FT-MIR 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 (< 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 CH₂ 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 CH₂, 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.