Rising atmospheric CO₂ reduces nitrogen availability in boreal forests

Anthropogenic nitrogen (N) pollution is a cause of eutrophication globally¹. However, recent datasets indicate that some ecosystems may be experiencing widespread oligotrophication-declining N availability-which is suggested to be a response to elevated atmospheric carbon dioxide (CO₂)². Plant N isotope (δ¹⁵N) chronologies have served as primary evidence for oligotrophication, but there is wide disagreement whether rising CO₂ or temporal changes in N deposition explain these patterns^3-6. Here we construct δ¹⁵N tree-ring chronologies using archived samples from Sweden's 23.5-million-hectare forest area from 1961 to 2018. The study area spans a 1,500-km latitudinal distance where N deposition varies fourfold, but where rising CO₂ is spatially uniform. Our data show declining δ¹⁵N chronologies throughout Sweden, including forests in the far north where atmospheric N deposition rates are very low. Linear mixed-effects models showed that rising CO₂ is the strongest predictor of δ¹⁵N values, whereas N deposition variables, temperature and forest basal area had lower explanatory power. Our findings suggest that elevated atmospheric CO₂ is causing oligotrophication in boreal forests, which has implications for predicting their future role as sinks in the global carbon cycle^7-9.