Relationships between alien plant species and their aboveground effects have been relatively well studied, but little is known about the effects of invasive plants on belowground faunal communities. Nematodes are abundant, ubiquitous and diverse soil biota, and alterations of their community compositions can illustrate changes in belowground ecosystems. In 2016 and 2017, we determined the response of species diversity, community composition and trophic composition of the soil nematode communities to invasion by the alien plant Solidago gigantea in two ecosystems, forest and grassland, where invasion takes place. Nematode abundance was higher and number of identified nematode species was lower at invaded than uninvaded sites, indicated by lower species diversity, regardless of ecosystem. Herbivorous nematodes were the most affected trophic group. Herbivore abundance was higher at invaded than uninvaded sites and in grassland than forest. The herbivorous species Boleodorus thylactus, Geocenamus sp., Helicotylenchus spp., Paratylenchus bukowinensis, Pratylenchoides crenicauda and Rotylenchus robustus were more abundant at the invaded sites. Abundances of nematodes in the other tropic groups were limited or not affected. The invasion did not significantly affect the ecological and functional indices, except for the Channel Index in 2016. Differences were observed in values of Enrichment Index (indicator of resource availability), Channel Index (indicator of ascendant bacterial/fungal decomposition channel) and Basal Index (indicator of depleted-perturbed soil food webs) between grassland and forests. We can thus conclude that invasion by S. gigantea significantly alters nematode community indicators (abundance, species diversity and specific trophic groups); however, this effect seems to be significantly influenced by the type of ecosystem where invasion takes place.