Whilst invasive plants can modify community and ecosystem attributes, there is little information on their impact across different trophic levels. Here, we examined the effects of Gunnera tinctoria Molina (Mirb.; Gunneraceae; “Chilean-rhubarb”) on the soil detrital food chain, the major pathway for energy flow in terrestrial ecosystems and the recycling of nutrients. Measurements of soil physio-chemistry properties and the natural abundances of C and N isotopes, along with assessments of microbial and earthworm populations were used to verify if G. tinctoria drives the soil food web and how C is transferred from decomposing litter to worm populations. Based on the C isotope compositions of plant material and detritivorous worms (i.e., earthworms and enchytraeid worms), all worms contribute to G. tinctoria litter decomposition, irrespective of their feeding preference. The increased role of larger earthworm populations in litter decomposition was also associated with changes in the soil microbial community, with some bacterial genera being absent from invaded areas. Of particular interest was a three-fold increase in Nitrospira spp. in invaded areas, likely due to an increase in earthworm-related ammonia production, with this being rapidly converted to nitrate. Given the poor representation of other ammonia and nitrite oxidizing soil bacteria, many of the Nitrospira spp. present in invaded soils may perform complete ammonia oxidation (i.e., COMAMMOX). Our results indicate a greater role for native earthworms in decomposition processes in areas invaded by G. tinctoria, resulting in modifications in the other soil biota and an increase in soil nitrate.