Tree functional diversity influences belowground ecosystem functioning

•Leaf traits influenced enzymatic activities, dsDNA, soil C and N.•Plant functional diversity influenced soil enzymatic activities.•Synergistic effects of tree mixtures on soil enzymatic activities were found.•We registered an overall decrease in soil C and N twelve years after tree planting.

Many ecosystem processes in forest ecosystems are influenced by tree species richness and tree functional diversity (FD). Several studies, mainly in grasslands, have already underlined a positive effect of plant species richness on soil carbon (C) storage, but evidence for such a relationship for forests is scarce and not much is known about the role of FD. In this study, we investigated the impact of trees with contrasting functional litter traits on soil C and nitrogen (N) storage in a forest plantation on a former grassland. In addition, we also investigated the impact of increasing FD on six different soil enzymes, considered as proximate agents of potential microbial mineralization processes. We found synergistic effects of tree mixtures on soil enzymatic activities at the highest FD levels and an overall increase in soil mineralization potential with FD within tree mixtures. Moreover, we registered an overall decrease in soil C and N stocks 12 years after tree planting. Our results suggest that the selection of tree species and mixtures based on functional traits influencing soil C storage is fundamental for the success of climate change mitigation strategies employing tree plantations on abandoned pastures or grasslands.