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

P>Photosynthetic carbon (C) isotope discrimination (Delta(Alpha)) labels photosynthates (delta(A)) and atmospheric CO2 (delta(a)) 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 Delta(Alpha). We investigated the relationship between Delta(Alpha) and the C isotope signals from stem (delta(W)), soil (delta(S)) and ecosystem (delta(E)) respired CO2 to environmental fluctuations, using novel tuneable diode laser absorption spectrometer instrumentation in a mature maritime pine forest. Broad seasonal changes in Delta(Alpha) were reflected in delta(W,)delta(S) and delta(E). However, respired CO2 signals had smaller short-term variations than Delta(A) and were offset and delayed by 2-10 d, indicating fractionation and isotopic mixing in a large C pool. Variations in delta(S) did not follow Delta(A) 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.