Above and belowground biodiversity in adjacent and distinct serpentine soils

In this study we analysed the diversity of vascular plants, soil microorganisms and microarthropods on two serpentine outcrops in the Northern Apennines (Italy). The soils of the two sites are at different stages of evolution, as shown by soil depth and soil organic matter (SOM) content: the first site is a serpentine grassland clearing within sub-montane vegetation, while the second is a serpentinite scree rich in specialised flora only, including rare hyperaccumulator plant species such as Noccaea caerulescens and Alyssum bertolonii. The aboveground biodiversity was analysed via floristic relevés. The bacterial diversity was estimated through 16S rDNA profiling of the Ni hyperaccumulator N. caerulescens at the rhizosphere level, which thrives in both sites. Microarthropod communities were characterized by extracting and identifying organisms from the soil. The number of individuals per taxon, Acari/Collembola ratio, biodiversity indices and QBS-ar index were calculated. The two sites showed a clear difference as regards plant community: only three plant species were present in both sites, namely Euphorbia cyparissias, belonging to Euphorbiaceae, Galium spp., belonging to Rubiaceae, and N. caerulescens, belonging to Brassicaeae. Belowground diversity was positively correlated with vegetation cover and SOM content. In particular, among the bacteria colonising the rhizosphere of N. caerulescens (present in both sites), Actinobacteria, which exhibit K-strategist attributes and are more successful in resource-limited, crowded environments, appeared to be more abundant in the site with lower SOM and higher content of Ni. Bacterioidetes, which are specialised in degradation of cellulose, chitin and plant detritus, were instead more abundant in the site with higher SOM. As for soil microarthropod communities, the site with higher concentration and bioavailability of heavy metals and lower SOM showed poorer and less structured community. In conclusion, the data observed with plant, microbial and microarhropod communities identify different micro-habitats, in spite of proximal geographical position and the common metal-rich substrate.