BACTERIA AND OTHER MICROORGANISMS OF THE SOIL - The Soil - page 134

it is desirable that nitrification should occur when plants are actively growing and
able to take up nitrates as soon as they are formed (see chapter 9 ) . Specific chemical
inhibitors of nitrification (e.g. Didin, N-Serve) are now available commercially, and
the practice of including such inhibitors in ammonium-containing fertilizers is gain-
ing ground. It is now possible to apply such fertilizers in autumn or winter and for
nitrification to be inhibited until, say, early spring when the sown crop begins to grow.
Nitrification is also prevented for long periods by fumigation of the soil; this treat-
ment may be performed in special cases, such as in tree nurseries or in citrus growing.
Another way in which nitrogen can be lost from soils is as a gas. The conversion
of nitrate, via nitrite, to nitrogen gas is brought about by a variety of anaerobic bacter-
ia by a reduction process known as 'denitrification', as briefly referred to above (see
Nitrogen Cycle ) . Sometimes very small amounts of the gas nitrous oxide (N 2 O) and
nitric oxide (NO) are also produced. Oxygen, like nitrogen, is very sparingly soluble
in water, and so microbes which utilize dissolved oxygen have to rely on atmospheric
oxygen being constantly dissolved in the soil water to replenish their supply. Anaer-
obic conditions may readily arise in soil due to the rapid consumption of oxygen in
the respiration of the many microorganisms and small soil fauna. Applying slurries of
organic manure also promotes anaerobiosis because liquid manure contains both mi-
croorganisms and organic material that is easily utilized. Displacement of the air from
soil fissures and pore spaces by water-logging, or even the mechanical compaction of
soil by machinery, also can cause anaerobic conditions. The prerequisites, then, for
active denitrification are the presence of organic substrates to provide carbon and en-
ergy for microorganisms, nitrate which is produced by the nitrification of ammonia or
urea fertilizers, and wet, anaerobic conditions. In a sense, denitrification is the reverse
of nitrogen fixation and it leads to losses of nitrogen gas; together with the leaching
of nitrates, these losses can be serious economically and represent a waste of the ni-
trogen supplied by fertilizers.