A comparison of the compositional differences between humic fractions isolated by the IHSS and exhaustive extraction procedures

Abstract

Humic substances (HSs), consisting, on the basis of solubilities in aqueous acid and basic media, of humic acids (HAs), fulvic acids (FAs), and humin (Hu), are the major components of soil organic matter (SOM). Most studies of soil/natural organic matter (SOM/NOM) have been carried out on extracts of soils in dilute sodium hydroxide solutions, the solvent used to extract the Standards of the International Humic Substances Society (IHSS). However, Hu, the major component in the classical definition of HSs, is insoluble in aqueous base and is not isolated by the traditional IHSS method. Recently, a sequential exhaustive extraction (SEE) process has been shown to be capable of isolating and separating the major components of the classically defined HSs from the soils of the temperate and tropical regions. The SEE system was used in the present study to isolate the HA/FA and Hu fractions from a subtropical volcanic Taiwanese soil. Chemical and compositional properties of these extracts were then compared with similarly obtained isolates from soils from the different climatic regions. Increases in the aliphatic relative to aromatic carbon contents were observed for both the HA and FA fractions when the pH values of the extraction media were increased. HAs and FAs isolated using the SEE method have spectroscopic profiles similar to those from the IHSS isolate; however, the cumulative extraction efficiency (%) of the SEE method (65 %) for the volcanic soil was much higher than for the traditional IHSS method (33 %). When the residual volcanic soil, following extractions once, three, and eight times with 0.1 M NaOH were then extracted with dimethyl sulphoxide (DMSO) plus concentrated sulphuric acid (the final solvent in the SEE sequence) it was seen that the content of crystalline polymethylene hydrocarbon (33 ppm (13)C-NMR resonance in the Hu (or DMSO/acid)) extract increased relative to the amorphous methylene (30 ppm). That highlights the difficulty in dissolving the more highly ordered hydrocarbon structures that would be expected to have closer associations with the mineral colloids. Although the SEE procedure isolated all of the HAs and FAs from the Yangmingshan soil, extractability of the Hu from the volcanic soil in the DMSO/acid solvent was low (21 %), and contrasted with the much higher yields from temperate and tropical regions. The decreased Hu extraction may arise from its associations with the extensive iron and aluminium hydroxide mineral colloids in the soil. The Hu from this sub-tropical soil was different from the Hus isolated from other soil types, indicating the need to isolate and characterise these recalcitrant organic material in order to understand the organic carbon components in soils in greater detail. Such results would indicate that more attention should be given to mineral colloids in soils, and to the organo/mineral associations that will have an important role in the stabilities of OM in the soil environment.

Interactions between humin and potentially toxic metals: Prospects for its utilization as an environmental repair agent

“…The E 4 /E 6 ratio was ascertained by measuring approximately 2.0 mg of humin in 10 mL of a NaHCO 3 0.05 mol L À1 solution and later reading the absorbance at 465 and 665 nm on a DR 3900 spectrometer [13,22].…”

“…Among the naturally occurring substances that have this characteristic and that are globally distributed in aquatic and land ecosystems, humic substances (HS) stand out [9][10][11][12][13].…”

“…HS can be fractioned based on their solubility into fulvic acids, the fraction of HS soluble at different pH levels; humic acids, which are insoluble at acidic pH and soluble at basic pH; and humin, which exhibits low solubility at different pH levels [13].…”

Humic substances and its distribution in coffee crop under cover crops and weed control methods

“…Low E 4 /E 6 ratios are considered to be indicative of the relatively higher degree of aromatization while high ratios indicate more aliphatic structures in the organic fractions (Chang et al, 2014).…”

Chemical composition of soil organic carbon changed by long-term monoculture cropping system in Chinese black soil

“…Litter decomposition increased the proportions of carboxyl C, alkyl C and aromatic C, and decreased that of O-alkyl C in forest (Zech et al 1997). Balanced fertilizer increased alkyl-C/O-alkyl-C ratio and decreased aromaticity in maize-wheat rotation system (Xu et al 2017a), and further improved the aliphatic nature of humin (Chang et al 2014, Xu et al 2017b. All these suggested that soil management would change the structural composition of SOC.SOC derives mainly from plant residues and thus land-cover change will lead to variation in the molecular nature of SOC (Wickings et al 2012) due to quality and composition of various plant residues.…”