Synergistic effects of microbial and animal decomposers on plant and herbivore performance

Decomposers drive essential ecosystem functions, such as organic matter turnover and nutrient cycling, thereby functioning as key determinants of soil fertility and nutrient uptake by plants. However, knowledge of interacting effects of functional dissimilar decomposer groups, such as microorganisms and animals, on aboveground functions is scarce.We set up a microcosm experiment to investigate single and combined effects of microbial (the fungus Fusarium graminearum) and animal decomposers (the earthworm Aporrectodea caliginosa) on the performance of winter wheat (Triticum aestivum) and aphids (Rhopalosiphum padi) in a full factorial design. We tested the shape of response of every variable in order to explore if interacting impacts of decomposers are under-additive (logarithmic fit), additive (linear fit) or over-additive (quadratic and exponential fit).Both microbial and animal decomposers increased the majority of the studied plant and herbivore performance parameters. While decomposers had additive effects on five plant performance variables they had over-additive effects on seven plant variables and three herbivore variables.The dominance of over-additive effects suggests positive interactions between microbial and animal decomposers. Facilitation in the decomposition process most likely synergistically increased nutrient supply for plants and food availability and quality for aphids.The present study indicates that functionally dissimilar decomposer groups of different kingdoms synergistically impact plant performance. Further, these beneficial effects propagated to herbivores suggesting that belowground functional diversity and positive interactions alter essential aboveground ecosystem functions over several trophic levels.