Habitat, Diversity and Functions of Various Soil Organisms

Earthworms are extremely important soil macro-fauna, especially in the upper 15-35cm of soil. There are more than 1,800 species. Their numbers vary widely in different soils, though in arable soils they may range from 30-300 per square metre, with a biomass of 110-1100 kg/ha. They tend to dominate soil fauna in regions with at least 800 mm of annual rainfall and prefer a well aerated, moist habitat such as medium-textured well-drained soils. Their numbers decrease as a result of predators, ammonia fertilisers, certain insecticides and soil disturbance through tillage. However, minimum tillage which leaves crop residues and mulch encourages earthworms.

Earthworms ingest and grind up organic matter and soil minerals, digest and assimilate certain elements, and eject soil casts which are stable soil aggregates significantly higher in bacteria, organic matter and available plant nutrients The extensive system of burrows, up to 1-2 metres deep, serves to increase aeration and drainage. They are of major importance in soil development and agricultural production as they enhance soil fertility and productivity through increasing availability of mineral nutrients to plants and integrating undecomposed surface residues, into the soil, hence reducing loss of nutrients, increasing O.M. content and improving soil structure. In one year in one hectare of land, earthworms may ingest between (22 to 450 tons/acre). They rely on organic matter as a source of food and thrive where farm manure or plant residues have been added to the soil.

Termites: There are some 2,000 species of termites or "white ants" found in 2/3 of the world's land but predominantly in grassland, savannah and forests in tropical and subtropical regions. They dominate the soil fauna in the drier tropics (<800 mm rain per year). They have a complex social life in nests or mounds above or below the ground, each home to over 1 million termites.

In building their "cities", including the network of passages which may spread 20-30 metres beyond the mound, termites transport soil from lower layers to and above the surface soil level, thereby bringing about extensive mixing of soil materials and the plant residues they use as food. They have significant effects on soil formation in terms of volume of soil moved though their effect on soil productivity is generally less beneficial than that of earthworms as the digestive processes of termites, aided by micro-organisms in their gut, tend to be more efficient.

Termite deposits are largely built from subsoil, thus commonly have a lower organic matter and nutrient content than the surrounding undisturbed topsoil, except if the subsoil is richer. They generally feed on rotting woody materials and plant residues although some eat living woody material and sound deadwood. They disrupt crop production by their mounds or nests and tend to obtain surface residues from wide areas of land and incorporate them in localised areas thus hindering the build up of a protective layer of crop residue mulch in croplands. (The bacterial metabolism in the guts of termites accounts for a substantial fraction of the global production of the greenhouse gas, methane.)

Soil Micro-fauna :Of the abundant microscopic animal life in soils, three groups are of some importance: nematodes, protozoa, and rotifers. The soil micro-fauna resemble those in water bodies as the soil provides a habitat that is intermittently aquatic. Nematodes: These generally invisible, unsegmented roundworms, commonly called threadworms or eelworms, are found in almost all soils, often in very large numbers and great diversity. Protozoa: Single celled protozoa are the most varied, numerous and simplest form of animal life, sometimes classified as Protista. More than 250 species have been isolated in soils, sometimes as many as 40 or 50 of such groups occur in a single sample of soil. They generally thrive best in moist well-drained soils and are most numerous in surface horizons, the biomass ranging from 20 to 200 kg/ha.

Nematodes: Many live on decaying organic matter (saprophyes) or are predatory on other micro-organisms. Some, such as the parasitic genus Heterodera, infest the roots of plants and if the resulting wounds become heavily infested by secondary pathogens this results in stunting and decreased yield especially to soyabeans, solonaceous crops and fruit trees. Nematodes have been controlled by long rotations (e.g. 5 years) with non-host crops, including for example canola (rapeseed) which produce root exudates with nematocidal properties, use of genetically resistant varieties and soil fumigation. In recent years and through international efforts, nematocides such as methyl bromide are being strictly restricted because of toxic environmental effects.

Protozoa: Some protozoa are predators on soil bacteria and other microflora, especially in the area immediately around the plant roots (rhizosphere). Generally they are not a major factor in organic matter decay or nutrient release, however, many serious plant and animal diseases are attributed to protozoan infections.

Rhizosphere: The living roots of higher plants are treated here as soil organisms as they grow and die in the soil and affect the chemical and physical properties of the soil and the soil nutrient availability. In an average cereal crop, after harvest some 2,500-4,500 kg/ha of root residues may remain, some 15-40% of the above ground crop.

Roots absorb water and soluble nutrients from the soil solution directly but also release significant quantities of organic compounds at the surface of young roots thereby supplying carbon and energy to other soil organisms They open up the soil, cause soil agreggation and help to maintain soil organic matter through the cycle of growth, death and decay. Through their processes including the exusion and release of organic compounds, plant roots are extremely important soil organisms.

Soil Algae: Several hundred species of algae form three general groups - green , yellow-green and diatoms- have been isolated from soils, but a small number are prominent throughout the world. They consist of eukaryotic cells: have a nuclei within a nuclear membrane. Algal populations typically range from 10,000 - 100,000 cells per gram of soil. Green algae prefer moist, non flooded acidic soils while diatoms prefer well drained land rich in organic matter. Blue-green algae, Cyanobacteria, are prokaryotes and are usually classified as bacteria Soil Algae contain chlorophyll enabling them like plants to carry out photosynthesis if exposed to light and moisture. They produce substantial O.M in some fertile soils and certain algae excrete polysaccharides which increase soil aggregation.

Soil Fungi are extremely diverse, some 700 species of some 170 genera have been identified which can be divided into 3 groups: yeasts, molds, and mushroom fungi. The symbiotic association between certain fungi and the roots of higher plants, called mycorrhizae ("fungus root") is very important. Yeasts are single celled organisms that live mainly in waterlogged anaerobic soils. Molds (microscopic) and mushrooms (macroscopic) are made up of filaments of cells or hyphae which may mass together to form mycelia.

Soil Fungi have plant like eukaryotic cells but no capacity for photosynthesis. They depend on living or dead OM for carbon and energy and, especially the molds, play a very important role in the transformation of soil constituents. Mushrooms prefer moist conditions rich in organic matter, as associated with forest and grass vegetation. In addition to the above ground fruiting body an extensive system of hyphae permeate the soil and play an important role in the breakdown of OM and woody tissue. Molds play a much more important role in soils than mushroom fungi and predominate in acid surface soils, where bacteria and actinomycetes offer only mild competition. Molds tolerate acid conditions and are important in decomposing organic residues in acid forest soils. Many genera of molds are found in soils, four of the most common are: Penicillium, Mucor, Fusarium and Aspergillus.

One of the most ecologically and economically important activities of soil fungi is the symbiotic association between certain fungi and the roots of higher plants, called mycorrhizae ("fungus root"). Research findings of recent years have brought to light the significant role mycorrhizae play in helping most plants, including most crops, absorb nutrients from the soil. Indeed many plants cannot survive without this relationship. The fungi obtain sugars directly from the plant, the fungi extend the plant root system, some 5-15 cm from the root, and provide maybe 10 times more absorptive surface. They assist in uptake of nutrients that are relatively immobile and in low concentrations, especially phosphorous but also other minerals (Zn, Cu, Ca, Mg, Me, Fe). They protect plants form excessive uptake of salts and toxic metals in saline, acid or contaminated soils and may even protect the plant from certain soil borne diseases.

There are 2 types of mycorrhizal associations of considerable practical importance. The ectomycorrhiza group includes hundreds of different fungal species associated primarily with trees, such as pine, birch, hemlock, beech, oak, spruce and fir. These fungi, stimulated by root exudates, cover the surface of feeder roots with a fungal mantle. The endomycorrhiza group, the most important of which are called vesicular arbuscular (VA) mycorrhizae, penetrates the root cell walls and forms hyphal masses within the root cells. There are over 100 identified species of VA mycorrhizae associated with most agricultural and horticultural crops except for Cruciferae (cabbage canola etc) and Chenopodiaceae (sugar beet , spinach, etc) and many forest trees. The mycorrhiza also play a role in stabilising soil aggregate structure and in enhancing nodulation and N fixation by legumes. They may be disrupted by tillage and periods of fallow and favour crop rotations which provide continuous host plants.

Soil Actinomycetes are extremely numerous and may reach hundreds of millions per gram of soil and in weight may exceed bacteria, for example up to 5000kg per ha in surface soils. They resemble molds in that they are filamentous and often profusely branched. They are smaller and similar to bacteria in being unicellular. They are prokaryotes and are classified with bacteria in the kingdom Monera.

Soil Actinomycetes are of great importance in the decomposition of soil organic matter and certain ones can fix atmospheric nitrogen into ammonium which is available to higher plants. They are important in the later stages of decay as they can reduce to simpler forms even the more resistant compounds, such as cellulose, chitin, and phospholipids. Many species produce antibiotic compounds which kill other microorganisms.

Soil Bacteria are extremely diverse, single celled prokaryotic organisms with typically some 20,000 different species in one gram of soil. A biomass of some 400-5,000 kg/ha is commonly found in the tilled layer of fertile soil. Their capacity for rapid reproduction allows bacteria to adjust their activities quickly in response to changes in their environment.

Soil bacteria are either autotrophic or heterotrophic. The autotrophs obtain their energy from sunlight or the oxidation of inorganic substances such as ammonium, sulfur, and iron and most of their carbon from carbon dioxide. They are vital in controlling nutrient availability to higher plants. Most are heterotrophs and along with actinomycetes and fungi account for the general breakdown of OM in soils. . Anaerobic bacteria allow decomposition in wetlands, releasing methane and nitrous oxide into the environment.

They possess a broad range of enzymatic capabilities including the breakdown of substances such as insecticides and organic toxins. They also dominate in enzymatic transformations such as oxidation and reduction of selected chemical elements including nitrification (liberation of available nitrates from annomium). A second critical process that depends largely on bacteria is nitrogen fixation, the biochemical combination of atmospheric nitrogen with hydrogen to form organic nitrogen compounds usable by higher plants. The process can be carried out by bacteria in soils independent of plants, but the amount of nitrogen fixed is much greater if the bacteria are intimately associated with plant roots. Rhozobium and Bradyrhizobium spp (rhizobia) are the best known which infest and form nitrogen fixing nodules on the roots of the legume family which includes many economically important crop, forage and pasture plants.

Cyanobacteria (blue-green algae) contain chlorophyll and can photosynthesise. They are especially numerous in rice paddies and other wetland soils, and when such lands are flooded, appreciable amounts of atmospheric N are fixed by these organisms. Especially recognised for N fixation is the cyanobacteria growing within the leaves of the aquatic fern Azolla.