Carbon substrate utilization and microbial biomass in European forest soils are related to tree species diversity

Tree species influence biogeochemical cycling through element deposition (throughfall, litterfall), root decomposition and exudates, and through their influence on the microbial activities in the soil. Yet, the effect of mixing tree species on soil functioning is unclear, in particular concerning the microbial diversity and activity in soils.
Here we synthesize results from the Exploratory Platform of the FunDivEUROPE project (http://www.fundiveurope.eu/). This network of 209 comparative plots covering tree diversity levels of 1 to 5 species was established in existing mature forests in 6 European regions. These six focal regions represent a gradient of major European forest types from boreal to Mediterranean forests. The aims of this study were to determine the soil microbial biomass and metabolic diversity of soil bacteria for these 6 European forest regions, presenting each a tree species richness gradient and to analyse the impact of tree species richness and the role of other controlling factors.
We analysed the relation between tree species diversity, the proportion of coniferous tree species and soil factors (pH, soil organic carbon, water soluble carbon and nitrogen) and the carbon substrate utilisation pattern of soil bacteria (BIOLOG Ecoplate), soil microbial biomass (fumigation-extraction), hot water carbon and nitrogen in the forest floor and the upper mineral soil horizon (linear mixed models, GLM for multivariate abundance data, discriminant correspondence analysis).
Mean values of microbial biomass carbon ranged from 3264 (Italy) to 8717 (Finland) mg kg-1 in the forest floor and from 465 (Italy) to 3748 (Finland) mg kg-1 in the mineral soil. Statistical models predicted microbial biomass to increase in both soil layers by 7-8% with each step increase in tree diversity. Increased proportion of conifers was linked to a decrease in the number of carbon substrates used by soil bacteria. The types of carbon sources used were dependent on region, proportion of conifers, soil pH and water-soluble carbon and nitrogen.