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= Humin = Humin(HU) refers to the extracted part of soil organic matter(SOM) that remains insoluble in aqueous solutions at all PH levels. Humin, together with humic acids(HA), fulvic acids(FA) form the humic substances of SOM. Humin is considered to have 50% or more of the organic carbon in mineral soils. More than 70% of that is lithified sediments.

Description
Humin is the least understood fraction of humic substances as its lack of solubility and intractable nature. For many years, humin was considered to be similar to the humic substances (HSs) in terms of the basic structure features and chemical composition. Humin was thought as a more recalcitrant form of HSs. However, over the last decades, evidence has shown that humin differs from other two fractions significantly in terms of contained components whose chemical compositions differed significantly from that of HSs, the presence of increased aliphatic and decreased aromatic and carboxyl contents.

In recent years, a new isolation procedure called NMR( Nuclear magnetic resonance spectroscopy) technique has enabled more in-depth study of the composition humin composition. In general, humin consists large portion of the aliphatic hydrocarbon and wax derived substances, and some carbonhydrate- and peptide- derived materials. Humin components are considered as macro organic substances as their molecular weights (MW) range from 100,000 to 10,000,000. In contrast, carbonhydrates’ molecular weight range from 500 to 100,000.

Potential sources of humin components
In order to have a better understanding of the origin of humin, it becomes essential to know the potential precursors based on the composition and structures and the processes from which humin components can be derived. Below is a shortlist of the potential precursors, with an emphasis on the biomolecular structures that most likely will contribute to the formation of humin.

Potential precursors like cutins, cutans, suberins, suberans, lipids, algaenans, and bacterans, and latex exudates will mainly contribute to the insoluble and nonhydrolyzable components of the humin fraction as they are widely distributed, and highly resistance to microbial degradation.

Other precursors like lignins, tannins, and melanins make substantial contributions to the formation of humic acids and fulvic acids, but at the same time make a limited contribution of the humic fraction.

Soil fertility and soil stability
The retention of nutrient cations and water retention are commonly recognized as the key functions of SOM. As a part of humic substances, humin also plays a role in retaining nutrients and water, but not very significantly. However, the humin fractions are well suited to adsorb and retain various hydrophobic lipid compounds like natural fats, oils and waxs. In addition, it also can absorb biocides, which can be used for pest control and plant disease control as well as protect plants from accidental contaminates like heavy metals.

Although some of the humin fraction is found to be closely bound with clay minerals within soil, humin fractions have a higher functional group content and greater flexibility than other two humic substances fractions, which still would benefit the soil formation process and maintenance of soil structure.

Carbon sequestration
Humin has more than 50% of the organic carbon of mineral soil, which make it has a high potential for carbon sequestration. Conversely, it means the resistant plant material that are the precursors of humin fractions would have longer turnover times and more effective to store carbon. Therefore, the use of those plants would greatly increase the humin formation and reduce the amount of carbon released from the plant residue decomposition process.

Heavy metal adsorption
Humin plays a vital role in pollutants sequestration. Humin has various function groups, which include a series of esters, methoxy alkanes and polar aromatic groups. And also, humin is extremely porous and has large surface area. Those characteristics of humin make it a potential adsorbent. It can absorb various toxic metals, such as Cu2+, Cr3+, Pb2+, Cd2+ and Ni2+. Studies show that humin has a lower adsorption capacity and a higher adsorption reversibility of Cu(II) compare to Humic acid (HA).

Natural- derived solid humin has high potential to act as a redox mediator for anaerobic bioremediation of contaminated soil even groundwater. As a redox mediator, humin donates electrons to anaerobic respiring microorganisms to help them bio-remediating oxidized pollutants.