In this article we will discuss about the factors that govern the rate of decomposition of organic matter. Also learn about the microorganisms that are associated with organic matter decomposition.
(i) Litter Quality:
Structural and chemical properties of litter make up its quality. However, it is less specific term reflecting decomposition rate. Dubey and Pandey (1996) have recorded the higher number of fungal taxa from decomposing litter of Cupressus torulosa than Pinus roxburghii litter decomposing in respective forests of Kumaon Himalaya.
In contrast, the litter of bryophytes are decomposed at a very slow rate because of presence of lignin like complex chemicals in its thalli.
(ii) Temperature:
Temperature governs the growth and microbial activity in its natural habitats. Singh (1989) studied the fungal communities associated with decomposition of leaf litter of oak in different forests along an elevation transect.
At different elevations temperature differs. They found that species diversity and fungal counts on oak leaf litter were markedly affected by the environment changes brought about by the native leaf litter. The pattern of fungal species occurring on litter changed with the progress of decay of substrate.
(iii) Aeration:
In the pores of soil, sufficient amount of oxygen is present which is required by aerobic flora. In water-logged conditions where O2 becomes a limiting factor aerobic microorganisms will be absent, and only anaerobic microorganisms will grow and decompose the organic matter. Soil texture affects aeration and the later affects microorganisms.
(iv) Soil pH:
Soil pH is governed by the presence of cations and anions. Certainly these affect microbial growth. For example, actinomycetes prefer to grow above soil pH 7, bacteria below 7, and fungi between pH 5 to 6.
(v) Inorganic Chemical:
The concentration of already available inorganic substances also affect the rate of decomposition of added matter on soil. In addition after decomposition from humus the elements N, P, K, Na, Mg, Ca, etc. are released in soil. Some amount is taken up by the growing microorganisms and the remainder is made available to plants.
(vi) Moisture:
Soil moisture varies according to water holding capacity (WHC) of a given soil. However, water is required to carry out the physiological processes. Significance of water in soil for various activities of microorganisms has nicely been discussed by Griffin (1974) in his book Ecology of Soil Fungi.
Rai and Srivastava (1982) have found the half life of litter in a tropical mixed dry deciduous forest about 173 days. Moisture plays a critical role in determining the activity of microorganism in decomposition.
Microorganisms Associated with Organic Matter Decomposition:
(i) Cellulose Decomposers:
Following are the decomposers of cellulose:
Fungi:
Altemaria, Aspergillus, Chaetomium, Coprinus, Fomes, Fusarium, Penicillium, Rhizopus, Trichoderma, Trametes, Verticillium, etc.
Bacteria:
Bacillus, Cellulomonas, Clostridium, Corynebacterium, Cytophaga, Pseudomonas, Vibrio, etc.
Actinomycetes:
Micromonospora, Nocardia, Streptomyces, Streptosporangium, etc.
(ii) Hemicellulose Decomposers:
Following is a list of decomposers of hemicelluloses:
Fungi:
Alternaria, Aspergillus, Chaetomium, Fusarium, Glomerella, Penicillium, Trichoderma, etc.
Bacteria:
Bacillus, Cytophaga, Erwinia, Pseudomonas, etc.
Streptomycetes:
Streptomyces, etc.
(iii) Lignin Decomposers:
Many of the basidiomycetes are capable of degrading lignin but the reaction is slow, because of the slow growth of microorganisms. Aerobic bacteria are also able to bring about some degradation of lignin, but little attention has been paid.
Examples of some of the lignin decomposers are given below:
Fungi:
Agaricus, Armillaria, Clavaria, Clitocybe, Coprinus, Ganoderma, Phaliota, Pleurotus, Polyporus, Poria, Trametes, Ustulina, etc.
Bacteria:
Species of Arthrobacter, Flavobacterium, Micrococcus, Pseudomonas, Xanthomonas, etc.
Organic Matter Dynamics in Soil:
When organic matter is present in soil it is gradually rendered into a uniform, dark coloured amorphous mass by microorganisms which is designated as humus. Humus serves as a source of energy for the development of various groups of microorganisms and as a result of decomposition of CO2, NH3 and other products are given off.
Fungi along with the other microbial groups chiefly bacteria and actinomycetes decompose the organic matter in soil and release the nutrients which are locked up in complex form in organic matter. However, the process of decomposition starts when the plants are in senescent stage.
After coming in the contact of soil some other groups of microorganisms colonize the litter. At this stage termites too play a role in physical breaking of the litter. Thereafter, microorganisms of different groups colonize the substrate depending upon its chemical composition. Thus microbial succession occurs on the decomposing material till it fully disappears in elemental forms.
The event of sequential appearance of microorganisms on a substrate with respect to time is called succession. Mostly succession of fungi on decaying material has been worked out in detail. Garrett (1981) has given a general trend of fungal succession on a substrate as given in Table 30.4.
During the course of decomposition, the water-soluble components are metabolised first. Thereafter, cellulose and hemicellulose disappear gradually. The lignin’s disappear in the last stage of decomposition because these are resistant to decomposition. During the course of disappearance of one component and start of the other, the spectrum of microflora i.e. decomposers is changed.
