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

Leaf ¹⁵N signature is a powerful tool that can provide an integrated assessment of the nitrogen (N) cycle and whether it is influenced by rising atmospheric CO₂, concentration. We tested the hypothesis that elevated CO₂ significantly changes foliage δ¹⁵N in a wide range of plant species and ecosystem types. This objective was achieved by determining the δ¹⁵N of foliage of 27 field-grown plant species from six free-air CO₂, enrichment (FACE) experiments representing desert, temperate forest, Mediterranean-type, grassland prairie, and agricultural ecosystems. We found that within species, the δ¹⁵N of foliage produced under elevated CO₂ was significantly lower (P<0.038) compared with that of foliage grown under ambient conditions. Further analysis of foliage δ¹⁵N by life form and growth habit revealed that the CO₂ effect was consistent across all functional groups tested. The examination of two chaparral shrubs grown for 6 years under a wide range of CO₂ concentrations (25-75 Pa) also showed a significant and negative correlation between growth CO₂, and leaf δ¹⁵N. In a select number of species, we measured bulk soil δ¹⁵N at a depth of 10 cm, and found that the observed depletion of foliage δ¹⁵N in response to elevated CO₂, was unrelated to changes in the soil δ¹⁵N. While the data suggest a strong influence of elevated CO₂, on the N cycle in diverse ecosystems, the exact site(s) at which elevated CO₂ alters fractionating processes of the N cycle remains unclear. We cannot rule out the fact that the pattern of foliage δ¹⁵N responses to elevated CO₂, reported here resulted from a general drop in δ¹⁵N of the source N, caused by soil-driven processes. There is a stronger possibility, however, that the general depletion of foliage δ¹⁵N under high CO₂, may have resulted from changes in the fractionating processes within the plant/mycorrhizal system.