Sequential exhaustive extraction of a Mollisol soil, and characterizations of humic components, including humin, by solid and solution state NMR

Abstract

A comprehensive sequential extraction procedure was applied to isolate soil organic components using aqueous solvents at different pH values, base plus urea (base-urea), and finally dimethylsulfoxide (DMSO) plus concentrated H 2 SO 4 (DMSO-acid) for the humin-enriched clay separates. The extracts from baseurea and DMSO-acid would be regarded as 'humin' in the classical definitions. The fractions isolated from aqueous base, base-urea and DMSO-acid were characterized by solid and solution state NMR spectroscopy. The base-urea solvent system isolated ca. 10% (by mass) additional humic substances. The combined base-urea and DMSO-acid solvents isolated ca. 93% of total organic carbon from the humin-enriched fine clay fraction (<2 mm). Characterization of the humic fractions by solid-state NMR spectroscopy showed that oxidized char materials were concentrated in humic acids isolated at pH 7, and in the base-urea extract. Lignin-derived materials were in considerable abundance in the humic acids isolated at pH 12.6. Only very small amounts of char-derived structures were contained in the fulvic acids and fulvic acids-like material isolated from the base-urea solvent. After extraction with baseurea, the 0.5 M NaOH extract from the humin-enriched clay was predominantly composed of aliphatic hydrocarbon groups, and with lesser amounts of aromatic carbon (probably including some char material), and carbohydrates and peptides. From the combination of solid and solution-state NMR spectroscopy, it is clear that the major components of humin materials, from the DMSO-acid solvent, after the exhaustive extraction sequence, were composed of microbial and plant derived components, mainly long-chain aliphatic species (including fatty acids/ester, waxes, lipids and cuticular material), carbohydrate, peptides/proteins, lignin derivatives, lipoprotein and peptidoglycan (major structural components in bacteria cell walls). Black carbon or char materials were enriched in humic acids isolated at pH 7 and humic acids-like material isolated in the base-urea medium, indicating that urea can liberate charderived material hydrogen bonded or trapped within the humin matrix.

Use of biochar and oxidized lignite for reconstructing functioning agronomic topsoil: Effects on soil properties in a greenhouse study

“…We used 70% wt/wt as the amount of organic C that occurs as humin organic C in the native topsoil (Stevenson 1994;Kononova 1966). We used biochar C or humalite C as a surrogate for humin C because humin, like biochar C and humalite C, is considered the most recalcitrant of the soil fractions (Song et al 2008). Labile organic mix C was added to the subsoil substrate to equal 2.25 times the difference between soil organic C and humin organic C in the native topsoil.…”

Distribution of chemical compartments of soil organic matter and c stocks of a cambisol from south Brazil as affected by Pinus afforestation

“…15 The HUI fraction is also referred to by some authors as the fraction comprising the "humic acid-like" and "fulvic acid-like" SOM. 14 The residual soil containing the HUH fraction was washed three times with distilled water, dried at 60 °C and kept for further analyses.…”

“…Both SOM fractions, C W and C HCl , are not considered part of the stabilized humic substances and most likely comprise a less recalcitrant SOM. 14 Further fractionation of the humic substances was performed on the soil sample containing the colloidal SOM (C CO ), i.e., soil sample free of POM and of non-humic substances. The separation of humic acid (AH) and fulvic acid (FA) was achieved by alkaline extraction, where 50 mL of NaOH 0.5 mol L -1 were added to 10 g of soil containing colloidal SOM (considered humified SOM here) sample and the suspension mechanically shaken for 3 h. The alkaline extract containing the extractable humic substances (C EHS ) was separated by centrifugation (10 min, 15,000 g) and this procedure was repeated until the supernatant became limpid.…”

“…12 After the extraction of humic and fulvic acids by means of alkaline solution, mixed solutions are used, such as NaOH + Urea, DMSO + H 2 SO 4 , DMSO + HCl and Acetone + HCl, and thereby different intra-aggregate organic bonds are broken stepwise. [13][14][15] To date, most of the studies on the impact of exotic species plantation on SOM, comprising Brazilian regions, have focused mainly on its stocks or have emphasized Eucalyptus plantations. In a recent study investigating the impact of Pinus introduction on SOM of a Leptosol located in South Brazil, an increase in the chemical recalcitrance and carboxylic groups in the SOM structure was observed in addition to a decrease in C stocks, after 30 years of plantation.…”

Gênesis de espodossolos amazônicos: um estudo sobre a estrutura e a mobilidade da matéria orgânica

“…Pode-se observar picos na região de 30 a 34 ppm que pode ser atribuída ao carbono alquil sem substituição que incluem cadeias lineares de carbono metilenos; um pico em 56 ppm pode ser atribuído a grupos metóxila presente na lignina que aparecerá novamente em torno de 150 ppm com um ou dois picos, mais também essa região pode ser atribuída ao carbono alquil com N-substituído em proteínas e peptídeos; picos na região 65 a 95 ppm referente a presença de carbono ligado a substituintes de oxigênio e/ou carbonos ligados a grupos de éteres; picos na região de 110 a 160 ppm atribuído a presença de carbono ligados a grupos aromáticos e fenólicos, bem com um pico na região próximo a 175 ppm que pode se referir a presença de carbonos ligados a grupos carboxílicos, amidas e ésteres (Dignac et al 2002;Simpson e Simpson, 2009 (Chen et al 2002;González-Pérez et al 2004;González-Pérez et al 2008;Song et al 2008;. Os ácidos húmicos apresentaram maiores valores de alifaticidade quando comparados à aromaticidade, com exceção dos horizontes: P3 Bh 280-310 que apresentaram maiores valores de aromaticidade.…”