THE EFFECT OF ATMOSPHERIC CO2 CONCENTRATION ON CARBON ISOTOPE FRACTIONATION IN C3 LAND PLANTS

Experiments growing plants under elevated pCO₂ in chamber and field settings, as well as meta-analyses of published data, have yielded a wide range of estimates for the effect of pCO₂ on the net isotopic discrimination (Δδ¹³C_p) between plant tissue (δ¹³C_p) and atmospheric CO₂ (δ¹³C_CO2). Here we present new data from plant growth chambers in which we grew a total of 191 C₃ plants (128 Raphanus sativus plants and 63 Arabidopsis thaliana) across fifteen levels of pCO₂ ranging from 370 to 4200 ppm. Three types of plant tissue were harvested and analyzed for carbon isotope value: above-ground tissues, below-ground tissues, and leaf-extracted nC₃₁-alkanes. We observed strong hyperbolic correlations (R ≥ 0.94) between the pCO₂ level and Δδ¹³C_p for each type of plant tissue analyzed; the linear relationships previously suggested by experiments across small (10 to 350 ppm) changes in pCO₂ (e.g., 300 to 310 ppm or 350 to 700 ppm) closely agree with the amount of fractionation per ppm increase in pCO₂ calculated from our hyperbolic relationship. In this way, our work is consistent with, and provides a unifying relationship for, previous work on carbon isotopes in C₃ plants at elevated pCO₂. Our results suggest that it may be possible to reconstruct changes in paleo-pCO₂ level using our new hyperbolic relationship if δ¹³C_CO2 value and initial pCO₂ level can be independently quantified. Several implications also arise for the reconstruction of water availability and water-use efficiency in both ancient and recent plant Δδ¹³C_p values across periods of changing pCO₂ level. For example, the change in Δδ¹³C_p implied by our relationship for the rise in pCO₂ concentration observed since 1980 is of the same magnitude (= ~0.7‰) as the isotopic correction for changes in δ¹³C_CO2 required by the input of ¹³C-depleted carbon to the atmosphere. For these reasons, only the portion of a terrestrial carbon isotope excursion that persists after accounting for changes in pCO₂ concentration should be used for the interpretation of a change in paleo-environmental conditions.