Nitrogen (N) enrichment has been pinpointed as a main driver for biodiversity change. Most of our knowledge of effects of increased N availability on ecosystems comes from northern Europe and America. Most other ecosystem types have been neglected. Although Mediterranean ecosystems are N-limited biodiversity hotspots, very little is known about the effects of N enrichment in these systems. In contribution to filling this gap, our study examined the short-term effects of N enrichment in a N-manipulation (doses and forms) field study of a severely nutrient-limited Mediterranean ecosystem located in a Natura 2000 site in Portugal. N availability (dose and forms) was modified by the addition of 40 and 80 kg N ha−1 year−1 as NH4NO3 or 40 kg N as NH4 + ha−1 year−1 (control plots are not fertilized) since January 2007. The studied ecosystem was highly N responsive, i.e., visible changes were seen within one year of N additions: vascular plant and soil microbial diversity (soil bacteria and arbuscular mycorrhizal fungal spores) increased. Also the N concentration in leaves and litter increased, while the carbon-to-nitrogen (C/N) ratio of leaves and litter decreased.