Are δ13C values of n-alkanes affected by atmospheric CO2 concentrations? Results from a free-air CO2 enrichment (FACE) experiment.

Abstract:

Compound-specific carbon isotope (δ¹³C) analyses of leaf waxes (i.e., n-alkanes) can be linked to large-scale shifts in vegetation, such as dominant taxa, functional types, life-forms and photosynthetic pathways that are usually coupled with environmental changes in climate. However using these δ¹³C values to interpret finer-scale ecosystem properties, including climate attributes such as CO₂ concentrations, is difficult owing to uncertainty in the magnitude of internal biosynthetic fractionations that determine the δ¹³C of waxes relative to that of bulk leaf material. We investigated the composition, abundance and δ¹³C of n-alkanes in the aboveground biomass of a C4 grass and a C3 grass exposed to experimentally controlled CO₂ at ambient [490ppm] and elevated [630ppm] levels within natural grassland in Wyoming. The δ¹³C values of bulk tissues were predictably different based on the C3 and C4 photosynthetic pathways, but the difference between bulk tissue and n-alkanes (ε_lipid), for both C₂₉ and C₃₁, was consistently greater in the C4 grass. The magnitudes of these ε_lipid values were large (- 7‰ to -15‰) relative to those found in most other studies. CO₂ concentration of the growing environment also had a significant effect on n-alkane δ¹³C values, with consistently higher values of ~ 2‰ under elevated CO₂ found in both species and in both a wet and a dry year. These results underscore the importance of recognizing potential abiotic effects on leaf wax δ¹³C values, in addition to the biotic drivers their variation, when interpreting climate from leaf-wax biomarkers of terrestrial ecosystems.