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

Abstract Forests provide vital ecosystem services, particularly as carbon sinks for nature-based climate solutions. However, the global impact of elevated atmospheric carbon dioxide (CO2) levels on carbon and nitrogen interactions of forests remains poorly quantified. We integrate elevated CO2 experimental observations and biogeochemical cycle model to elucidate the synergies between enhanced nitrogen and carbon cycling in global forests under elevated CO2. Elevated CO2 levels alone increase net primary productivity by 26% (95% CI, 21-30%) and leaf C:N ratio by 32% (18-46%), while stimulating biological nitrogen fixation by 72% (27-136%) and nitrogen use efficiency by 22% (8-38%). Under the elevated CO2 middle road scenario for 2050, forest carbon sink is projected to increase by 0.32 billion tonnes (PgC), with forest products increasing by 4 million tonnes (Tg) nitrogen, reactive nitrogen loss to the environment decreasing by 8 Tg, and fertilizer input decreasing by 4 Tg nitrogen relative to the baseline scenario. The monetary impact assessment of the direct elevated CO2 impact on forests represents a social value of US$292 billion. These findings should inform the development of forest management strategies for future climate change adaptation and mitigation.