Decomposers are the final step in the food chain and they reduce the organic matter into inorganic forms so that it is once again available to the producers. The upper layer of soil is the main site for the action of decomposers. In this way, decomposers provide energy and nutrients to the producers.
The upper layer of soil consists of detritus which includes fallen dried leaves and dead remains of plants and animals. This can be found as litter fall or root detritus.
The process of decomposition includes three steps:
a. Fragmentation of Detritus:
Fragmentation of detritus takes place with the help of invertebrates like snails and earthworms. As they feed on detritus they are called as detritivores. When detritus passes through their alimentary canal it gets fragmented or pulverised. This increases the surface area of detritus. They also add certain growth substances which help in the growth of microbes. A few detritivores are caprophagic that is they can break down the faecal pellets of animals.
b. Leaching:
This is the process by which simple and water soluble compounds percolate down to lower layers of soil with moving water.
c. Catabolism:
During this process certain bacteria and fungi release extracellular enzymes. These enzymes help in converting decomposing detritus to inorganic forms. The enzymes are specific and work on a specific detritus, for example Pseudomonas bacteria decomposes proteins into ammonia and simple nitrogen compounds.
The result of the process is humification and mineralisation. Humification means formation of humus which is dark coloured amorphous substance formed from dead and decomposing material. It is highly resistant to microbes because of which it undergoes slow decomposition. Humus is a reservoir of nutrients. Mineralisation refers to the addition of inorganic substances like CO2, H2O and minerals like NH4+, Ca++, Mg++, K+, etc. to soil.
There are times when these nutrients instead of getting mineralised, get bound to the biomass of decomposers. Due to this they become temporarily unavailable to other organisms. This is known as immobilisation of nutrients (Fig. 4).
Factors Affecting Decomposition:
Climatic factors and chemical quality of detritus are the deciding factors of the rate of decomposition. Climatic factors include soil moisture, temperature and oxygen. For rapid rate of decomposition, the temperature should be more than 25°C with sufficient amount of humidity. If the temperature is very high with dryness or the temperature is very high with plenty of moisture the rate of decomposition slows down.
As far as chemical quality of detritus is concerned, detritus which is rich in lignin and chitin and shows slow rate of decomposition whereas, detritus rich in nitrogenous compounds and deficient in lignin shows much faster rate of decomposition.