Nitrogen (N) is one of the key elements affecting the diversity,dynamics and functioning of terrestrial, freshwater and marineecosystems. Many organisms have adapted to low levels of N(Vitousek et al. 1997); moreover, the availability of N stronglyinfluences the growth and abundance of organisms (Vitousek andHowarth 1991). Anthropogenic N deposition sources, like agricul-ture or combustion of fossil fuels, thus exert strong effects onthe biodiversity and functioning of many ecosystems (Sala et al.2000). In N-limited ecosystems, such as forests, changes in herba-ceous layer biodiversity and composition are of special interestdue to their functional relevance and fast response to changes in Navailability