Denitrification


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denitrification

[dē‚nī·trə·fə′kā·shən]
(chemistry)
(microbiology)
The reduction of nitrate or nitrite to gaseous products such as nitrogen, nitrous oxide, and nitric oxide; brought about by denitrifying bacteria.

Denitrification

 

a process occurring widely in nature whereby nitrates are reduced to molecular nitrogen by bacteria. Denitrification takes place with the formation of nitrites and nitrogen oxide according to the scheme

2HNO3→2HNO2→N2O→N2

The bacteria obtain the energy necessary to reduce nitrates from the oxidation of organic matter (carbohydrates, alcohols, and organic acids), and the nitrate oxygen is an electron and hydrogen acceptor. The denitrification that takes place during the oxidation of glucose can be expressed by the equation

5C6H12O6 + 24KNO3→24KHCO3 + 6CO2 + 12N2 + 18H2O

There are also unusual species of denitrifying bacteria that reduce nitrates by oxidizing sulfur or molecular hydrogen. Denitrification is severely inhibited or ceases completely in the presence of molecular oxygen. It should not be confused with the reduction of nitrates to ammonia, a process associated with the assimilation by microorganisms of nitrates as a source of nitrogen. Many bacteria possess this ability as well as actinomycetes and fungi, which in general are incapable of inducing nitrification. Denitrification must be distinguished from pseudonitrification, in which a purely chemical reaction between nitrites and ammonium salts, amines, or amides takes place in a bacteria culture or in nature and which results in the release of molecular nitrogen. For example, NH4Cl + HNO2→N2 + HCl + 2H2O. One gram of soil contains tens and hundreds of thousands of dentrifying bacteria. However, denitrification can take place vigorously in soil only under certain conditions: when there is a sufficient quantity of nitrates and nitrogen-free organic matter readily decomposed by microorganisms, at optimum pH (7.0-8.2) and temperature (25°-30°C), and, most important, under anaerobic conditions. That is why denitrification is very intensive in moist, poorly aerated soils. During denitrification the amount of nitrogen in the soil decreases owing to the release of molecular nitrogen and traces of nitrous oxide. This results in a decrease in soil productivity. Seventy-five percent of nitrate nitrogen escapes from the soil in the form of molecular nitrogen ten days after nitrates and plant residues have been added to clayey soil. Good aeration of the soil (by cultivation), a decrease in the moisture content of the soil at certain times (through drainage), and the creation of conditions for the better use of soil nitrates by cultivated plants are measures that help reduce denitrification.

A. A. IMSHENETSKII

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