Investigating forest soil biodiversity is essential to increase our understanding of ecosystem functions, assess potential consequences of global change, and thus optimize future decision-making processes. This cumulative PhD thesis contributes to this field by elucidating responses of bacterial and fungal forest soil communities, and their associated functions, in relation to tree diversity using a trait-based ecological approach with a focus on microbial living strategies. The three main chapters investigated microbial communities, using PCR-amplicon molecular methods, bioinformatics and novel statistics in the frame of the SoilForEUROPE project funded through the 2015–2016 BiodivERsA COFUND call for research proposals. Links between above-belowground biodiversity are crucial to understand forest functionality. For instance, studies on relationships of tree diversity and tree identity with microbial diversity reveal shifts in litter decomposition, nutrient cycling, primary production and the regulation of greenhouse gas emissions. These kinds of studies commonly compare microbial populations of different tree taxonomical groups. However, the effects of different tree taxa on microorganisms are mediated by tree morphology, physiology, phenology and genetics. Therefore, the use of specific plant traits to study biodiversity has become more frequent, adding a mechanistic understanding of compositional or functional shifts and interactions with soil microbial communities. This generalizable approach provides a common currency to compare similar microbial communities from different regions or environments with few microbial taxa in common. Microbial communities are also filtered by other processes such as global drivers, stochastic events, abiotic and biotic factors in addition to the mentioned tree traits. This environmental filtering process results in a functional microbial community structure, also with their own set of traits to increase their population size through higher performance and as response the capacity to affect their own ecosystem. Furthermore, it is expected that a particular set of microbial traits represents the life history strategies that favored a particular community under specific environmental conditions. This thesis correlates tree traits with bacterial and fungal communities by using a wide-ranged European forest platform with 64 plots of four different latitudinal regions. The SoilForEUROPE design also included multispecies and monospecific forests comprising 13 main tree species and 33 different tree species compositions. All these conditions supplied a diversity of environments to improve our knowledge of microbial soil diversity and above-belowground interactions. The here presented thesis encompasses five individual chapters. Chapter 1 provides the research context, project presentation and the main approach used. The Chapters 2 and 3 were developed in association with colleagues from the University of Freiburg and investigate four major European forest types: boreal forests (Finland), hemi‐boreal forests (Poland), mountainous beech forests (Romania) and thermophilous deciduous forests (Italy). Chapter 4 focuses purely on temperate forest plots and Chapter 5 compiles and concludes the results and presented ecological meanings. In particular, Chapter 2 evaluated the influence of tree species composition and diversity on fungal diversity and community composition, and highlights the relationships of fungal guilds and enzymatic activities with tree traits in detail, while also taking environmental variables into account. We demonstrated, how guilds like fungal saprotrophs mirror the litter quality, while tree root traits are often linked to an increasing number of fungal symbiotrophs. We found that forest types of higher latitudes, which are dominated by fast tree communities, correlated with high carbon‐cycling enzymatic activities. In contrast, Mediterranean forests with slow tree communities showed high enzymatic activities related to nitrogen and phosphorus cycles. In Chapter 3, we investigated links between bacterial communities, their functionality and root trait dispersion. Bacterial diversity revealed no major changes across the root functional dispersion gradient. In contrast, predicted gene profiles linked to plant growth activities suggested an increasing bacterial functionality from monospecific to multispecies forest. We also exposed that in multispecies forests, the bacterial functionality declines with the increasing functional dispersion of the roots. We further revealed important effects of the tree species identity on bacterial community composition, but we did not find significant relationships with root functional dispersion. However, bacterial network analyses indicated that multispecies forest have a higher complexity in their bacterial communities, which points towards more stable forest systems with greater functionality. Chapter 4 aimed to explore microbial communities of different soil depths from 0 to 30 cm across forests covering deciduous, evergreen and mixtures plots. Microbial abundance and diversity were especially affected by soil depth and by the presence of evergreen trees. Results showed higher accuracy to detect niche preference by using taxonomy levels than metabolic pathways or fungal guilds as features of a machine learning model. We found that bacterial communities are primarily shaped by soil depth in contrast to fungal community, which were rather influenced by the forest composition. Results also supported the importance of mixed forest to maintain nutrient cycling and a broad diversity of metabolites compared to monospecific forest and this differences where particular perceived in the upper 10 cm of soil. Chapter 5 concludes the thesis and presents a few remarks highlighting microbial strategies that might be favored under a particular soil forest composition. Overall, this thesis not only revealed the ecological patterns of soil forest microbial communities, but also provides a practical tool with necessary information to support decision-making and enlarge the schemes to conserve soil biodiversity.