Do invertebrate decomposers affect the disappearance rate of litter mixtures?

We designed an experiment using litter bags with fine and coarse mesh size to analyse interacting effects between invertebrate decomposers and the number of litter species on litter disappearance rates. We used litter of nine broad-leaved tree species to compare disappearance rates of litter from single species with mixtures of two to six species. Species composition of litter and invertebrates interacted strongly in their effects on litter disappearance rate. Contribution of invertebrates to litter disappearance increased with time mainly for litter which disappeared slower in the absence of invertebrates. Disappearance rates were positively correlated with initial N content and negatively correlated to initial C content of litter. These relationships were stronger in the presence of invertebrates, suggesting that their activity is positively related to initial litter chemistry. Number of component litter species, however, had no effect on disappearance rate irrespective of the activity of invertebrates. Using individual rates of disappearance for single species, we calculated the expected rates of disappearance for each of the experimental mixtures of leaf litters. We found that mixtures of several species of leaf litter resulted in significant deviations from the expected values. These deviations showed a significant effect of the number of component litter species. However, this result was caused by a strong negative deviation of one single mixture of six species. The presence of invertebrates resulted in even greater deviations from the expected values, suggesting an important contribution of invertebrates to the effects of litter mixing on litter disappearance rates. Hence, our results underline the importance of idiosyncratic effects of species traits in mixtures. Our results suggest that the influence of invertebrate decomposers interacts with litter chemistry during decomposition, but is not affected by litter species richness per se.