Change in heathland dominant plants strongly increases C mineralization potential despite marginally affecting microbial community structure

Abstract

Purpose In many ecosystems, the identity of the dominant plant is changing because of global change. If the new dominant species has different litter and root traits than the one it replaces, it is likely to have an influence on soil microbial communities and the functions they perform. We used a grass-encroached heathland, where dwarf shrubs are replaced by grasses with different ecological traits, as a case study to explore this question. We hypothesized that grass colonization of heathland would improve litter quality, which would favor soil copiotroph microbes and increase C mineralization rate. Methods We established a 13-plot field observatory spanning across a 0-100% gradient of grass cover percentage. In each plot, we characterized plant, fungal and bacterial communities, using a combination of ARISA (taxonomic diversity), metabarcoding plus hierarchical modelling of species communities (community structure), FDA assay (metabolic activity) and Biolog ecoplates (functional diversity and rate of C mineralization). Results Our results show that microbial taxonomic and functional diversities are not affected by grass colonization. Microbial communities were also similar at high phylogenetic level, including for ericoid mycorrhizas and typical oligo- and copiotrophic species. At a finer phylogenetic level, some abundant extremophilic OTUs (e.g Acidothermus bacteria) were progressively replaced by fungal black yeasts. Functional response of microbial communities was more obvious. The C mineralization potential significantly increased across the grass gradient. Conclusion Change in dominant plant traits may induce drastic functional changes in microbial communities despite having only a very minor effect on their diversity or structure.