Formation of Humus

Formation of Humus

	Formation of Humus 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: Aerobic conditions Carbohydrate + O₂ → CO₂ + H₂O + energy Anaerobic conditions Carbohydrate + CO₂ → Acid or alcohol + Energy.
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. Between 60 and 80 per cent of the carbon from most plant residues is evolved as CO₂ within a year of deposition; 5 to 15 per cent is incorporated into the microbial biomass and the rest remains in soil humus. Water-soluble, readily available substrates such as sugars, amino acids and pyrimidines are quickly metabolised during decomposition (generally within a few hours or days) while less reactive compounds are digested more slowly.
This imbalance leads to accumulation of more recalcitrant compounds (i.e. humus) in the soil. Early in decomposition, simple, phenolic compounds, bits of lignin and melanin (lignin-like molecules) and other phenolic polymers are transformed by b-oxidation of side chains, addition of hydroxyl groups, oxidation of methyl groups and decarboxylation. The more reactive compounds are oxidised and then converted to radicals, which stabilise by linking into dimers or forming quinones (oxidised polyphenols).
The less reactive compounds add onto the polymers through nucleophilic addition to quinones. 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.
	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 was considered, the presence of live roots stimulated the decomposition of organic material because roots increased microbial activity. In summary, through a series of complex reactions, microbes mediate the conversion of organic material such as leaves and twigs to the dark humus which colours and glues soils. Without these processes, the soils would be loose, non-cemented dusts and no life would be able to grow on them, and the world would be a very different place.

Home | Site map | Submit Article | Directory | Search