Microorganism

The responses of three decomposer groups (earthworms, springtails and microorganisms) to manipulations in plant species diversity (1, 2, 4, 8), plant functional group diversity (1, 2, 3, 4) and functional group identity (grasses, legumes, small herbs, tall herbs) were studied in a microcosm experiment. Separate and combined treatments with earthworms and springtails were set up. Two earthworm species representing major functional groups of earthworms in grasslands were investigated, the endogeic species Aporrectodea caliginosa (Savigny) and the anecic species Lumbricus terrestris L. For springtails three species were investigated, the hemiedaphic species Heteromurus nitidus (Leleup), Folsomia candida (Willem) and the euedaphic species Protaphorura fimata (Gisin). Plant species and functional group diversity beneficially affected A. caliginosa (increase in body weight and incorporation of 15N from labelled litter) and P. fimata (density), presumably by changing the quality of belowground resources. In contrast, the biomass of L. terrestris decreased with plant species diversity but only in presence of legumes. For H. nitidus and F. candida the identity of plant functional groups was more important than plant species diversity per se. Also, the response of F. candida depended on earthworms. Microbial respiration was reduced by earthworms in more diverse plant communities, which correlated with root biomass. In contrast, microbial biomass was not affected by plant species diversity. The results suggest that belowground resource inputs from plant roots strongly modify decomposer performance and that the quality of the resources that enter the belowground subsystem is more important than their quantity. The responses of decomposers generally were not correlated with below- or aboveground plant productivity. In addition, the results document that effects of plant community composition on the performance of decomposer species depend on the presence of other decomposers.