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Nitrogen
Cycle -
Molecular
nitrogen
constitutes
about
78
per
cent
of
the
earth's
atmosphere,
but
it
is
chemically
inert
and
cannot
be
utilized
by
most
living
organisms.
Plants,
animals
and
most
microorganisms,
therefore,
depend
on
a
source
of
combined
nitrogen
such
as
ammonia,
nitrate
or
organic
nitrogen
compounds
for
their
growth.Nitrogen
undergoes
a
number
of
transformations
involving
organic,
inorganic
and
volatile
forms
of
nitrogen.
A
part
of
the
great
reservoir
of
atmospheric
nitrogen
is
converted
into
an
organic
form
by
certain
free
living
microorganisms
and
by
Plant
microbe
associations
which
then
make
this
element
available
for
plant
growth.
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Upon
death,
plants
and,
animals
undergo
microbial
decay
and
organic
nitrogen
is
released
as
ammonia
which
is
then
oxidized
to
nitrate
by
microorganisms.
The
nitrate form
of
nitrogen
is
mostly
used
by
plants
or
may
be
lost
by
leaching
or
reduced
to
gaseous
nitrogen
and
subsequently
lost
to
the
atmosphere.
The
nitrogen
cycle
mainly
includes
transformations
such
as
(1)
nitrogen
mineralization
in
which
nitrogen
containing
complexes
are
decomposed
and
converted
into
inorganic
compounds
for
use
by
plants,
and
(ii)
nitrogen
immobilization
in
which
nitrogen
compounds
are
assimilated.
Some
of
the
biochemical
changes
brought
about
by
microorganisms
in
the
nitrogen
cycle
are
discussed
below.Nitrogen
Mineralization:
In
the
process
of
mineralization
proteins,
nucleic
acids
and
their
components
are
degraded
by
microorganisms
with
the
eventual
liberation
of
ammonia
and
this
is
called
ammonification.
A
part
of
the
liberated
ammonia
is
assimilated
by
the
microorganisms
themselves.
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The
first
step
in
the
process
of
ammonification
is
the
hydrolysis
of
proteins,
nucleic
acids
and
other
organic
nitrogenous
compounds
into
amino
acids
(proteolysis).
The
amino
compounds
are
then
deaminated
to
yield
ammonia.
Ammonification
usually
occurs
under
aerobic
conditions,
while
under
anaerobic
conditions
protein
decomposition
leads
to
conversion
of
ammonia
into
amines
and
related
compounds.
These
amines
are
subsequently
oxidized
in
the
presence
of
oxygen
to
release
ammonia.
In
nature,
the
breakdown
of
nitrogenous
substances
is
brought
about
by
the
activity
of
a
multitude
of
microbial
species.Almost
all
bacteria,
actinomycetes
and
fungi
can
bring
about
proteolysis
and
the
amino
acids
so
produced
are
utilized
for
the
growth
of
these
organisms.
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Nitrification:
In
the
second
phase,
ammonia
is
converted
into
nitrate
and
this
process
is
called
nitrification.
Nitrification
occurs
in
two
steps;
first,
ammonia
is
oxidized
to
nitrite:This
change
is
brought
about
by
chemoautotrophic
bacteria
of
the
genera
Nitrosomonas,
Nitrosolobus,
Nitrosococcus
and
Nitrosospira.
These
bacteria
obtain
their
energy
requirement
by
the
oxidation
of
NH4+
to
NO2-
Of
these
nitrifying organisms,
Nitrosomonas
are
the
most
important
in
soils.
Besides
the
chemoautotrophic
bacteria,
some
heterotrophic
bacteria
such
as
Streptomyces
and
Nocardia
have
also
been
known
oxidize
ammonia
to
nitrite
In
the
second
step,
nitrite
is
oxidized
to
nitrate
and
this
reaction
is
dependent
on
the
activities
of
bacteria
belonging
mainly
to
the
genera
Nitrobacter.
Certain
fungi
belonging
to
the,
genera
Aspergillus,
Penicillium
and
Cephalosporium
can
also
carry
out
nitrification. Nitrosomonas,
first
converts
ammonia
to
hydroxylamine
which
is
then
transformed
into
some
undefined
intermediate,
possibly
a
compound
such
as
nitroxyl
(HNO). Nitrobacter
oxidizes
nitrite
to
nitrate
and
yields
two
electrons
for
each
molecule
of
NO2
transformed.
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The
last
step
in
the
sequence
is
visualized
as
involving
a
hydrated
nitrate
molecule
in
which
electrons
are
removed
to
yield
nitrate.Certain
bacteria
are
capable
of
using
nitrate
as
the
terminal
electron
acceptor
under
anaerobic
conditions.
As
a
consequence
of
such
nitrate
respiration,
nitrate
is
reduced
to
nitrogen
gas
or
nitrous
oxide.
This
process
is
called
as
denitrification
and
leads
to
the
loss
of
nitrogen
from
the
soil.
Denitrification
depletes
the
soil
of
an
essential
nutrient
for
plant
growth
and
therefore
is
not
a
desirable
reaction.
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Denitrification
occurs
mostly
in
waterlogged
anaerobic
soils
with
a
high
organic
matter
content
and
the
ability
to
carry
out
denitrification
is
restricted
only
to
certain
bacteria.
Fungi
and
actinomycetes
have
so
far
not
been
implicated.
Among
the
bacteria
important
in
denitrification
are
Thiobacillus
denitrificans,
Micrococcus
denitrificans,
species
of
Pseudomonas,
Bacillus,
Paracoccus,
Achromobacter
and
Serratia.The
enzymes
involved
in
various
steps
of
denitrification
reactions
are
called
as
nitrate,
nitrite,
nitric
oxide
and
nitrous
oxide
reductases. Nitrate
is
first
reduced
to
nitrite
which
is
then
transformed
to
NO.
The
NO
is
converted
to
N2
with
N2O
as intermediate.Although
an
undesirable
reaction
from
the
point
of
view
of
plant
nutrition,
denitrification
is
of
major
ecological
importance
since
with
out
denitrification
the
supply
of
nitrogen
on
the
earth
would
have
got
depleted
and
NO3
would
have
accumulated.
Also,
since
high
concentration
of
NO3
are
toxic,
denitrification
is
a
mechanism
by
which
some
of
the
nitrogen
is
released
back
to
the
atmosphere.
Nitrogen
Immobilization: Sometimes
when
plant
residues
or
pure
carbohydrates
are
added
to
the
soil,
there
is
a
rapid
decrease
in
the
amount
of
available
inorganic
nitrogen.
This
is
referred
to
as "Nitrogen
immobilization" which
results
from
the
microbial
assimilation
of
inorganic
nitrogen.
The
process
of
immobilization
involves
the
incorporation
of
ammonia
and
nitrate
into
microbial
protein
and
nucleic
acids
and
is
therefore
the
reverse
of
mineralization.
Mineralization
and
immobilization,
therefore,
run
counter
to
each
other.
On
the
death
of
microorganisms,
the
immobilized
nitrogen
is
however,
released
through
mineralization.
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