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Microbial community plays an important role in soils. These organisms are diverse and dynamic, performing beneficial activities such as decomposing organic matter, making nitrogen available and, sometimes, suppressing certain undesirable soil pathogens (Graham & Mitchell, 1997). The active part of soil is actually quite small, 1 to 5 percent by mass is microscopic organisms like bacteria and fungi. Yet the living microbial biomass most readily metabolises organic compounds, contributes significantly to short-term plant nutrition, and is the most responsive to land use (Scholes and Scholes, 1995).

The most numerous and common microorganisms are the bacteria, followed by the actinomycetes and fungi (Porteous, 2000). Bacteria consist of a very diverse and highly variable group of single-celled, prokaryotic organisms found in soils of every ecosystem. The bacterial population of soils is dominated by species of Pseudomonas, Arthrobacter, Bacillus, and others. Actinomycetes are another specialized group of soil bacteria. They look more like a fungus, but they are taxonomically classified as bacteria.

These organisms give freshly tilled soil that rich earthy smell, due to the presence of the genus Streptomyces, which may comprise 70 to 90 percent of the total actinomycete population in a given soil. Compared with bacteria and fungi, actinomycetes are generally much slower-growing organisms. Normally, large actinomycete populations are associated with soils containing significant organic matter or with compost piles.

Fungi are capable of forming an extensive network of mycelium, often called hyphae. Hyphae can be a few inches or several thousand feet in size. Broken into small pieces, hyphae enable fungi to reproduce and grow, which makes determining fungal populations difficult (Winfield, 1995). The presence or availability of preferred food sources for microorganisms tends to dictate their abundance. There may be hundreds of millions to billions of microbes in a single gram (Table 1).

The most numerous microbes in soil are the bacteria followed in decreasing numerical order by the actinomycetes, the fungi, adapted for exploiting the three-dimensional pore network of the soil, soil algae and cyanobacteria (photosynthetic microbes which can add small amounts of carbon to soil) and soil protozoa (unicellular soil organisms that decompose organic materials as well as consume large numbers of bacteria) (Sylvia et al., 1998). Soil microbes are important for soil structure also but their effect is more subtle. Soil microbes produce many gummy substances (polysaccharides, mucilages, etc.) that help to cement soil aggregates. This cement makes aggregates less likely to crumble when exposed to water.

Soil microbes are of paramount importance in cycling nutrients such as carbon, nitrogen, phosphorus, and sulphur (Dindal, 1990). They can regulate the quantities of N available to plants. It is only through the actions of soil microbes that the nutrients in organic fertilizers are liberated for plants and use by other microbes. This process of mineralization is the conversion of organic complexes of the elements to their inorganic forms (conversion of proteins to carbon dioxide CO2, ammonium NH4+, and sulphate SO4=). It is perhaps the single-most important function of soil microbes as it recycles nutrients tied up in organic materials back into forms useable by plants and other microbes (Internet 2).

Bacteria play crucial roles in soil formation, organic matter decomposition and remediation of contaminated soils, and are responsible for transformations of mineral nutrients such as nitrogen and sulphur in the soil. Bacteria have an important role in nutrient transformations. For example, the genera Desulfomonas is convert sulphate to hydrogen sulphide gas, which produces the distinct rotten-egg smell. The specialized group called the nitrobacteria (Nitrobacter and Nitrosomonas) are more beneficial, converting ammonium to nitrate (the most readily used form of nitrogen for plants). Without these bacteria, many nitrogen fertilizers would be unable to release their fertility.

Actinomycetes are very important because they degrade some of the more complex and difficult-to-digest organic molecules such as cellulose and chitin, which is eventually transformed into soil humus. Fungi are the most important decomposers of structural plant compounds (cellulose and lignin – but note that lignin is not broken down when oxygen is absent). Fungal filaments also stabilize soil structure because these threadlike structures ramify throughout the soil literally surrounding particles and aggregates like a hairnet.

Fungi play an active role in organic matter degradation and are crucial for cementing soil particles together to form aggregates. One beneficial process carried out exclusively by soil microbes is nitrogen fixation, the capture of inert N2 gas from the air for incorporation into the bodies of microbial cells. In one well-known form of this process, symbiotic nitrogen fixation, soil bacteria such as Rhizobium and Bradyrhizobium actually inhabit specialized structures on the roots of leguminous plants where they fix substantial quantities of nitrogen that becomes available to the host plant. Many steps in the nitrogen cycle take place in soils. Ammonification and denitrification are two of these processes. During ammonification, bacteria decomposers break down amino acids from dead animals and animal wastes into NH4OH. During denitrification, anaerobic bacteria break down nitrates, releasing N2 back into the atmosphere (Paul and Clark, 1996).

Another benefit of soil microbes is their ability to degrade pest control chemicals and other hazardous materials reaching the soil. Thus through the actions of the soil microflora, pesticides may be degraded or rendered non-toxic lowering their potential to cause environmental problems such as ground and surface water contamination (Graham and Mitchell, 1997). The presence of any particular species of bacteria is largely dependent on existing soil conditions and the availability of necessary foods for the bacteria to consume. Of all soil organisms, bacteria possess the greatest metabolic capabilities. They are capable of extremely fast growth and under certain ideal circumstances; the populations of some bacteria can double in just minutes.

Active decomposers, soil bacteria and fungi, operate within a certain temperature and pH range. Mesophiles, the most common bacterial type, prefers soil at 25-35 C (Alexander, 1977). Microorganisms thrive at an optimal pH near neutrality, but grow in a wider range of pHs. A neutral pH provides adequate supplies of inorganic nutrients, a balance of air and water-filled pore space (about 50-60% of water holding capacity) and abundant organic substrates (carbon and energy sources). When any one of these parameter get too far beyond the normal range some segment of the population will likely be stressed. For example, aerobic (oxygen requiring) bacteria will be at a disadvantage when a soil becomes waterlogged and available O2 is depleted through respiration of roots, microbes, and soil animals.

Fungi favour more slightly more acidic soil than bacteria. In soil more acid than pH 5.5, the flora is dominated by filamentous fungi (Alexander, 1971). Compared with bacteria, actinomycetes are less sensitive to environmental conditions such as heat or drought because they produce structures that allow them to survive these adverse conditions. However, actinomycetes are sensitive to pH and are almost absent below pH 5. Below this pH, very few bacteria and actinomycetes can survive, while many fungi grow very well. Fungi also survive unfavourable environments with specialized structures called sclerotia and conidia. These store food and other materials until conditions are favourable for them to germinate and form a new mycelium or a fruiting body.

The greatest population change from summer to winter was a reduction of 75 percent in the actinomycetes group. Aerobic bacteria declined 67 percent and anaerobic bacteria decreased 30 percent in both soils. The fungi remained unchanged throughout the year in …

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