Thursday 30 April 2020
Describe the nitrogen cycle
This is the cycling of nitrogen and its
compounds in nature; plants absorb nitrogen in form of nitrates and then
assimilate it into plant proteins; animals obtain this nitrogen in plant
proteins through feeding on plants; when the animals die and decompose, they
release the nitrogen in form of ammonia to the soil; free atmospheric nitrogen
is converted into nitrates through a process known as nitrogen fixation; the
process occurs in two ways: biological and nonbiological; biological fixation
of nitrogen is done by nitrogen-fixing bacteria; which are either free-living
or symbiotic; symbiotic bacteria are of the genus Rhizobium; and are found in root nodules of legumes (such as pea,
clover and alfalfa); the bacteria convert atmospheric nitrogen into ammonia; that
is used directly by the leguminous plants to form nitrogen containing organic
compounds (amino acids, nucleic acids, proteins); when plants die, the nodules
release ammonium compounds into the soil; which are then converted to nitrites;
by nitrifying bacteria of genus Nitrosomonas
and Nitrococcus (nitrite
bacteria) and then to nitrates by Nitrobacter
(nitrate) bacteria; free-living micro-organisms that fix nitrogen include
putrefying/saprophytic bacteria; (such as Azobacter
spp, Clostridium and some algae such
as Anabaena, Chlorella and Nostoc);
the organisms fix nitrogen into ammonia by break down of protein material in
dead organisms; the ammonia is converted to nitrites; then to nitrates;
However, denitrifying bacteria (e.g. Pseudomonas
denitrificans and Thiobacillus
denitrificans); break down/reduce nitrates to nitrites, ammonium compounds
and even gaseous nitrogen; a process known as denitrification; the process
helps to release free nitrogen into the air for recycling; nonbiological
nitrogen fixing is carried out by lightning during thunderstorms; the lightning
energy, causes atmospheric nitrogen and oxygen to combine forming oxides of
nitrogen; which dissolve in rain water to form nitrous acid/nitric acid; that
is washed down into the soil; the nitric acid formed reacts with other chemical
compounds dissolved in soil water; to form nitrates; the nitrates are then
utilized by plants;
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