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Soil Humus
Soil humus is a mixture of dark, colloidal organic compounds relatively resistant to decomposition. These compounds result from the decay of organic litter, and accumulate in the o and A horizons of soils. Soil humus helps to glue mineral particles into aggregates, giving structure to the soil and affecting soil stability.
Humus is formed during the decomposition of organic 'litter' (including pine needles, leaves, and animal droppings) in soils. This decay is mediated by microbes and the enzymes they excrete (which break certain specific bonds in organic matter). The main reactions of this decomposition are:
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aerobic conditions: Carbohydrate + O2 → CO2 + H2O + Energy
anaerobic conditions: Carbohydrate → CO2 + Acid or alcohol + Energy
Humus includes sugar amines, nucleic acids, phospholipids, vitamins, sulpholipids, polysaccharides and many other unclassified compounds. Fulvic acid and humin have similar structures. The COOH and phenolic OH groups are weakly acidic, which give humus its pH buffering ability, pH dependent charge and cation chelating ability. In addition, the toxicity of the phenolic subgroups which make up humus contributes to its resistance to microbial decomposition.
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The branched structure of humic molecules may also make them more resistant.
There are three main classifications of humus: fulvic acid, humic acid and humin. Humin is insoluble but fulvic and humic acids are soluble in dilute NaOH solution. Humic acids precipitate in acidic solution, but fulvic acids remain soluble. Humic molecules are incredibly varied in composition, but generally are characterised by:
Many active chemical functional groups exposed to the surrounding solution for reaction with other substances in the solution.
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A very large cross-linked and folded molecule with molecular weights in hundreds of thousands of grams per molecule.
Humus has a large surface area per unit of mass and is highly charged (similar to clay), and individual humus molecules are dynamic and constantly changing form (but may remain as humus for several thousand years).
These reactions are much complicated by the complex structure of the litter, and the products of decomposition include various nutrients, organic acids and amines (depending on the conditions and starting materials) in addition to the resistant residues or humus.
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Between 60 and 80 percent of the carbon from most plant residues is evolved as CO2 within a year of deposition; 5 to 15 percent is incorporated into the microbial biomass and the rest remains in soil humus.
There are different pathways of decomposition of organic matter to form humus. In one of the pathways, polyphenols are produced (either from the decomposition of lignin - a rigid polymer that, with cellulose, makes up woody material-or through microbial synthesis from nonlignin sources), then enzymatically oxidised to form quinones which polymerise creating humic macromolecules.
Early in decomposition, simple, phenolic compounds, bits of lignin and melanin (lignin-like molecules) and other phenolic polymers are transformed by beta-oxidation of side chains, addition of hydroxyl groups, oxidation of methyl groups and decarboxylation.
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The more reactive compounds are oxidised and then converted to radicals, which stabilise by linking into dimers or forming quinones (oxidised polyphenols). These linkages repeat to create humic macromolecules.
During the decomposition or humification of organic litter, the carbon to nitrogen (C/N) and carbon to sulphur (C/S) ratios decrease, indicating that relatively more carbon than nitrogen or sulphur is lost in the process.
Numerous factors control or influence the decomposition of organic matter. Among these are the properties, amount and stage of decay of the organic matter, and the availability of oxygen, temperature, soil moisture, nutrients and soil texture. Temperature and water availability affect the decomposition rate. As acidity increases, soil respiration decreases, leading to an accumulation of organic matter and a drop in the rate of litter decomposition. When soil moisture is considered, the presence of live roots stimulate the decomposition of organic material because the roots increase microbial activity.
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