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Background Plant diversity influences ecosystem functioning. A positive relation between plant diversity and productivity above- and belowground has been established. Aboveground, this effect has been shown to be due to complementarity effects, interactions between species in a mixture that lead species to, on average, produce more biomass than expected based on their productivity in monoculture. The mechanisms underlying complementarity effects and the positive diversity-productivity relation are predicted to lie belowground, e.g. resource partitioning and/or facilitation. The relation between plant diversity and decomposition is less clear, and research on the diversity-decomposition relation belowground is limited. This is an important gap in biodiversity knowledge, as the decomposition of plant litter is the major source of nutrients and carbon in terrestrial ecosystems, and most plant litter in grasslands is belowground. Methods This thesis explored the effect of plant diversity on belowground productivity and decomposition. Belowground complementarity effects were quantified in the Jena Trait Based Experiment, and the diversity of or plasticity in species-specific vertical root distribution as underlying mechanism was tested. The plant diversity- root decomposition relation was quantified in the Jena Experiment and the Jena Trait Based Experiment. The role of root traits and the soil environment as mediating factors were tested. Major findings Plant diversity had a positive effect on root biomass production, and this relation was attributed to complementarity effects. The diversity in species-specific vertical root distribution did not explain complementarity effects, and thus, is not likely a major mechanism underlying the diversity-productivity relation. Species altered their vertical root distribution in response to inter-specific neighbours. The direction of this change differed between functional groups: grasses became shallower in mixture, forbs became deeper. This change did not explain species-specific belowground relative productivity (relative to monoculture productivity). Therefore, plasticity in vertical root distribution was not a major factor underlying belowground complementarity effects or the diversity-productivity relation. Functional group composition, not plant diversity, had a consistent effect on root decomposition. The presence or abundance of grasses consistently reduced root litter quality and decomposition. In the Jena Experiment, plant diversity had a negative effect on root decomposition, mainly due to shifts in functional group composition over a diversity gradient. In the Jena Trait Based Experiment, root decomposition was unaffected by plant diversity, but decreased as the abundance of grass roots increased. Root traits were found to be important in explaining variation in root decomposition. Conclusions Plant diversity had a positive effect on belowground productivity, which could be attributed to complementarity effects. Functional group composition, not plant diversity, had consistent effects on root decomposition. Root traits were important in explaining root decomposition throughout this thesis. Root traits may also be important in explaining complementarity effects, however, the diversity of or plasticity in vertical root distribution did not. This thesis highlights the role of belowground interactions in facilitating the positive diversity-productivity relation, and the role of plant functional groups and root traits in explaining how plant diversity alters root decomposition.

Root interactions in a diverse grassland : the role of root traits in belowground productivity and decomposition