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42. Molecular characteristics of permanganate- and dichromateoxidation-resistant soil organic matter from a black-C-rich colluvial soil

Author

Suarez-Abelenda, Manuel, Kaal, Joeri, Camps-Arbestain, Marta, Knicker, Heike, and Macias, Felipe

Subjects
Soils -- Carbon content, Soil research, Chemical research, Permanganates -- Chemical properties -- Environmental aspects, Oxidation-reduction reaction -- Research, Agricultural industry, Earth sciences
Abstract

Samples from a colluvial soil rich in pyrogenic material (black C, BC) in north-west Spain were subjected to [K.sub.2][Cr.sub.2][O.sub.7] and KMn[O.sub.4] oxidation and the residual soil organic matter (SOM) was NaOH- extracted and analysed using analytical pyrolysis-gas chromatography-mass spectroscopy (Py-GC/MS) and solid-state [sup.13]C cross-polarisation magic angle spinning-nuclear magnetic resonance ([sup.13]C CP MAS-NMR) in order to study the susceptibility of different SOM fractions (fresh, degraded/microbial, BC and aliphatic) towards these oxidising agents. Untreated samples that were NaOH-extracted were also analysed. The Py-GC/MS and [sup.13]C NMR indicated that KMn[O.sub.4] promotes the oxidation of carbohydrate products, mostly from degraded/microbial SOM and lignocellulose, causing a relative enrichment of aliphatic and aromatic structures. Residual SOM after [K.sub.2][Cr.sub.2][O.sub.7] oxidation contained BC, N- containing BC and aliphatic structures. This was corroborated by a relatively intense resonance of aromatic C and some signal of alkyl C in [sup.13]C NMR spectra. These results confirm that dichromate oxidation residues contain a non-pyrogenic fraction mainly consisting of aliphatic structures. Additional keywords: [K.sub.2][Cr.sup.2][O.sup.7], KMn[O.sub.4], SOM oxidation, charcoal, Py-GC/MS, [sup.13]C NMR. Received 5 July 2013, accepted 3 October 2013, published online 6 March 2014, Introduction Soil organic matter (SOM) is a complex mixture of plant, animal and microbial tissue, both fresh and at different stages of decomposition (Stevenson 1994; Tabatabai 1996). The molecular structures [...]

Published
2014
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48. Origin of dissolved organic matter in the Harz Mountains (Germany): A thermally assisted hydrolysis and methylation (THM-GC–MS) study

Author

Kaal, Joeri, Plaza, César, Nierop, Klaas G.J., Pérez-Rodríguez, Marta, Biester, Harald, GeoLab Algemeen, Organic geochemistry, and Organic geochemistry & molecular biogeology

Subjects
Harz Mountains, THM-GC–MS, Source assessment, Soil Science, Dissolved organic matter
Abstract

Environmental change is increasing the concentration of dissolved organic matter (DOM) in catchments of the Northern Hemisphere. This study aims to assess the causes of high DOM concentrations in streams and reservoirs of the Harz National Park (Germany), by means of molecular characterization using thermally assisted hydrolysis and methylation (THM-GC–MS). In order to formulate proxies of the prevailing origin of the numerous THM products of polyphenols, carbohydrates, proteins, aliphatic macromolecules, resins and other DOM precursors, we created a reference sample set of potential sources (spruce, birch, blueberry, heather, peat moss, soils) from the area. Besides solid-state reference samples (bulk organic matter; BOM) we obtained and analyzed their leachates (water-extractable OM; WEOM). Finally, an existing THM-GC–MS dataset of the DOM from the Oder river, which crosses the boundary between peat and forest biomes in the Harz, was extended and explored chemometrically using Principal Component Analysis (PCA) to test the proxies for stream DOM assessment. The results show large differences between BOM and WEOM, which suggests that the solid-to-leachate transition is highly selective or significantly alters the major biomolecular constituents. THM compounds that tend to be more abundant in WEOM than in BOM are G-type phenolic compounds (1,2-dimethoxybenzenes, from lignin and tannin), nitrogen-containing moieties and benzene carboxylic acids, whereas WEOM is depleted in products of polysaccharides, syringyl lignin and aliphatic macromolecules (cutin and suberin). The lignin fingerprint of the WEOM also differs significantly from that of BOM, being depleted in the vast majority of the typical products of macromolecular lignin (G7, G8, G14, G15) and enriched in the acid moiety (G6, predominantly from vanillic acid), especially for spruce wood. THM chromatograms of DOM from the forest section of the Oder show an extraordinary abundance of G6, most probably from spruce-derived lignin. This may indicate a major role of DOM released from decaying spruce logs and forest soils. The results highlight both the potential and the pitfalls associated with source identification of DOM using THM-GC–MS.

Published
2020

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