Evaluating high time-resolved changes in carbon isotope ratio of respired CO2 by a rapid in-tube incubation technique

Recent insights into fractionation during dark respiration and rapid dynamics in isotope signatures of leaf- and ecosystem-respired CO2 indicate the need for new methods for high time-resolved measurements of the isotopic signature of respired CO2 (delta C-13(res)). We present a rapid and simple method to analyse delta C-13(res) using an in-tube incubation technique and an autosampler for small septum-capped vials. The effect of storage on the delta O-18 and delta C-13 ratios of ambient CO2 concentrations was tested with different humidity and temperatures. delta C-13 ratios remained stable over 72 h, whereas delta O-18 ratios decreased after 24 h. Storage at VC improved the storage time for delta O-18. Leaves or leaf discs were incubated in the vials, flushed with CO2-free air and respired CO2 was automatically sampled within 5 min on a mu Gas autosampler interfaced to a GV-Isoprime isotope ratio mass spectrometer. Results were validated by simultaneous on-line gas-exchange measurements of delta C-13(res) of attached leaves. This method was used to evaluate the short-term (5-60 min) and diurnal dynamics of delta C-13(res) in an evergreen oak (Quercus ilex) and a herb (Tolpis barbata). An immediate depletion of 2-4 parts per thousand from the initial delta C-13(res) value occurred during the first 30 min of darkening. Q. ilex exhibited further a substantial diurnal enrichment in delta C-13(res) of 8 parts per thousand, followed by a progressive depletion during the night. In contrast, T. barbata did not exhibit a distinct diurnal pattern. This is in accordance with recent theory on fractionation in metabolic pathways and may be related to the different utilisation of the respiratory substrate in the fast-growing herb and the evergreen oak. These data indicate substantial and rapid dynamics (within minutes to hours) in delta C-13(res), which differed between species and probably the growth status of the plant. The in-tube incubation method enables both high time-resolved analysis and extensive sampling across different organs, species and functional types. Copyright (C) 2007 John Wiley & Sons, Ltd.