Application of DRIFTS, 13C NMR, and py-MBMS to Characterize the Effects of Soil Science Oxidation Assays on Soil Organic Matter Composition in a Mollic Xerofluvent

Andrew J Margenot; Francisco J Calderón; Kimberly A Magrini; Robert J Evans

doi:10.1177/0003702817691776

Application of DRIFTS, ¹³C NMR, and py-MBMS to Characterize the Effects of Soil Science Oxidation Assays on Soil Organic Matter Composition in a Mollic Xerofluvent

Appl Spectrosc. 2017 Jul;71(7):1506-1518. doi: 10.1177/0003702817691776. Epub 2017 Mar 31.

Authors

Andrew J Margenot¹, Francisco J Calderón², Kimberly A Magrini³, Robert J Evans³

Affiliations

¹ 1 University of California-Davis, Davis, CA, USA.
² 2 USDA Agricultural Research Station, Akron, CO, USA.
³ 3 USDOE National Renewable Energy Laboratory, Golden, CO, USA.

PMID: 28361600
DOI: 10.1177/0003702817691776

Abstract

Chemical oxidations are routinely employed in soil science to study soil organic matter (SOM), and their interpretation could be improved by characterizing oxidation effects on SOM composition with spectroscopy. We investigated the effects of routinely employed oxidants on SOM composition in a Mollic Xerofluvent representative of intensively managed agricultural soils in the California Central Valley. Soil samples were subjected to oxidation by potassium permanganate (KMnO₄), sodium hypochlorite (NaOCl), and hydrogen peroxide (H₂O₂). Additionally, non-oxidized and oxidized soils were treated with hydrofluoric acid (HF) to evaluate reduction of the mineral component to improve spectroscopy of oxidation effects. Oxidized non-HF and HF-treated soils were characterized by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), ¹³C cross polarization magic angle spinning (CP-MAS) nuclear magnetic resonance (NMR) spectroscopy, and pyrolysis molecular beam mass spectrometry (py-MBMS), and for particle size distribution (PSD) using laser diffractometry (LD). Across the range of soil organic carbon (OC) removed by oxidations (14-72%), aliphatic C-H stretch at 3000-2800 cm^-1 (DRIFTS) decreased with OC removal, and this trend was enhanced by HF treatment due to significant demineralization in this soil (70%). Analysis by NMR spectroscopy was feasible only after HF treatment, and did not reveal trends between OC removal and C functional groups. Pyrolysis-MBMS did not detect differences among oxidations, even after HF treatment of soils. Hydrofluoric acid entailed OC loss (13-39%), and for H₂O₂ oxidized soils increased C:N and substantially decreased mean particle size. This study demonstrates the feasibility of using HF to improve characterizations of SOM composition following oxidations as practiced in soil science, in particular for DRIFTS. Since OC removal by oxidants, mineral removal by HF, and the interaction of oxidants and HF observed for this soil may differ for soils with different mineralogies, future work should examine additional soil and land use types to optimize characterizations of oxidation effects on SOM composition.

Keywords: DRIFTS; Diffuse reflectance infrared Fourier transform spectroscopy; H2O2; HF; KMnO4; LD; NMR; NaOCl; SOM; hydrofluoric acid; hydrogen peroxide; laser diffractometry; nuclear magnetic resonance spectroscopy; oxidation; potassium permanganate; py-MBMS; pyrolysis molecular beam mass spectrometry; sodium hypochlorite; soil organic matter.