Characterization of Bulk Soil Humin and Its Alkaline-Soluble and Alkaline-Insoluble Fractions

Abstract

Humic substances are the major components of soil organic matter. Among the three humic substance components (humic acid, fulvic acid, and humin), humin is the most insoluble in aqueous solution at any pH value and, in turn, the least understood. Humin has poor solubility mainly because it is tightly bonded to inorganic soil colloids. By breaking the linkage between humin and inorganic soil colloids using inorganic or organic solvents, bulk humin can be partially soluble in alkali, enabling a better understanding of the structure and properties of humin. However, the structural relationship between bulk humin and its alkaline-soluble (AS) and alkaline-insoluble (AIS) fractions is still unknown. In this study, we isolated bulk humin from two soils of Northeast China by exhaustive extraction (25 to 28 times) with 0.1 mol L-1 NaOH + 0.1 mol L-1 Na4P2O7, followed by the traditional treatment with 10 % HF-HCl. The isolated bulk humin was then fractionated into AS-humin and AIS-humin by exhaustive extraction (12 to 15 times) with 0.1 mol L-1 NaOH. Elemental analysis and solid-state 13C cross-polarization magic angle spinning nuclear magnetic resonance (13C CPMAS NMR) spectroscopy were used to characterize and compare the chemical structures of bulk humin and its corresponding fractions. The results showed that, regardless of soil types, bulk humin was the most aliphatic and most hydrophobic, AS-humin was the least aliphatic, and AIS-humin was the least alkylated among the three humic components. The results showed that bulk humin and its corresponding AS-humin and AIS-humin fractions are structurally differed from one another, implying that the functions of these humic components in the soil environment differed.

Humic substances and oxidable fractions of organic matter in different environments in Amazônia Maranhense

“…1 and 2). This pattern may have been provided, possibly, by the humin inherited from the forest, which was preserved from the action of biological activity by the initial development of the pasture, which tends to provide a greater amount of biomass that is easier for microorganisms to access (Ebeling et There is a predominance of the humin fraction in all areas and layers, which may be associated with its recalcitrance, low solubility, and the association with iron oxides (Li et al 2015), the latter very present in Plintosols (Santos et al 2018a). Other factors that may help to understand the predominance of the humin fraction would be the essentially aliphatic chemical nature of the molecule, derived from macromolecules highly resistant to biological degradation and coming from plants and/or microorganisms (Valladares et al 2011).…”

Optimized isolation method of humin fraction from mineral soil material

“…To isolate humin from soil material, different methods have been attempted (Hayes et al, 2017 ). They can be divided into following main groups: (1) digestion of mineral soil components with HF/HCl followed by alkali extraction of HA and FA (Tatzber et al, 2007 ; Zhang et al, 2017 ); (2) alkali extraction of HA and FA followed by extraction of humin by different organic solvents, as DMSO (Hayes, 2006 ) or MBIK (Rice and McCarthy 1989 ; Almendros et al, 1996 ); and (3) alkali extraction of HA and FA followed by HF/HCl digestion of mineral soil components (Kang & Xing, 2005 ; Li et al, 2015 ; Zhang & Katayama, 2012 ). Nevertheless, each of these methods has different limitations.…”

Agro-environmental applications of humic substances: A critical review

“…acid, humin is neither soluble in acidic nor in alkali aqueous solutions due to its large molecular weight (100,000 -10,000,000) (Li et al, 2015).…”