Rising atmospheric CO2 intensifies nitrogen limitation in boreal forests

Anthropogenic nitrogen (N) pollution has been emphasized as a cause of eutrophication globally. However, several recent datasets have suggested widespread oligotrophication may be occurring in some ecosystems, which is suggested to be a response to rising atmospheric carbon dioxide (eCO₂). Plant δ¹⁵N chronologies have served as primary evidence for oligotrophication, however, there has been wide disagreement whether eCO₂ or temporal changes in N deposition explain these patterns. We constructed δ¹⁵N tree ring chronologies across Sweden’s 23.5 million hectare productive forest area from the 1950s to 2010s. The study area spans a 1500 km latitudinal distance where N deposition varies four-fold, but where eCO₂ is spatially uniform. Our data revealed negative δ¹⁵N chronologies throughout Sweden, including forests in the far north where atmospheric N deposition rates are very low. Linear mixed effects models showed that eCO₂ was by far the strongest predictor of δ¹⁵N values, whereas N deposition variables, temperature, and forest basal area had much lower explanatory power. Our results clarify debates on the interpretation of previous δ¹⁵N chronologies, and provide clear evidence that eCO₂ is causing oligotrophication in boreal forests, which has implications for predicting their future role as sinks in the global carbon cycle.