Widespread foliage δ¹⁵N depletion under elevated CO₂: inferences for the nitrogen cycle

Leaf 15 N signature is a powerful tool that can provide an integrated assessment of the nitrogen (N) cycle and whether it is influenced by rising atmospheric CO 2 concentration. We tested the hypothesis that elevated CO 2 significantly changes foliage δ 15 N in a wide range of plant species and ecosystem types. This objective was achieved by determining the δ 15 N of foliage of 27 field‐grown plant species from six free‐air CO 2 enrichment (FACE) experiments representing desert, temperate forest, Mediterranean‐type, grassland prairie, and agricultural ecosystems. We found that within species, the δ 15 N of foliage produced under elevated CO 2 was significantly lower ( P <0.038) compared with that of foliage grown under ambient conditions. Further analysis of foliage δ 15 N by life form and growth habit revealed that the CO 2 effect was consistent across all functional groups tested. The examination of two chaparral shrubs grown for 6 years under a wide range of CO 2 concentrations (25–75 Pa) also showed a significant and negative correlation between growth CO 2 and leaf δ 15 N. In a select number of species, we measured bulk soil δ 15 N at a depth of 10 cm, and found that the observed depletion of foliage δ 15 N in response to elevated CO 2 was unrelated to changes in the soil δ 15 N. While the data suggest a strong influence of elevated CO 2 on the N cycle in diverse ecosystems, the exact site(s) at which elevated CO 2 alters fractionating processes of the N cycle remains unclear. We cannot rule out the fact that the pattern of foliage δ 15 N responses to elevated CO 2 reported here resulted from a general drop in δ 15 N of the source N, caused by soil‐driven processes. There is a stronger possibility, however, that the general depletion of foliage δ 15 N under high CO 2 may have resulted from changes in the fractionating processes within the plant/mycorrhizal system.