What is it about?
Tree species functional group is more important for soil carbon stock and soil nutrient status than tree species diversity across six major European forest types Seid Muhie Dawud, Karsten Raulund-Rasmussen, Sophia Ratcliffe, Timo Domisch, Leena Finér, François-Xavier Joly, Stephan Hättenschwiler, Lars Vesterdal Forests deliver a range of ecosystem services to society, such as climate regulation, by storing carbon (C) in biomass and in soil, while other services such as maintaining soil nutrient status support the provision of sustained wood production. Selection of tree species can influence soil properties, but it remains unclear whether conversion to more species-diverse forests can increase soil C stock and influence soil nutrient parameters. Mixed forests composed of tree species with complementary traits and growth habits would possibly explore resources more efficiently than single-species forests, thereby improving forest ecosystem functioning. Studies of wood production and aboveground C stocks have supported this idea, but the influence of tree species diversity on soil C stock and soil nutrient parameters remains unexplored for common forest types at the European scale. We used 209 plots from the FunDivEUROPE project covering six major forest types of Europe with a wide spatial distribution to explore the effects of tree species diversity (1 to 5 tree species) and tree species type (conifers vs. broadleaves) on soil C stock and soil nutrient status as indicated by C/N ratio and pH. We found a modest, but consistent positive effect of tree species diversity on soil C stocks across the six forest types, whereas effects on soil nutrient status were region-dependent. Tree species type was more important than tree species diversity; soil C stocks increased and soil nutrient status decreased with an increasing share of conifers in the forests, but the impacts differed between forest types. The results imply that targeted selection of tree species with desired characteristics (e.g. complementary traits for effective resource use) is a preferred management approach for influencing soil C stock, C/N ratio and pH in mixed forests rather than increasing the number of tree species per se. Such information is needed to evaluate consequences of ongoing species loss, but also to develop sound management approaches for the selection of tree species mixtures to support soil nutrient status and increasing C stocks in forest ecosystems adapted to climate change.