Chemical treatment of soils is routinely employed in the study of soil organic matter (SOM). These range from oxidative treatments for fractionating and removing SOM, as well as pretreatments like hydrofluoric acid (HF) to enhance structural analyses such as nuclear magnetic resonance (NMR) and diffuse reflectance infrared Fourier-transform (DRIFT) spectroscopies. To evaluate whether commonly employed chemical treatments remove structurally distinct fractions of SOM, treatments used in the literature were performed on an agricultural soil (2.2% total organic carbon) and the remaining SOM was structurally characterized by a suite of complementary techniques. The soil (mixed, super-active, nonacid, thermic Mollic Xerofluvent) was subjected to three distinct oxidation treatments commonly used in SOM fractionation: potassium permanganate (KMnO4), sodium hypochlorite (NaOCl), and hydrogen peroxide (H2O2). Additionally, non-oxidized and oxidized soils were treated with HF. Both non-HF and HF-treated soils were characterized by DRIFT spectroscopy and pyrolysis molecular beam mass spectrometry (py-MBMS), and HF-treated soils were analyzed by 13C cross polarization magic angle spinning (CP-MAS) NMR spectroscopy. Spectroscopic characterizations generally indicated differences in SOM structure by oxidation treatments, which did not necessarily track with differences in SOM removal (KMnO4 &lt; NaOCl &lt; H2O2), and suggested a strong effect of HF treatment on SOM structure. Differences occurred among oxidation treatments in altering relative functional group composition, including aliphatic C-H, aromatic C=C and carbonyl C=O (DRIFT), carbonyl, aromatic, O-alkyl and alkyl C (NMR) and low- and high-molecular weight structural units, including lignin-like and heterocylic N (py-MBMS). SOM removal was associated with decrease in absorbance of DRIFT aliphatic C-H stretch (3000-2800 cm-1), but a similar relationship was not evident for NMR-characterized total alkyl C (110-0 ppm). HF treatment resulted in a significant increase in signal intensity (py-MBMS) and differences in intensity and occurrence of infrared peaks (DRIFT) relative to paired non-HF treated samples. These results suggest selective removal of SOM may occur depending on the treatment and method of structural characterization, and that the use of HF for sample pretreatment may significantly influence characterizations of SOM.

Effects of chemical treatments on soil organic matter composition identified by DRIFT, NMR and py-MBMS characterization

249th National Meeting (2015)

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