<scp>FT</scp> ‐ <scp>MIR</scp> Spectroscopic Analysis of the Organic Carbon Fractions in Australian Mineral Soils

ABSTRACT Soil organic carbon (C) is heterogeneous. It exists in various forms along a decomposition continuum, from labile fast‐cycling compounds to more persistent forms that can reside in the soil for centuries to millennia. The soil organic C fractionation methods account for this complexity by separating soil organic C into distinct groups with similar turnover. Here, we aimed to (a) fractionate 401 mineral soils from three depths (0–10, 10–20, 20–30 cm) with small organic C concentrations (&lt; 2.5% mean C) and varying textures and mineralogy using a granulometric method to derive the particulate organic C in macroaggregates (POC mac ), the particulate organic C in microaggregates (POC mic ) and the mineral‐associated organic carbon (MAOC), (b) test whether mid‐infrared (MIR) spectra (4000–450 cm −1 ) can discriminate the C fractions and characterise their distinct organic and mineral functional groups and (c) examine which mineral and organic functional groups concomitantly occur in the spectra of the C fractions to provide insights into their composition. A canonical variate analysis showed that the MIR spectra use information from mineral and organic absorptions to discriminate the organic C fractions. Closer investigation of specific regions of the MIR spectrum showed, as might be expected, that absorptions relating to quartz were more pronounced in the POC mac and POC mic fractions, and clay mineral absorptions were dominant in the MAOC fraction. The stretching vibrations of alkyl CH 2 bonds (2930, 2860 cm −1 ) were the most prominent organic absorptions, particularly in the MAOC fraction, followed by absorptions from amide groups (1525, 1630 cm −1 ). Our findings demonstrate that MIR spectroscopy can characterise the compositional differences between the organic C fractions and identify co‐occurrences of organic functional groups, such as alkyl CH 2 , with clay minerals. They suggest associations between organic molecules and clay minerals that contribute to soil organic C persistence. Future research and applications should combine fractionation with MIR spectroscopy to enhance the resolution of soil C analyses and the reliability of fractionations.