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Organic Matter Breakdown: Releasing Nutrients for Uptake

How is organic matter broken down in the soil?

Organic matter break down is a biological process because it is the soil organisms (microorganisms, earthworms, microarthropods, ants, beetles etc) that perform the chemical and physical transformations. Break down of organic matter involves physical fragmentation, chemical alteration of organic matter and finally release of mineral nutrients. Different organisms are involved with the different stages of the decomposition.

Leaf and root systems are naturally colonised by microorganisms. Break down starts almost immediately after the organism, or part of it, dies. These colonies of microorganisms use enzymes to oxidise the organic matter to obtain energy and carbon (C). Earthworms and other larger soil animals, such as mites, collembola (springtails), and ants assist in the decomposition of organic matter by incorporating it into the sub-surface soil, where conditions are usually more favourable for decomposition than on the soil surface. The soil animals fragment the organic material, which increases its surface area and allows it to be further colonised and decomposed by microorganisms.

Soil organic matter

Can plants immediately take up nutrients released from decomposition?

Mineralisation is the biological process in which organic compounds are chemically converted to other simpler organic compounds or inorganic forms, such as ammonium or phosphate, by soil microorganisms. When microorganisms mineralise a protein molecule, for example, the molecule may undergo several changes to simpler organic molecules before the carbon is converted to carbon dioxide and the nitrogen, phosphorus and sulphur become readily available to plants as ammonium (NH4+), phosphate (PO43-) and sulphate (SO42-).

Bacteria and fungi are responsible for most of the mineralisation of organic matter in soil by releasing enzymes that oxidise organic compounds. This process releases energy and carbon, some of which is used by the microbes for constructing cellular components such as cell walls and membranes.

How long does organic matter take to break down?

The chemical compounds in organic matter undergo breakdown at different rates. The first organic compounds to be broken down are those that have simple cellular structures, such as amino acids and sugars. Cellulose breaks down more slowly and phenols, waxes and lignins will remain in the soil for the longest time. The chemical structures of these molecules are very complex; very strong chemical bonds hold the molecules together.

Stems and leaves do not break down at the same rate because they are made up of various combinations of molecules that differ in the strength of their chemical bonds. Specialised enzymes are required for their degradation. Leaves generally have more cellulose and less lignin than stems. Cellulose is a simpler, chain-like molecule that can be decomposed more rapidly than lignin which has a complex, folded structure. When lignin is linked with cellulose within plant cell walls, the cellulose becomes more difficult to degrade because it is less accessible to enzymes. Under most conditions it might take three times as long to degrade stems as it does to degrade leaves.

What is the C:N ratio of organic matter and why is it important?

The C:N ratio refers to the amount of carbon relative to the amount of nitrogen present in organic matter. There is always more carbon than nitrogen in organic matter. It is usually written as a single number, because it expresses how much more carbon than nitrogen there is. For example if the ratio is 20, this means that there are 20 atoms of carbon for each atom of nitrogen in that kind of organic matter. If the ratio is 100, it means that there are 100 atoms of carbon for each atom of nitrogen. When the number is low it means that the organic matter has a high concentration of N, while if the ratio is a large number, it means that there is considerably more carbon than nitrogen. The C:N ratio does not tell us what form the carbon and nitrogen are in, just the amounts of C relative to N.

The C:N ratio of the organic matter is important to consider in relation to the availability of nitrogen (and other nutrients) after breakdown has taken place. Incorporating organic matter that has a high C:N ratio into soil, for example, will probably cause some nitrogen deficiency in the crops/plants, at least in the short-term, unless additional N is added. If there is a shortage of nitrogen, the plants may suffer first because microorganisms can access nitrogen in soil more easily than plants. Legumes which are effectively nodulated can fix nitrogen to avoid the impact of nitrogen immobilisation.

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