Stable isotope analysis of dissolved organic carbon in soil solutions using a catalytic combustion TOC analyzer – IRMS with cryofocusing interface

摘要

Stable carbon isotopes are a powerful tool to assess the origin and dynamics of carbon in soils. However, direct analysis of the 13C/12C ratio in the dissolved organic carbon (DOC) pool has proved to be difficult. Recently, several systems have been developed to measure isotope ratios in DOC by coupling a total organic carbon (TOC) analyzer with an isotope ratio mass spectrometer. However these systems were designed for the analysis of fresh and marine water and no results for soil solutions or 13C-enriched samples have been reported. Because we mainly deal with soil solutions in which the difficult to oxidize humic and fulvic acids are the predominant carbon-containing components, we preferred to use thermal catalytic oxidation to convert DOC into CO2. We therefore coupled a high-temperature combustion TOC analyzer with an isotope ratio mass spectrometer, by trapping and focusing the CO2 cryogenically between the instruments. The analytical performance was tested by measuring solutions of compounds varying in the ease with which they can be oxidized. Samples with DOC concentrations between 1 and 100 mg C/L could be analyzed with good precision (standard deviation (SD) 0.6), acceptable accuracy, good linearity (overall SD = 1) and without significant memory effects. In a 13C-tracer experiment, we observed that mixing plant residues with soil caused a release of plant-derived DOC, which was degraded or sorbed during incubation. Based on these results, we are confident that this approach can become a relatively simple alternative method for the measurement of the 13C/12C ratio of DOC in soil solutions.