The mining and burning of oil shale results in massive quantities of tailings and ash which is heaped in large hills in northeastern Estonia. The ash hills are a novel, harsh environment with alkaline soils containing high levels of heavy metals, phenols, and oil residue. Microbial colonization precedes and in harsh environments facilitates plant colonization in primary succession. However, the composition and functional diversity of early microbial communities is often unknown. Ash is deposited in tiers such that the oldest ash is at the bottom of the hill, with each tier representing a successive deposition. Sampling at different elevations serves as a surrogate for a chronosequence. We thus established a transect from the top to the base of the ash hill, collected soil samples every 5 meters, and assessed plant community composition at each sampling location. We also collected soils from the old-growth forest adjacent to the ash hill. We utilized 454 sequencing on DNA isolated from the soil samples to assess bacterial (16S rRNA) and fungal (mtLSU) biodiversity. Our sampling design allows the investigation of microbial community succession and the influence of plant colonization and plant community diversity on microbial community composition and function.
We estimated microbial biodiversity based on 95% sequence similarity to identify unique operational taxonomic units (OTUs). There was minimal overlap in the microbial community in the old-growth forest soils and at any sampling location on the ash hill. Microbial communities in the youngest ash soil have relatively low biodiversity dominated by saprotrophic bacterial and fungal taxa common to mine spoils throughout Europe. Interestingly, the earliest plant colonizers are obligate biotrophic terrestrial orchids, contrary to most models of primary plant succession. Microbial biodiversity and functional diversity increase sharply at the site of early plant colonization with both saprobic and biotrophic microbes present in the soil. Plant community complexity increases with the age of the ash soils. The microbial taxa present in these oldest ash soils are similar to those at the early plant colonization site. However, there is a pronounced shift in the functional composition: symbiotic mycorrhizal fungi dominate the soil microbial community. We suggest microbial community composition on the ash hills is shaped by environmental tolerance and available carbon sources. Early colonizers must tolerate a caustic environment with scant carbon resources. Once plants establish, sugars in the rhizosphere become the dominant carbon source.