Do soil animals control litter diversity effects on decomposition?

Plant litter diversity can be functionally important for decomposition and nutrient recycling. The underlying mechanisms for diversity effects, however, are poorly understood, in particular potential interactions across trophic levels. Large litter feeding soil animals might control litter diversity effects through altered food selection and consumption rates in response to changing litter species composition. In recent field and laboratory experiments in an European temperate forest, an Amazonian tropical rainforest, and a Mediterranean forest, we tested the interactions between changing litter diversity and macrofauna. Results indicate that leaf litter decomposition from individual tree species change in the presence of other species, and that macrofauna can strongly influence litter diversity effects on litter turnover and nutrient release. In the temperate forest, for example, we observed a more rapid decomposition of relatively recalcitrant litter in mixtures, which is driven by millipede presence. Contrary to our expectation, macrofauna had a relatively weak overall influence on mass loss rates of litter mixtures in the tropical rainforest. However, nutrient release rates from litter mixtures decreased when macrofauna was present. Macrofauna abundance is particularly high in Mediterranean forests. Ongoing experiments test the complementarity hypothesis of macrofauna species diversity within a single functional group, and whether complementarity depends on litter diversity. We conclude that the interactions between litter species diversity and soil animals can have important implications for nutrient supply rates and may influence the fate of carbon and nutrient cycling in forest ecosystems, but the relative importance may vary among different ecosystems.