  Nitrogen (N) deposition is a primary driver of species loss in plant
communities globally. However, the mechanisms by which high N availability
causes species loss remain unclear. Many hypotheses for species loss with
increasing N availability highlight density-dependent mechanisms, i.e., changes
in species interactions. However, an alternative set of hypotheses highlights
density-independent detrimental effects of nitrogen (e.g., N toxicity). We
tested the role of density-dependent and density-independent mechanisms in
reducing species performance. For this aim, we used 120 experimental plant
communities comprised of annual species growing together in containers under
four fertilization treatments: (1) no nutrient addition(, (2) all nutrients
except N (P, K, and micronutrients), (3) Low N, and (4) high N. Each
fertilization treatment included two sowing densities to differentiate between
the effects of competition (N * density interactions) and other detrimental
effects of N. We focused on three performance attributes: the probability of
reaching the reproduction period, biomass growth, and population growth. We
found that individual biomass and population growth rates decreased with
increasing sowing density in all nutrient treatments, implying that species
interactions were predominantly negative. The common grass had a higher biomass
and population growth under N enrichment, regardless of sowing density. In
contrast, the legume showed a density-independent reduction in biomass growth
with increasing N. Lastly, the small forb showed a density-dependent reduction
in population growth, i.e., the decline occurred only under high density. Our
results demonstrate that density-dependent and density-independent mechanisms
operate simultaneously to reduce species performance under high N availability.
Yet, their relative importance varies among species and life stages.
