The effects of alien plants on whole nutrient cycles have been scarcely studied, despite the increasing evidence on their impact on nutrient pools and fluxes. Carpobrotus edulis, a dangerous invasive plant in coastal areas worldwide, is considered an ecosystems engineer which, by changing many soil properties, benefits its own invasion and hampers the restoration of the invaded habitats. To study, for the first time, the ‘true’ impact of C. edulis on the soil N cycle, we used a paired 15N labelling experiment and a Ntrace compartment model to estimate the gross N fluxes in the 0–5 cm and 5–10 cm soil layers of non-invaded and C. edulis invaded areas of two temperate-humid coastal rocky locations. Carpobrotus edulis invasion generally increased NH4+ immobilization (INH4, 1.19–4.48 ×), presumably due to a lower N availability for the microbiota. The invasion also decreased autotrophic nitrification (ONH4, 0.20–0.79 ×), either by a direct effect over soil microbiota or by the acidification triggered by C. edulis. Unexpectedly, the dissimilatory nitrate reduction (DNRA) was the exclusive NO3− consuming process modelled on most of the studied soils, although the incubation was aerobic. Apparently, the high organic C content of these soils induced a higher O2 consumption and the formation of anaerobic microsites where the DNRA could have taken place. The lower NO3− availability of invaded soils could explain their lower DNRA rates (0.04–0.70 ×) compared to native soils. Both DNRA and ONH4 were more affected in the 0–5 cm layer, but the invasion also significantly affected N rates in the 5–10 cm layer. Overall, net nitrification and mineralization generally decreased in the invaded soils. This study shows that the invasion of C. edulis alters soil gross and net N fluxes in a 0–10 cm depth through its effects on soil properties and microbiota.