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

Radiocarbon (C-14) has been widely used for quantification of fossil fuel CO2 (CO2ff) in the atmosphere and for ecosystem source partitioning studies. The strength of the technique lies in the intrinsic differences between the C-14 signature of fossil fuels and other sources. In past studies, the C-14 content of CO2 derived from plants has been equated with the C-14 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 Delta C-14(res) values, and from the bulk organic matter we determine-Delta C-14(born) values. A significant difference between Delta C-14(res) and Delta C-14(bom) (P < 0.01) was observed for all investigated plants, ranging from -42.3 parts per thousand to 10.1 parts per thousand. The results show that Delta C-14(res) values are in agreement with mean atmospheric Delta(CO2)-C-14 for several days leading up to the sampling date, but are significantly different from corresponding bulk organic Delta C-14 values. We find that although dark respiration is unlikely to significantly influence the estimation of CO2ff, an additional bias associated with the respiration rate during a plant's growth period should be considered when using Delta C-14 in plants to quantify atmospheric CO2. (C) 2017 Elsevier Ltd. All rights reserved.