Elevated CO₂ reduces losses of plant diversity caused by nitrogen deposition

The interactive effects of rising atmospheric carbon dioxide (CO ₂) concentrations and elevated nitrogen (N) deposition on plant diversity are not well understood. This is of concern because both factors are important components of global environmental change and because each might suppress diversity, with their combined effects possibly additive or synergistic. In a long-term open-air experiment, grassland assemblages planted with 16 species were grown under all combinations of ambient and elevated CO₂ and ambient and elevated N. Over 10 years, elevated N reduced species richness by 16% at ambient CO₂ but by just 8% at elevated CO₂. This resulted from multiple effects of CO₂ and N on plant traits and soil resources that altered competitive interactions among species. Elevated CO₂ thus ameliorated the negative effects of N enrichment on species richness.