Evaluation of pre-treatment procedures for improved interpretation of mid infrared spectra of soil organic matter

This study evaluated the effectiveness of dry ashing of soils and subsequent spectral subtraction procedure in comparison with alternative chemical methods in accentuating organics for diffuse reflectance infrared Fourier transform (DRIFT) spectroscopic analysis of soil organic matter composition. Chemical oxidation of soil with sodium hypochlorite (NaOCl) and demineralisation by hydrofluoric acid (HF) treatment were assessed as possible alternatives to the dry ashing method. Surface samples (0-10 cm) of four contrasting soils i.e., Ferralsol, Luvisol, Vertisol and Solonetz were used in the study. We observed the spectral changes before and after pre-treatments using DRIFT spectroscopy of bulk soils and their respective four separated density fractions (< 1.8, 1.8-2.2, 2.2-2.6, and > 2.6 g cm− 3). Dry ashing of soils altered the absorption bands of gibbsite and goethite in the Ferralsol; however, phyllosilicate bands in the Luvisol, Vertisol and Solonetz remained largely unaffected. The organic bands were incompletely removed by the dry ashing method and shifts in some major organic bands were also noticed. Thus the accuracy of the dry ashed spectral subtraction was limited due to the mineral and organic bands alteration. The NaOCl treatment oxidised labile organic carbon (OC) (e.g., aliphatics) without any mineral alteration, but the rest of the OC fractions remained largely unaffected. Therefore, the result of spectral subtraction of the NaOCl treated samples from original spectra was ambiguous. The HF treatment removed minerals efficiently from both bulk soils and density fractions, which noticeably highlighted the organic bands with little or no mineral interferences in the spectra of the treated samples. HF treatment also removed a small portion of the total OC that was associated with minerals. We conclude that HF treatment was more effective than dry ashing and NaOCl oxidation to accentuate organics in the soil DRIFT spectra, since it provided mineral interference free spectra with in general least affected organic bands. Thus, HF treatment of soil offers a potential pre-treatment method for improved spectroscopic characterisation of soil organic matter.