Earthworms for bioconversion
The potential of earthworms to stabilize the organic refuse into useful components has been known only recently. Mary Appelhof, the ‘Worm Woman’, with her experiments in the early 1970s, popularized the idea of using worms for waste conversion and brought it to the public’s attention. In addition to this work, the publication of the “Proceedings of a Workshop on the Role of Earthworms in the Stabilization of Organic Residues” in 1981, 100 years after Darwin's study, was also responsible for the accelerated interest on the use of earthworms in breaking down organic wastes within and outside of the United States. Edwards (1998) reported five earthworm species (D. veneta, E. eugeniae, P. excavatus andP.hawayana and E. fetida) to be the most potential earthworms for breakdown of organic refuse. Generally most organic wastes can be broken down as such, except for those, which might need some pre-digestion prior to feeding. Earthworms are highly adaptable to different types of organic waste, provided, the physical structure, pH and the salt concentration are not above the tolerance level (Seenappa, et al, 1995). In most of the cases, the feedstock is thermophilically composted in windrows (turned twice weekly), for 15 to 30 days before being fed to earthworms.
Earthworms convert the smelly organic matter into a dark, odourless, homogeneous material called castings or vermicast which is an ideal plant growth supplement. It is often referred to as 'Black Gold' by gardeners. Earthworms feed partly on the waste itself, but mostly on the microorganisms produced during decomposition. Their movement through the waste assists the break down and aeration of the material, providing ideal conditions for microbes to flourish, which in turn accelerates the decomposition rate of the organic matter. The waste entering the earthworm gut is subjected to biochemical break down by the enzymes secreted in the gut wall of the animal and by the microorganisms therein. The resulting product is a colloidal humus that acts as a slow release fertilizer. The nutrients are easily available to plants, but resist leaching. The rate of decomposition also depends on the type of litter.
Fragmentation and breakdown
The rate of organic matter breakdown depends mainly on the type of litter. Soft plant and animal residues may be decomposed by the soil micro-flora. Tougher plant leaves, stems and root material do not break down easily; they are first disintegrated by the soil animals, including earthworms. Earthworms thus have an important role in this initial process of the organic matter cycle. Soils with few earthworms have a well-developed layer of un-decomposed organic matter lying on the soil surface. Many types of leaves are not acceptable to the earthworms when they first fall on the ground, but require a period of weathering before they become palatable. It is believed that this weathering leaches the water-soluble poly-phenols from the leaves. These tiny creatures are responsible for the translocation of the accumulated organic debris from the soil surface to the subsurface layers and during this process much of the organic materials are ingested, macerated and excreted. Earthworms are also known to contribute several kinds of nutrients in the form of nitrogenous wastes (Lakshmi and Vijayalakshmi, 2000).
Consumption and Humification
Earthworms are reported to consume more organic matter from the soil surface than the other entire smaller soil animals put together (Ronald and Donald, 1977) .The amount they turn over depends on the availability of total suitable organic matter. If the soil physical conditions are suitable, the abundance of earthworm increases until the food becomes a limiting factor. The smaller earthworms that feed on the litter produce cast that are almost entirely fragmented litter, whereas the larger species consume large proportion of soil, and there is less organic matter in their casts. The final process in organic matter decomposition is the humificaton, in which the large organic particles are converted into a complex amorphous colloid containing phenolic materials. Only about one fourth of the organic matter becomes converted to humus. The major contributions of earthworms are in breaking up of organic matter, combining it with soil particles and, enhancing microbial activity. They also mix the humified material into soil.
Nitrogen mineralization
Earthworms greatly increase the soil fertility, and part of this must be due to the increased amounts of mineralized nitrogen that they make available for the plant growth. There have been reports of increase in the amount of nitrogen in the soil in which the earthworms are reared. This may be due to the decay of the bodies of dead earthworms, which are rich in proteins.
Govindan (1998) reported that earthworm body contains 65% protein, 14% fats, 14%carbohydrates and 3% ash. Similarly, Ronald and Donald (1977) reported that 72% of the dry weight of an earthworm is protein and that the death of an earthworm will release up to 0.01 g of nitrate in the soil. Also, earthworms consume large amount of plant organic matter that contains considerable quantities of nitrogen, and much of this is returned to the soil in their excretions.
Effects on the C/N ratio
Plant roots in general cannot assimilate the mineral nitrogen unless the Carbon/Nitrogen (C/N) ratio is in the order of 20:1 or lower. Earthworms help to lower the C/N ratio of fresh organic matter during respiration (Ronald and Donald, 1977b). To assess the role of earthworm in lowering the C/N ratio, the consumption of the carbon must be measured, and this can be done approximately, by measuring the respiration. But the disadvantage of laboratory studies is that they do not always reflect the actual situation. There was a remarkable reduction in C/N ratio of vermicompost than in the compost.