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Title: Application of DRIFTS, NMR, and py-MBMS to characterize the effects of soil science oxidation assays on soil organic matter composition in a Mollic Xerofluvent

Author
item MARGENOT, ANDREW - University Of California
item Calderon, Francisco
item MAGRINI, KIMBERLY - National Renewable Energy Laboatory
item EVANS, ROBERT - National Renewable Energy Laboatory

Submitted to: Applied Spectroscopy
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/28/2016
Publication Date: 3/31/2017
Publication URL: https://handle.nal.usda.gov/10113/5801865
Citation: Margenot, A., Calderon, F.J., Magrini, K., Evans, R. 2017. Application of DRIFTS, NMR, and py-MBMS to characterize the effects of soil science oxidation assays on soil organic matter composition in a Mollic Xerofluvent. Applied Spectroscopy. 71(7):1506-1518. doi: 10.1177/0003702817691776.

Interpretive Summary: In this experiment, we used different chemical oxidation treatments to test how increasing oxidation intensity affects the chemistry of soil organic matter. This is important, because milder oxidations are thought to represent the soil organic matter that supplies nutrients to crops in the short-term, while stronger oxidations should leave behind the soil C that is recalcitrant and tends to stay in the soil for long periods of time. Soil organic matter removal was in the order: KMnO4 < NaOCl < H2O2. Different assays were used to determine the chemical changes upon oxidation: Infrared spectroscopy (FTIR), nuclear magenetic resonace (NMR), and analytical pyrolysis. Our results show that the assays differ in their capability to see the chemical changes caused by the oxidants. Removal of minerals from the samples aided in the interpretation of the assay results, but the chemical used (HF) can introduce biases due to carbon loss and chemical reactions.

Technical Abstract: To evaluate whether commonly employed chemical treatments remove structurally distinct fractions of soil organic matter (SOM), a Mollic Xerofluvent under agricultural use was subjected to three distinct oxidation treatments: potassium permanganate (KMnO4), sodium hypochlorite (NaOCl), and hydrogen peroxide (H2O2). Additionally, non-oxidized and oxidized soils were treated with hydrofluoric acid (HF) in order to demineralize the samples. Oxidized non-HF and HF-treated soils were characterized by diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy, 13C cross polarization magic angle spinning (CP-MAS) nuclear magnetic resonance spectroscopy (NMR), and pyrolysis molecular beam mass spectrometry (py-MBMS), and for particle size distribution (PSD) by laser diffractometry (LD). Structural characterizations indicated (1) differences in SOM structure by oxidation treatments, which did not necessarily track with differences in SOM removal (KMnO4 < NaOCl < H2O2) and depended on the method of characterization, and (2) a strong effect of HF treatment on SOM characterizations. Oxidation treatments altered relative functional group composition, including aliphatic C-H, aromatic C=C and carbonyl C=O (DRIFTS), carbonyl, aryl, and alkyl C (NMR), and low- and high-molecular weight structural units (py-MBMS). HF treatment affected the intensity and occurrence of IR absorbance peaks (DRIFTS) relative to paired non-HF treated samples. HF also increased signal intensity of py-MBMS, concurrent with an increase in particle size fineness (LD) and the detection of fragments representing undecomposed biomass relative to markers of humified SOM (py-MBMS). Our results show that although HF can enable NMR analysis, it can remove 13-31% of the SOM, which might bias interpretation of ensuing SOM characterization. Stoichiometric, structural and particle size characterizations identify an interaction of H2O2 and HF potentially mediated by organomineral associations, suggesting oxidant-specific effects of HF.