Functional diversity of soil microbial communities under Scots pine, Norway spruce, silver birch and mixed boreal forests

• Plant diversity did not affect functional diversity of soil microbial communities.
• Forest vegetation affected physiological profiles of soil microbial communities.
• Differential use of carboxylic acids accounted for most variation in community-level physiological profiles.

Functional structure and diversity of forest soil microbial communities may depend on plant diversity. The objective of our study was to compare community level physiological profiles (CLPPs) of microbial communities from uppermost soil horizons under Scots pine (Pinus sylvestris), Norway spruce (Picea abies), silver birch (Betula pendula) and mixed pine–birch–spruce stands representing boreal forests differing in the identity of the dominant tree species and diversity of overstory and understory plant species in the system. The samples of O and A soil horizons were measured for pH, organic carbon, and total nitrogen. Microbial analyses included determination of microbial biomass (Cmic) and measurement of CLPPs with the MicroResp™ system. The studied forest types differed significantly in plant species diversity with birch and mixed forests being the most and the pine forest the least diverse. Functional microbial diversity only slightly differed between the studied forest types and was not related to plant diversity. Despite similar functional diversity of soil microbial communities there were significant differences in CLPPs in the O horizons under the pine, spruce and birch forests indicating that plant cover affects the rate at which particular organic compounds are degraded. Differences in physiological profiles between the birch, pine and spruce forest soils were mainly due to variable decomposition of readily decomposable organic compounds represented mainly by carboxylic acids.