Compound‐specific δ13C analysis of monosaccharides from soil extracts by high‐performance liquid chromatography/isotope ratio mass spectrometry

Abstract

Carbohydrates represent up to 25% of soil organic matter and derive from fresh plant input or organic matter transformation within the soil. Compound-specific isotope analysis (CSIA) of monosaccharides (sugars) extracted from soil provides a powerful tool to disentangle the dynamics of different carbohydrate pools of soils. The use of high-performance liquid chromatography/oxidation/isotope ratio mass spectrometry (HPLC/o/IRMS) allows isotope measurements without the need for derivatisation and thus increasing accuracy and precision of the isotopic measurement, compared with gas chromatography/combustion/isotope ratio mass spectrometry (GC/c/IRMS). The CSIA of soil carbohydrates was performed using a HPLC/o/IRMS system. The chromatographic and mass spectrometric subunits were coupled with a LC-Isolink interface. Soil sugars were extracted after mild hydrolysis using 4 M trifluoroacetic acid (TFA). Chromatographic separation of the sugars was achieved using a low strength 0.25 mM NaOH solution as mobile phase at a flow rate of 250 μL min-1 at 10 °C. The chromatographic conditions allowed the baseline separation of the seven most abundant sugars in soil. Complete removal of TFA from the soil hydrolysate ensured chromatographic stability. The accuracy was better than 0.66 ‰ for amounts of >2.5 nM sugar on column. The sugars extracted from an agricultural soil appeared to be more enriched in 13 C than the soil organic carbon, and to have a similar isotopic signature to the soil microbial biomass. The proposed method proved to be suitable for the analysis of the common sugars in soil extracts and represents a precise tool for the study of carbohydrate dynamics. Copyright © 2013 John Wiley & Sons, Ltd.