Elevated CO2 levels promote both carbon and nitrogen cycling in global forests

Forests provide vital ecosystem services, particularly as carbon sinks for nature-based climate solutions. However, the impact of elevated atmospheric carbon dioxide (CO2) levels on carbon and nitrogen interactions of forests remains poorly quantified. We integrate experimental observations and biogeochemical models to elucidate the synergies between enhanced nitrogen and carbon cycling in global forests under elevated CO2. Elevated CO2 alone increases net primary productivity (+27%; 95% CI: 23–31%) and leaf C/N ratio (+26%; 95% CI: 16–39%), while stimulating biological nitrogen fixation (+25%; 95% CI: 0–56%) and nitrogen use efficiency (+32%; 95% CI: 5–65%) according to a global meta-analysis. Under the elevated CO2 middle-road scenario for 2050, the forest carbon sink is projected to increase by 0.28 billion tonnes (PgC yr−1), with reactive nitrogen loss decreasing by 8 Tg yr−1 relative to the baseline. The monetary impact assessment of the elevated CO2 impact on forests represents a societal value of US$271 billion. Using a global meta-analysis approach, the authors show that elevated CO2 alone can increase primary productivity and leaf C/N ratio and stimulate nitrogen fixation and nitrogen use efficiency. They project increasing carbon sink and decreasing reactive nitrogen loss under climate change.