Density-dependent and independent mechanisms jointly reduce species performance under nitrogen enrichment

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 (mesocosms) comprised of annual species growing together in containers under four fertilization treatments: 1) no nutrient addition (control), 2) all nutrients except N (P, K, and micronutrients), 3) low N (3 gN m⁻²) + other nutrients, and 4) high N (15 gN m⁻²) + other nutrients. 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 Avena barbata had a higher biomass and population growth under N enrichment, regardless of sowing density. In contrast, the legume Trifolium purpureum showed a density-independent reduction in biomass growth with increasing N. Lastly, the small forb Silene palaestina 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.