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 patterns3–6. 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 cycle7–9.
