Relationship between plant and soil microbial communities along a successional gradient in a chalk grassland in north-western France

Plant and microbial communities were compared through their structure and their functions in a chalk grassland in north-western France. Multivariate statistical analyses allowed us to establish floristic patterns and to identify four stages of succession from short grasslands to early stages of forest. In each stage, several measurements were made on vegetation (floristic data, biomass weight and lignin content), on soil (total carbon, soluble organic carbon, total nitrogen, polysaccharides and phenols contents, lignin content in litter) and on soil microbial communities (microbial biomass, functional structure assessed by using BIOLOG analysis, genetic structure of bacterial and fungal communities assessed by using Amplified Ribosomal DNA Restriction Analysis (ARDRA) for 16S and 18S rDNA: ARDRA 16S and 18S). Although the organic matter resources that support soil microbial communities are larger in older stages of plant succession (high plant and litter biomass and nitrogen content in plant leaves in forest stages), results show that biomass and potential level of enzymatic activity of microbial communities are negatively correlated with the presence of shrubs and positively associated with grassland habitats. These microbiological variables also present complex interactions with other factors and are positively correlated with soil humidity, soil soluble carbon and polysaccharides contents and negatively correlated with the presence of lignin in plant biomass and litter. The results show out that microbial communities are independent from the successional gradient of vegetation, in terms of functional (BIOLOG analysis) and genetic structure (ARDRA 16S and 18S). This study provides a first step in understanding the link between the biodiversity (number and type of plant species, genetic structure of microbial community) and function (biomass production, enzymatic catabolism) in chalk grassland ecosystems.