Advancing soil organic matter and mineral characterization by mid-infrared spectroscopy using spectral subtraction

The composition of soil organic matter (OM) and mineral phases is essential for soil functions. Diffuse reflectance Fourier transform mid-infrared (DRIFT-MIR) spectroscopy allows rapid characterization of soils, but its effectiveness is often limited by the complex interactions and interferences between OM and mineral phases. This study systematically evaluated spectral subtraction after OM removal by thermal dry oxidation (loss on ignition (LOI)), chemical wet oxidation using sodium hypochlorite (NaOCl), and hydrogen peroxide (H 2 O 2 ). We derived sample specific subtraction factors to determine the mineral- and OM-enhanced spectra of each sample. Six soil samples with contrasting textures (clay, loam and sand) and organic carbon content (0.8–6.8%) as well as the presence of inorganic carbon (up to 0.5% carbon) were analyzed. Our results showed that LOI at 550°C for 60 min was most effective for removing OM (>98% of initial organic carbon). In contrast, NaOCl and H 2 O 2 resulted in lower OM removal (72–95% of initial organic carbon) and created artifacts in the mineral- and OM-enhanced spectra. In a spiking test, milled and dried summer wheat root litter was added to a model mineral soil composed of six phyllosilicate clay minerals, calcium carbonate and goethite. For LOI and NaOCl, dehydroxylation and oxidation reactions of the mineral phase were clearly traceable, but LOI proved most suitable for recovering the original OM and mineral spectral signatures. Our study shows that spectral subtraction is an efficient approach to disentangle OM and mineral spectra composition from complex soil matrices by DRIFT-MIR. We provide guidance on how spectral subtraction can be applied to different soil types to rapidly investigate OM dynamics and mineral composition.