Enzymatic versus nonenzymatic denitrification Public Deposited



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  • Denitrification is classically defined as the microbial reduction of nitrate and nitrite with the liberation of molecular nitrogen and, in some instances, nitrous oxide. The sequence of reactions in which nitrogen is evolved as an end-product is essentially a respiratory mechanism in which nitrate and/or nitrite replaces molecular oxygen. The organisms which are capable of such activities are termed facultative aerobes. However, denitrification is not the only means by which microorganisms reduce nitrate and nitrite. Microorganisms also reduce these anions to the ammonium level for incorporation as cellular nitrogen. The process of denitrification by organisms is a biological one and is supplemented in the soil by another series of reactions not involving biological mechanisms. This form of denitrification is nonbiologically mediated via nonenzymatic avenues of soil nitrogen loss. One of the end-products of this nonenzymatic process is different. The characteristic gaseous end-products are molecular nitrogen and nitric oxide whereas the enzymatic route is characterized by the end-products nitrogen and nitrous oxide. Studies were carried out in an attempt to determine the magnitude of nitrogen loss, mediated via biological (enzymatic) and nonbiological systems (nonenzymatic) to verify, using appropriate tracer techniques, the origin of the nitrogen gases evolved in each case, and to provide a further evaluation of the effect of pH as well as on other soil characteristics and environmental factors on nitrite and nitrate decomposition. By using sterile soils amended with ¹⁵N labeled sodium nitrite it was shown that nonbiological denitrification is significant in the nitrogen cycle. To compare this nonenzymatic process with enzymatic denitrification, several soil types as well as a bacterium isolated from marine sediment were used. This gram negative, polarly flagellated bacterium was found to be unique in that it degrades nitrite and nitrate, producing nitrogen gas. Gases evolved from nitrite under sterile conditions were nitrogen and nitric oxide, but no gas was evolved from nitrate. From nonsterile soil, nitrate yielded nitrogen and nitrous oxide. This suggests that nitrite, a critical intermediate in nitrification and denitrification, is degraded nonbiologically and that nitrate, unless reduced to nitrite, cannot be degraded nonbiologically. Hydrogen ion concentration alone cannot fully explain the instability of nitrite in a sterile soil system. It was found that cation exchange capacity, water tension, organic matter, and clay fraction as well as pH are involved in both enzymatic and nonenzymatic denitrification. The reaction sequences of enzymatic and non-enzymatic denitrification appear to be different and unrelated. It is concluded that nonbiological route(s) of soil nitrogen loss must be given equal emphasis in the classical denitrification pathway. Because nitrogen represents a major end-product of the non-enzyme mediated breakdown of nitrite-nitrogen, it becomes difficult to differentiate between the relative contribution of these routes of soil nitrogen loss. However, it does become clear that with nitrate and nitrite-nitrogen, greater field losses of nitrogen occur than had been previously considered possible, particularly since pH represents only one soil factor influencing the conversion of nitrate or nitrite salts to the gaseous state.
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