The Life of Organic Matter in the Soil

Decomposition – The After Life of Organic Matter in the Soil.

There are different metabolic pathways that organisms employ to obtain energy from organic materials in the soil environment. The balance between these modes of decomposition determines important soil characteristics and the availability of essential plant nutrients.

Under aerobic conditions, respiring organisms such as saprophytic fungi, nematodes, protozoa and certain bacteria use oxygen to release energy from plant and animal matter. This is the primary type of decomposition that occurs in aerated topsoil and surface litter, and promotes the rapid turn over of nutrients. Organic acids produced in the process also liberate minerals from the soil matrix. While aerobic decomposition and organic acid mineralisation may release nutrients, making them available for plant uptake, nutrients in ionic states are also subject to leaching.

The carbon, in organic soil compounds that are decomposed aerobically, is lost as carbon dioxide

There are certain environments however, as found within soil aggregates or the stomachs of earthworms for example, where oxygen is deficient, so different metabolic pathways are required. Under these conditions, a large group of bacteria and yeasts, collectively termed facultative anaerobes, can switch from aerobic respiration to anaerobic respiration or fermentation in order to acquire energy from organic compounds. The foods used by these facultative anaerobes include plant exudates, microbial metabolites, and the by products of previous decomposition activity. In the absence of oxygen, they recruit alternative molecules and ions as electron acceptors in order to derive energy from these foods. The conversion of nitrate to ammonium and the forms of manganese and iron that are plant available are very much dependant on these reductive reactions.

In the latter stages of decomposition, through the actions of facultative anaerobes, organic materials are reduced to minute factions that become increasingly resistant to further breakdown. As such, not all the inherent carbon is released. These factions adhere to the surfaces of clay and silt particles in relatively stable organo-mineral particles, with charged surfaces, that attract and hold onto plant nutrients. Organo-mineral particles bind together as micro aggregates, which confer a conducive environment for the further perpetuation of lasting organic compounds. This is the predominant mechanism by which longer-term soil carbon or humus is built.

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