Influence of recent vegetation on labile and recalcitrant carbon soil pools in central Queensland, Australia: evidence from thermal analysis-quadrupole mass spectrometry-isotope ratio mass spectrometry

The effect of a recent vegetation change (<100 years) from C4 grassland to C3 woodland in central Queensland, Australia, on soil organic matter (SOM) composition and SOM dynamics has been investigated using a novel coupled thermogravimetry-differential scanning calorimetry-quadrupole.mass spectrometry-isotope ratio mass spectrometry (TG-DSC-QMS-IRMS) system. TG-DSC-QMS-IRMS distinguishes the C isotope composition of discrete SOM pools, showing changes in labile, recalcitrant and refractory carbon in the bulk soil and particle size fractions which track the vegetation changes. Analysis of evolved gases (by QMS) from thermal decomposition, rather than observed weight loss, proved essential in determining the temperature at which SOM decomposes, because smectite and kaolinite clays contribute to observed weight losses. The δ13C analyses of the CO2 evolved at different temperatures for bulk soil and particle size-separates showed that most of the labile SOM under the more recent woody vegetation was C3-derived carbon whereas the δ13C values in the recalcitrant SOM showed greater C4 contributions. This indicated a shift from grass (C4)- to tree (C3)-derived carbon in the woodland, which was also supported by the two-phase 13C enrichment with depth, i.e. C3 vegetation dominated the top soil (0–10 cm), but the C4 contribution increased with depth (more gradual). This is perturbed by the inclusion of charcoal from forest fires (14C age incursions) and by the deep incorporation of C3 carbon due to root penetration.