The intensive industrial activities of the 20th century have left behind highly contaminated wasteland soils. It is well known that soil parameters and the presence of pollutants shape microbial communities. But in such industrial waste sites, the soil multi-contamination with organic (polycyclic aromatic hydrocarbons, PAH) and metallic (Zn, Pb, Cd) pollutants and long-term exposure may induce a selection pressure on microbial communities that may modify soil functioning. The aim of our study was to evaluate the impact of long-term multi-contamination and soil characteristics on bacterial taxonomic and functional diversity as related to the carbon cycle. We worked on 10 soils from northeast of France distributed into 3 groups (low anthropised controls, slag heaps, and settling ponds) based on their physico-chemical properties (texture, C, N) and pollution level. We assessed bacterial taxonomic diversity by 16S rDNA Illumina sequencing, and functional diversity using Biolog® and MicroResp™ microtiter plate tools. Although taxonomic diversity at the phylum level was not different among the soil groups, many OTUs were influenced by metal or PAH pollution, and by soil texture and total nitrogen content. Functional diversity was not influenced by PAH contamination while metal pollution selected microbial communities with reduced metabolic functional diversity but more tolerant to zinc. Limited microbial utilisation of carbon substrates in metal-polluted soils was mainly due to the nitrogen content. Based on these two observations, we hypothesised that reduced microbial activity and lower carbon-cycle-related functional diversity may have contributed to the accumulation of organic matter in the soils that exhibited the highest levels of metal pollution.

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Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous soil organic pollutants. Although PAH-degrading bacteria are present in almost all soils, their selection and enrichment have been shown in historically high PAH contaminated soils. We can wonder if the effectiveness of PAH biodegradation and the PAH-degrading bacterial diversity differ among soils. The stable isotope probing (SIP) technique with 13C-phenanthrene (PHE) as a model PAH was used to: (i) compare for the first time a range of 10 soils with various PAH contamination levels, (ii) determine their PHE-degradation efficiency and (iii) identify the active PHE-degraders using 16S rRNA gene amplicon sequencing from 13C-labeled DNA. Surprisingly, the PHE degradation rate was not directly correlated to the initial level of total PAHs and phenanthrene in the soils, but was mostly explained by the initial abundance and richness of soil bacterial communities. A large diversity of PAH-degrading bacteria was identified for seven of the soils, with differences among soils. In the soils where the PHE degradation activities were the higher, Mycobacterium species were always the dominant active PHE degraders. A positive correlation between PHE-degradation level and the diversity of active PHE-degraders (Shannon index) supported the hypothesis that cooperation between strains led to a more efficient PAH degradation.

Polycyclic aromatic hydrocarbons (PAHs) and metals are contaminants of industrial brownfield soils. Pollutants can have harmful effects on fungi, which are major actors of soil functioning. Our objective was to highlight fungal selection following long-term contamination of soils. Fungal diversity was assessed on 30 top-soil samples from ten sites gathered in three groups with different contamination levels and physico-chemical characteristics: 1) uncontaminated controls, 2) slag heaps displaying high PAH and moderate metal contaminations, and 3) settling ponds displaying high metal and intermediate PAH contaminations. Although fungal abundance and richness were similar among the soil groups, the diversity and evenness indices were lower for the slag heap group. Fungal diversity differed among soil groups at the phylum and OTU levels, and indicator species were identified. The relative abundance of Agaricomycetes, Saccharomycetes, Leotiomycetes and Chytridiomycota was higher in the control soils than in the two groups of contaminated soils. Cryptomycota LKM11 representatives were favoured in the slag heap and settling pond groups, and their relative abundance was correlated to the zinc and lead contamination levels. Dothideomycetespositively linked to PAH contaminationand Eurotiomycetes were specific to the slag heap group. Pucciniomycetes and especially Gymnosporangium members were favoured in the settling pond soils.