|Abstract or Summary
- 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.