1. Earthworms are certainly beneficial to soil environment. Approximately 7000 species of earthworms reported worldwide are probably the most important macroanimals in most soils. These hermaphroditic macroanimals eat detritus, soil organic matter, and microorganisms found on these materials. Furthermore, earthworms do not eat living plants or their roots and so do not act as pests to crops. More specifically, Eisenia foetida earthworms hasten decomposition of litter but do not mix it into the mineral soil. Endogeic earthworms such as the “red worm,” live mainly in the upper 10-30 cm of mineral soil where they make shallow, largely horizontal burrows. Other large anecic earthworms make vertical, relatively permanent burrows as much as several meters deep. Earthworms literally eat their way through the soil, creating systems of burrows which are important pathways by which plant roots can penetrate dense soil layers. Their physical activity has earned earthworms the title of “nature’s tillers.” Their impact on water infiltration can be dramatic. Another aspect of earthworms as they make their way through soils is casting. Earthworms ingest soils and after passing through the gut, these soils are expelled as globules of soil called casts. The casting behavior of earthworms generally enhances the aggregate stability of the soil and also present significantly higher bacteria, organic matter and available plant nutrients compared to the bulk soil. 2. Mycorrhizae or “fungus root,” is an association between certain fungi and the roots of higher plants. The symbiosis is one of the most ecologically and economically important activities of soil fungi. Furthermore, in natural ecosystems many plants are dependent on mycorrhizal relationships and cannot survive without them. Instead of having to compete with all other soil heterotrophs for decaying organic matter, the mycorrhizal fungi obtain sugars directly from the plant’s root cells. Although this represents an energy cost to the plant receive some extremely valuable benefits from the fungi. Fungal hyphae grow out into the soil some 5 to 15 cm from the infected root, reaching farther and into smaller pores than could the plant’s own root hairs increasing the efficiency. This provides nearly 10 times as much absorptive surface as the root system of an uninfected plant. Mycorrhizae als enhance the ability of plants to take up phosphorus and other nutrients that are relatively immobile and present in low concentrations in the soil solution. It is also worth noting that mycorrhizae can improve water uptake making plants more resistant to drought and salinity stress. Finally, there is evidence that mycorrhizae also protect plants from certain soilborne diseases and parasitic nematodes. This is accomplished through the production of antibiotics that alter the root epidermis which compete with fungal pathogens for infection sites. For all these reasons, the use of mycorrhizae can be a powerful tool in land restoration projects as well as in some agricultural situations. 3. Soil fauna and flora are very important to plant productivity and the ecological functioning of soils. The most significant contribution of soil fauna and flora to higher plants is the decomposition of dead leaves, roots and other plant tissues. Soil organisms also assimilate wastes from animals, human sewage and other organic materials added to the soils. Microbes synthesize new compounds and stabilize soil structure which contribute to humus formation. The bacteria, archaeans, and fungi assimilate some of the (N)itrogen, (P)hosphorous, and (S)ulfur in organic materials they digest. Excess amounts of these nutrients may be excreted into the soil solution in inorganic form either by microflora or by the nematodes and protozoa that feed on them. Thus, the soil food web converts organically bound forms of N, P and S into mineral form that can be taken up once again by higher plants. Soil flora and fauna can also breakdown compounds that are toxic to plants. Soil ecosystems include organisms that not only are unharmed by these compounds butt can produce enzymes that allow them to use these toxins as food. Some toxins are xenobiotic, meaning they are compounds foreign to biological systems and resist attack by commonly occurring microbial enzymes. However, the detoxifying activity of Prokaryotes and fungi is by far the greatest in the surface layers of soil, where microbial numbers are concentrated in response to the greater availability of organic matter and oxygen. Microorganisms can also stimulate and fix important nutrients that are necessary for plant growth. For example the availability of essential elements such as iron and manganese are determined largely by microbial action. Microbial oxidation also controls the potential for toxicity in soil contaminated with selenium or chromium. Furthermore, the fixation of elemental nitrogen gas, which cannot be used directly by higher plants, into compounds usable by plants is one of the most important microbial processes in soils. Actinomycetes in the in the genus Frankia fix major amounts of nitrogen in forest ecosystems; cyanobacteria are important in flooded rice paddies, wetlands, and deserts; and rhizobia bacteria are the most important group for the capture of gaseous nitrogen in agricultural soils. The greatest amount of nitrogen fixation by these organisms occurs in root nodules or in other associations with plants. The final player in making significant contributions to plant productivity and ecological functioning of soils are the rhizobacteria. These bacteria are especially adapted to living in the zone called the rhizoplane. This zone is immediately around the plant root to include the rhizosphere soil and root surface itself. The rhizoplane supports a dense population of these bacterial microorganisms almost completely encrust root surfaces. Little interaction between the soil and root can take place without some intervening microbial influence. Certain rhizobacteria even promote plant growth in other ways such as enhanced nutrient uptake or hormonal stimulation.
Posted on: Wed, 02 Apr 2014 04:43:50 +0000