from the same layer in the aquifer, the adaptation period to p-nitrophenol varied from a few days to as long as 6 weeks. In most cases the concentration of xenobiotic added, over the range from a few nanograms to micrograms per gram, made no difference in the response. Most-probable-number counts demonstrated that adaptation is accompanied by an increase in specific degrader numbers. This study has shown that diverse patterns of response occur in the subsurface microbial community.     

Adaptation to and biodegradation of xenobiotic compounds by microbial communities from a pristine aquifer

The ability of subsurface microbial communities to adapt to the biodegradation of xenobiotic compounds was examined in aquifer solids samples from a pristine aquifer. An increase in the rates of mineralization of radiolabeled substrates with exposure was used as an indication of adaptation. For some compounds, such as chlorobenzene and 1,2,4-trichlorobenzene, slight mineralization was observed but no adaptation was apparent during incubations of over 8 months. Other compounds demonstrated three patterns of response. For m-cresol, m-aminophenol, and aniline intermediate rates of biodegradation and a linear increase in the percent mineralized with time were observed. Phenol, p-chlorophenol, and ethylene dibromide were rapidly metabolized initially, with a nonlinear increase in the percent mineralized with time, indicating that the community was already adapted to the biodegradation of these compounds. Only p-nitrophenol demonstrated a typical adaptation response. In different samples of soil from the same layer in the aquifer, the adaptation period to p-nitrophenol varied from a few days to as long as 6 weeks. In most cases the concentration of xenobiotic added, over the range from a few nanograms to micrograms per gram, made no difference in the response. Most-probable-number counts demonstrated that adaptation is accompanied by an increase in specific degrader numbers. This study has shown that diverse patterns of response occur in the subsurface microbial community.     

Effect of the carbon source on assessment of degrading bacteria with the spread-plating technique duringin situ bioremediation

Spread-plating belongs to traditional microbiological methods employed for quantification of subsurface microflora during bioremediation projects in the Czechia. Concentration of degrading organisms is estimated from the number of colonies grown on agar plates supplied with contaminant as the sole carbon source. The data obtained duringin situ bioremediation of the Dačice site contaminated by cutting oil suggests that changes in the composition of the carbon source in the subsurface may cause a discrepancy between laboratory data and situation in subsurface.     

Mode of action of an oleophilic nutrient in enhancing 14C-hexadecane mineralization by a marine bacterial community

Hexadecane mineralization by a bacterial consortium isolated from a non heavily contaminated coastal seawater was largely increased by an enrichment with Inipol EAP 22 (10% w/w). In the conditions of the experiment, the efficiency of this oleophilic nutrient is linked to the organic N and P sources (urea and lauryl-phosphate) and, to a lesser extent, to the presence of a readily available carbon source (oleic acid). Otherwise, the micro emulsion nature of the formulation and its physical properties seem to play a major role in accelerating hexadecane mineralization.     

Enrichment of dissimilatory Fe(III)-reducing bacteria from groundwater of the Siklós BTEX-contaminated site (Hungary)

Dissimilatory iron-reducing bacteria are commonly found in microbial communities of aromatic hydrocarbon-contaminated subsurface environments where they often play key role in the degradation of the contaminants. The Siklós benzene, toluene, ethylbenzene, and xylene (BTEX)-contaminated area is one of the best characterized petroleum hydrocarbon-contaminated sites of Hungary. Continuous monitoring of the microbial community in the center of the contaminant plume indicated the presence of an emerging Geobacter population and a Rhodoferax phylotype highly associated with aromatic hydrocarbon-contaminated subsurface environments. The aim of the present study was to make an initial effort to enrich Rhodoferax-related and other dissimilatory iron-reducing bacteria from this environment. Accordingly, four slightly different freshwater media were used to enrich Fe(III) reducers, differing only in the form of nitrogen source (organic, inorganic nitrogen or gaseous headspace nitrogen). Although enrichment of the desired Rhodoferax phylotype was not succeeded, Geobacter-related bacteria were readily enriched. Moreover, the different nitrogen sources caused the enrichment of different Geobacter species. Investigation of the diversity of benzylsuccinate synthase gene both in the enrichments and in the initial groundwater sample indicated that the Geobacter population in the center of the contaminant plume may not play a significant role in the anaerobic degradation of toluene.     

A comparison of the ability of forest and agricultural soils to mineralize chlorinated aromatic compounds

Soils were sampled from two agricultural fields, two relatively pristine forests, and one suburban forest in Ontario, Canada. The ability of these soils to mineralize 2,4-dichlorophenoxyacetate, 3-chlorobenzoate, 4-chlorophenol, 2,4-dichlorophenol, pentachlorophenol, and atrazine was determined using ¹⁴C-labeled substrates. Direct preexposure was necessary before atrazine mineralization could be detected; however, it was not necessary for degradation of any of the other chemicals. 2,4-dichlorophenoxyacetate and pentachlorophenol mineralization was much higher in the agricultural soils relative to the pristine forest soils, but 3-chlorobenzoate and 2,4-dichlorophenol mineralization rates showed the opposite trend. Mineralization of 4-chlorophenol was about equivalent in all soils. Suburban forests soils were indistinguishable from agricultural soils with respect to their degradation of 2,4-dichlorophenoxyacetate and chlorobenzoate. Additionally, they were better able than any of the soils to withstand the toxic effects of pentachlorophenol. Pentachlorophenol mineralization was highly variable in the pristine forest soils, ranging from about 6 to 50%. Abiotic factors such as pH, soil type, and organic and moisture content did not account for these significant site differences. The selective forces responsible for these differences, and the possible differences in microbial populations are discussed.     

Enhancing the mineralization of [U-14C]dibenzo-p-dioxin in three different soils by addition of organic substrate or inoculation with white-rot fungi

The potential for aerobic mineralization of [U-¹⁴C]dibenzo-p-dioxin (DD) was investigated in samples of three different agricultural soils already contaminated with polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/F) by industrial activities. The influence of amendments, i.e. wheat straw and compost, and of soil treatment by inoculation with lignolytic fungi, grown on wheat straw substrate, was tested. All the soils tested contained an indigenous DD-mineralizing microflora. The soil characterized by the highest organic matter content and the highest content of soil microbial biomass displayed the best DD mineralization of 36.6% within 70 days, compared with the two organic-matter-poor soils with an endogenous DD mineralization of 19.5% and 23.3% respectively. Amendments with compost increased DD mineralization up to 28% in both soils with low organic matter and microbial biomass content, but did not affect mineralization in the organic-matter-rich soil. Addition of wheat straw had no constant influence on DD mineralization in the soils tested. The best DD mineralization resulted from inoculation with lignolytic white-rot fungi (Phanerochaete chrysosporium, Pleurotus sp. Florida, Dichomitus squalens) and with an unidentified lignolytic fungus, which was isolated originally from a long-term PCDD/F-contaminated soil. A mineralization of up to 50% within 70 days was reached by this treatment. The influence of inoculated fungi on mineralization differed between the soils investigated. Received: 14 April 1997 / Received revision: 24 June 1997 / Accepted: 29 June 1997