Functional diversity affects decomposition processes in experimental grasslands

1. A number of biodiversity experiments have shown that plant diversity plays a significant role for ecosystem functioning. However, diversity effects on processes involving multi-trophic interactions such as litter decomposition are rather rare. In these experiments, plant diversity is usually categorized into taxonomic units or functional groups. Continuous measures of functional diversity that are based on measurable traits, in contrast, may be a more flexible way to analyse the functional significance of biodiversity. 2. Litter decomposition is a key process in ecosystem biogeochemistry. To understand the consequences of altered biodiversity for ecosystem functioning, it is thus crucial to quantify any potential diversity effects on decomposition processes. 3. I performed several complementary decomposition trials within the BIODiversity and Ecological Processes in Terrestrial Herbaceous ecosystems (BIODEPTH) experiment, which established a gradient of plant species richness and number of functional groups. I hypothesized that decomposition rates increase with increasing plant diversity due to non-additive litter mixing effects and more favourable microenvironmental conditions. 4. Decomposition rates of both standard materials and community-specific litter increased with the number of functional groups and with a continuous measure of functional diversity. Species richness, in contrast, had no or rather small positive effects on decomposition. Presence of nitrogen-fixing legumes strongly enhanced decomposition, via effects on both litter quality and on the decomposition microenvironment. 5. The predominance of functional diversity rather than of species richness effects suggests that profound knowledge on functional attributes of plant species is the key to understand and to predict biodiversity-decomposition relationships.