Δ14CO2 from dark respiration in plants and its impact on the estimation of atmospheric fossil fuel CO2

Abstract

Radiocarbon (¹⁴C) has been widely used for quantification of fossil fuel CO₂ (CO_2ff) in the atmosphere and for ecosystem source partitioning studies. The strength of the technique lies in the intrinsic differences between the ¹⁴C signature of fossil fuels and other sources. In past studies, the ¹⁴C content of CO₂ derived from plants has been equated with the ¹⁴C content of the atmosphere. Carbon isotopic fractionation mechanisms vary among plants however, and experimental study on fractionation associated with dark respiration is lacking. Here we present accelerator mass spectrometry (AMS) radiocarbon results of CO₂ respired from 21 plants using a lab-incubation method and associated bulk organic matter. From the respired CO₂ we determine Δ¹⁴C_res values, and from the bulk organic matter we determine Δ¹⁴C_bom values. A significant difference between Δ¹⁴C_res and Δ¹⁴C_bom (P < 0.01) was observed for all investigated plants, ranging from -42.3‰ to 10.1‰. The results show that Δ¹⁴C_res values are in agreement with mean atmospheric Δ¹⁴CO₂ for several days leading up to the sampling date, but are significantly different from corresponding bulk organic Δ¹⁴C values. We find that although dark respiration is unlikely to significantly influence the estimation of CO_2ff, an additional bias associated with the respiration rate during a plant's growth period should be considered when using Δ¹⁴C in plants to quantify atmospheric CO_2ff.

Keywords: Carbon cycle; Dark respiration; Fossil fuel CO(2); Plant; Δ(14)C.