Nitrogen (N) deposition is a primary driver of species loss in plant communities globally and especially in drylands. However, the mechanisms by which high levels of N cause species loss remain unclear. Many hypotheses for species loss with increasing N availability highlight density dependent mechanisms, i.e., changes in species interactions. Alternatively, another set of hypotheses highlights density-independent detrimental effects of nitrogen (e.g., N toxicity, changes in soil microbiome).
We tested the role of density-dependent and density-independent mechanisms in reducing species performance in a dry Mediterranean system. For this aim, we used 120 experimental plant communities (mesocosms) comprised of annual species growing together in containers filled with soils with varying water availability under four fertilization treatments: (1) no nutrient addition (control), (2) all nutrients except N, (P, K, and micronutrients), (3) Low N (3gN m-2 ) + other nutrients, and (4) high N (15gN m-2 ) + other nutrients. Each fertilization treatment included two sowing densities to differentiate between the effects of competition (which arise only at high density) 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 dominant 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 palaestinea) 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.