LFS:SoilWeb/Interactions Among Soil Components/Organic Matter and Soil Air Interactions

Organic matter decomposition

Decomposition occurs most rapidly in well aerated soils. When organic plant residues are incorporated into such a soil, three general reactions occur:

Carbon compounds are enzymatically oxidized to produce carbon dioxide, water, energy, and decomposed biomass.

Elements essential to plant nutrition, such as N, P, and S, are released and/or immobilized by a series of specific reactions that are relatively unique for each element.

Compounds are formed that are very resistant to microbial actions.

In poorly aerated soils, decomposition occurs more slowly. This is because many of the microbial organisms involved in decomposition are aerobic, that is they can only survive (and decompose) in the presence of oxygen. When oxygen is not abundant, there are fewer organisms available to do the work and partially decomposed organic residues build up in the soil.

The time required for decomposition and mineralization is highly variable, and can range anywhere between a few days and years. The rate of decomposition depends on:

The environmental conditions in the soil including water content, temperature, pH, and aeration.
The quality of the added residues as a food source for soil organisms.

See Soil Organisms for related information.

Aerobic and Anaerobic Respiration and Fermentation

Respiration is a process carried out by heterotrophic organisms to obtain useful energy from the oxidation of organic matter. Recall that in oxidation reactions, electrons are donated whereas in reduction reactions electrons are accepted. Oxidation and reduction reactions always occur together because a substance can only donate (accept) electrons if another substance can accept (donate). It is helpful to recall the following:

Oxidation-reduction (redox) reactions require the presence of electron donors and acceptors. The difference between aerobic and anaerobic respiration is the type of electron acceptor available:

In aerobic respiration, microbes and plants use oxygen to metabolize organic compounds. Oxygen is the strongest electron acceptor and yields the most energy from oxidation.

In anaerobic respiration, oxygen is absent so soil microbes use different electron acceptors, such as Fe³⁺, Mn⁴⁺, NO₃^- or SO₄^2-, to metabolize organic compounds. These secondary electron acceptors produce less energy from oxidation than oxygen. Their reduced oxidation states (e.g. NO₂^-) are often toxic to plants and soil microbes.

Another means of metabolizing is fermentation:

In fermentation both oxygen and secondary electron acceptors are absent, so soil microbes metabolize by rearranging organic molecules into more stable compounds, releasing some of molecule's bound energy. This process produces less energy than either aerobic or anaerobic respiration. Some soil organisms lack the enzymes needed for this process or they may be unable to function when large quantities of the reduced product accumulate.