Photosynthetic carbon isotope discrimination and its relationship to the carbon isotope signals of stem, soil and ecosystem respiration

Photosynthetic carbon (C) isotope discrimination (ΔΑ) labels photosynthates (δA) and atmospheric CO2 (δa) with variable C isotope compositions during fluctuating environmental conditions. In this context, the C isotope composition of respired CO2 within ecosystems is often hypothesized to vary temporally with ΔΑ. We investigated the relationship between ΔΑ and the C isotope signals from stem (δW), soil (δS) and ecosystem (δE) respired CO2 to environmental fluctuations, using novel tuneable diode laser absorption spectrometer instrumentation in a mature maritime pine forest. Broad seasonal changes in ΔΑ were reflected in δW,δS and δE. However, respired CO2 signals had smaller short-term variations than ΔA and were offset and delayed by 2–10 d, indicating fractionation and isotopic mixing in a large C pool. Variations in δS did not follow ΔA at all times, especially during rainy periods and when there is a strong demand for C allocation above ground. It is likely that future isotope-enabled vegetation models will need to develop transfer functions that can account for these phenomena in order to interpret and predict the isotopic impact of biosphere gas exchange on the C isotope composition of atmospheric CO2