Detritivores strongly influence soil community structure and ecosystem processes in a variety of ways, including nutrient cycling, above ground plant composition, primary and secondary production, as well as decomposition rates and carbon storage. Earthworms in particular are known to have an important impact on soil fertility and leaf litter decomposition. In this study we examined the interactions between soil macrofauna (earthworms) and leaf litter decomposition using laboratory microcosms. Specifically, we asked, how does resource quality affect interspecific and intraspecific interactions among soil fauna (earthworms), and what were the subsequent effects on organic matter decomposition (e.g., leaf decomposition, soil respiration)? Inter- and intraspecific interactions of two naturally coexisting earthworm species, Lumbricus rubellus and Octolasion lacteum, were examined in a factorial design, comprising twelve treatments: 0, 1, 2, 4, 8 earthworms in single and mixed species combinations. We combined a substitutive and additive design to specifically isolate the effects of inter- versus intraspecific between earthworm species. The experiment lasted six weeks, while L. rubellus and (or) O. lacteum fed on single or mixed leaf litter species of Liriodendron tulipfera and Quercus rubra. As predicted, there was preferential feeding of L. tulipfera over Q. rubra. L. rubellus biomass increased with a growth rate of approximately 4 mg/day in single or low-density microcosms with L. tulipfera as the leaf resource. However overall biomass decreased in mixed-litter interspecific and high-density intraspecific competition assays. Earthworms directly or indirectly increased soil respiration rates in all treatments with a maximum of threefold increase in the eight-earthworm treatment. Mixed earthworm species microcosms exhibited higher respiration rates than single species ones indicating a strong interaction effect. The results of this work highlight the complexity of the biotic interactions between and among species at two trophic levels, and the implications for organic matter decomposition in soils.