DECOMPOSITION
Introduction
Decomposition is the reduction of energy rich organic matter by consumers,
largely decomposers and detritivores to CO2, H2O and inorganic nutrients.
Decomposition involves the loss of heat energy and the conversion of organic
nutrients into inorganic ones. It is the result of various processes including
fragmentation, mixing, change in physical structure, ingestion, egestion and
concentration accomplished by diverse organisms linked together in highly tangled
food webs.
It involves all consumers to some extend because the passage of organic
matter through the digestive system results in degradation of organic matter and the
contribution of changed and partially decomposed material in the form of faeces as a
substrate for other feeding groups. True decomposition is accomplished mainly by
decomposers, the bacteria and fungi and by detritivores which feed on dead matter
of all kind.
Decomposers
Organisms most commonly associated with decomposition are the microflora —
the bacteria and the fungi. Bacteria may be aerobic requiring oxygen as the electron
acceptor or they may be anaerobic, able to carry on their metabolic functions without
oxygen by using some inorganic compound as the oxidant (commonly found in
aquatic mud and sediment and in the lumen of ungulate herbivores). Fermentation ie
conversion of sugar to organic acids and alcohol is less efficient in the breakdown of
organic matter. It lowers the pH of the substrate and favors fungal activity. Many
decomposer bacteria are facultative anaerobes – they use oxygen when it is present
but in its absence they can use some inorganic compounds as their energy source.
The dominant microfloral decomposers of animal matter are bacteria while that
of plant matter are fungi.
Fungi extend their hyphae over and into the detritus to withdraw nutrients.
Bacteria and fungi secret enzyme into plant and animal material. Some of the
products are absorbed as food and the remainder is left for other organisms to
utilize. Once one group has exploited the material to its capacity another group of
bacteria or fungi able to utilize the remaining material more resistant to
decomposition like lignin, cellulose succeed them. A succession of microflora occurs
in the detritus until the material is finally reduced to inorganic nutrients.
Detritivores
Decomposition is aided by the fragmentation of detritus caused by the litter
feeding invertebrates – the detritivores. They fall into four main category :
(a) Microfauna : Body length less than 100 micron. Represented by protozoans
(b)Mesofauna : Body length ranges between 100 micron to 2mm. Represented
by mites, spring tail and potworms
(c)Macrofauna : Body length ranges between 2mm to 20mm. Represented by
nematodes, caddis fly larva and may fly and stone fly nymphs
(d) Megafauna : Body length greater than 20mm. Represented by snails,
earthworms and millipedes
These detritivores feed on plant and animal remains as well as on faecal
material. In aquatic and semi-aquatic ecosystem this group includes molluscs and
crabs.
Microbivores
Energy and nutrients incorporated in bacterial and fungal biomass do not
go unexploited in the decomposer world. Microbivores feed on bacteria and fungi.
This group is made up of protozoa such as anoebus; springtails (colombola),
nematodes, larval forms of beetles (coleoptera), flies (diptera) and mites (acarina).
Microbes act as regulators of decomposition. They may also reduce
microbial population or they may promote microbial activity by preventing the
microbes from overpopulation and by maintaining them at a level of maximum
productivity. Thus, Microbivores prevent aging and senescence of bacterial and
fungal population. On the other hand, they also stimulate growth by dispersing
fungal spores.
Formation of humus
As the amount of energy decreases in time, the least decomposable
material, largely derived from lignin is left behind as humus. Humus is structurally
non-cellular, dark colored chemically complex organic material whose characteristic
constituents are humin, a group of unchanged plant chemicals and other organic
compounds such as fluric acid and humic acid.
_____***_____
