Rising CO₂ concentrations reduce nitrogen availability in alpine grasslands

Alpine grasslands, local biodiversity hotspots with very high nature conservation and cultural value, belong to one of the most affected ecosystems by global change. Yet, the potential effects of others than global warming factors on alpine plant functioning are poorly understood. To address this gap, we made use of 359 herbarium specimens from nine vascular plant species collected in the Bavarian Alps, Germany, extending back 200 years (1807–2018) to reconstruct historical changes in foliar N content and stable isotope composition (δ¹⁵N), indicators of plant response to long-term N atmospheric deposition and rising atmospheric CO₂ concentrations ([CO₂]). These changes were interpreted in terms of three competing hypotheses (eutrophication, oligotrophication and photorespiration), representing alternative explanations for the response of plants to changes of N and CO₂ availability. Foliar δ¹⁵N decreased significantly over time but an explanation by an increased input of reactive N from long-distance transport ('eutrophication' hypothesis) was unlikely because foliar N contents decreased significantly as well. An increased carbon gain due to increasing [CO₂] (‘oligotrophication’) also was unlikely because instantaneous water use efficiency remained unchanged and indicated no increase in C gain. The detected patterns agreed well with the ‘photorespiration’ hypothesis that biochemically links N assimilation and C assimilation. Increasing concentration of ambient CO₂ that decreases photorespiration explained decreasing δ¹⁵N values (R² = 0.84, p < 0.001) and decreasing N contents (R² = 0.40, p < 0.036). Our results suggest that increasing [CO₂] by suppressing photorespiration reduces N availability to alpine plants. These findings contradict the generally accepted assumption of negative effects of eutrophication on alpine grasslands caused by air-borne N deposition. We conclude that increasing [CO₂] should be considered as an alternative driver of long-term changes in alpine ecosystems, as it affects directly the plant C:N stoichiometry, a key plant trait determining several important ecosystem processes.