collembola are one of the most abundant soil fauna in terrestrial ecosystems. they play essential roles in ecosystem processes like litter decomposition. ongoing biodiversity loss across taxa harms the stability and resilience of ecosystems and therefore threatens our sustainable development. recent evidence has shown that biodiversity loss negatively impacts ecosystem processes and functions such as productivity, soil microbes, and the production of fine roots. despite the critical importance of soil collembola, our understanding of the effects of plant diversity on soil collembola remains uncertain. the purpose of this dissertation is first to summarize previous studies and reveal the general response of collembola to plant species diversity across ecosystems. the second objective is to test whether tree mixtures affect the collembola community in young boreal forests and if these mixture effects change with water conditions and stand ages. in my first study, by conducting a meta-analysis of 623 paired observations of plant mixtures and corresponding monocultures from 40 studies, i examined the effects of plant mixtures on soil fauna abundance and diversity across global terrestrial ecosystems and summarized consistent responses of soil fauna to plant species diversity across soil depths, ecosystem types, and climate conditions. i found that the diversity of soil fauna was on average 10% greater in plant mixtures than expected from corresponding monocultures. in contrast, the abundance of fauna did not respond to plant mixtures. importantly, plant mixture effects on both soil fauna abundance and diversity significantly increased with plant species richness in mixtures. moreover, the effects of plant mixtures on soil fauna abundance increased over time in diverse species mixtures. [...]