Biodegradation of an organic chemical in soil is the modification or decomposition of the chemical by soil microorganisms to produce ultimately microbial cells, carbon dioxide, oxygen, and water.

Soil serves as the home for numerous microorganisms capable of degrading organic chemicals. The most predominant microorganisms in soil include bacteria, actino-mycetes, and fungi. One gram of surface soil can contain from 0.1 to 1 billion cells of bacteria, 10 to 100 million cells of actinomycetes, and 0.1 to 1 million cells of fungi (Dragun 1988b). The microorganism population in soils is generally greatest in the surface horizons where temperature, moisture, and energy supply is favorable for their growth. As the depth increases, the number of aerobic microorganisms decreases; however, anaerobic microorganisms can exist depending on availability of nutrients and organic material.

The biodegradation of an organic chemical by a microorganism is catalyzed by enzymes which are produced as part of the metabolic activity of the living organism. The biotransformation occurs either inside the microorganism via intracellular enzymes or outside the microorganism by the action of extracellular enzymes. After an organic chemical and an enzyme collide, an enzyme-chemical complex forms. Then, depending on the alignment between the functional groups of the chemical and the enzyme, a reaction product (modified or decomposed organic chemical) is formed by the removal of one or more functional groups by oxidation or reduction reactions (Dragun

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