High CO₂ dampens then amplifies N-induced diversity loss over 24 years

Rising levels of atmospheric carbon dioxide (CO₂) and nitrogen (N) deposition affect plant communities in numerous ways^1-11. Nitrogen deposition causes local biodiversity loss globally^12-14, but whether, and if so how, rising CO₂ concentrations amplify or dampen those losses remains unclear and is almost entirely unstudied. We addressed this knowledge gap with an open-air experiment in which 108 grassland plots were grown for 24 years under different CO₂ and N regimes. We initially found that adding N reduced plant species richness less at elevated than at ambient CO₂. Over time, however, this interaction reversed, and elevated CO₂ amplified losses in diversity from enriched N, tripling reductions in species richness from N addition over the last eight years of the study. These interactions resulted from temporal changes in the drivers of diversity, especially light availability, that were in turn driven by CO₂ and N inputs and associated changes in plant biomass. This mechanism is likely to be similar in many grasslands, because additions of the plant resources CO₂ and N are likely to increase the abundance of the dominant species. If rising CO₂ generally exacerbates the widespread negative impacts of N deposition on plant diversity, this bodes poorly for the conservation of grassland biodiversity worldwide.