Microbial Transformations of Nitrogen in Soil: Factors, Forms, and Impact on Plant Nutrition
MICROBIAL
TRANSFORMATIONS
OF
NITROGEN
I
n
t
r
o
d
u
c
t
i
o
n
Source
of
Nitrogen
used
by
the
plants
is
inert
gas
which
constitutes
about
78%
of
earths
atmosphere
The
ploughed
layer
of
majority
of
cultivated
soils contain
0.02%
to
0.04%
of
N
I
n
d
i
a
n
s
o
ils
h
a
v
e
v
e
r
y
l
o
w
N
i
t
r
o
g
e
n
c
o
n
ta
in
b
e
c
a
u
s
e
o
f
low
organic
matter
accumulation due to
tropical
climate.
S
o
i
l
s
d
e
v
e
l
o
p
e
d
u
n
d
e
r
hu
m
id
c
li
m
at
e
h
i
g
h
e
r
N
c
o
n
t
e
n
t
than those
formed
under
dry
climate.
On
an
average
black
soil(0.08%)
than
red
soils(0.05%)
Factors
Effecting
the
content
of N
in
Soil
1.
Climate
i.
Temperature:
Lower
the temperature
higher
is
the
N
content
due
to
more organic matter
addition
and
slow
rate
of
decomposition.
ii.
Water
supply:
Soil
N
content
increases
with
water
supply
up
to
field
capacity.
1.
Effect
of
cultivation:
N
content decreases
with
cultivation
due to
organic matter
loss
through
decomposition.
3.
Soil
texture:
Finer
the
texture,
higher
is
the
N
content.
4.
Depth
in
the
profile:
Surface
soil has
more
N
content
than
deeper
soils.
Forms
of
soil
nitrogen:
The
total nitrogen
content
of
soils
ranges
from less
than
0.02
%
in
sub
soils
to
more
than
2.5 %
in
peat
soils.
The
N
present
in
soil
can
generally
be
classed
as
inorganic
(around
2
%)
and
organic
(around
98
%).
1.
Inorganic
nitrogen
compounds:
The
inorganic
forms
of
soil
nitrogen
include
ammonium
(NH4
+
),
nitrate
(NO3
-
),
nitrite (NO2
-
),
nitric oxide
(NO)
nitrous
oxide
(N2O)
and
elemental
nitrogen.
NH4
+ ,
NO3
-
and
NO2
-
are
important
in
soil
fertility
and
represent
2
to
5 %
of
total
nitrogen.
2.
Organic
nitrogen
compounds
: occur
as
consolidated
amino
acids
or
proteins,
free
amino
acids,
amino
sugars
and
other
unidentified compounds
like
materials
that result
from
the
reaction
of
NH4
+
with lignin,
polymerisation
of
quinones
and
nitrogen
compounds,
the
condensation
of
sugars
and
amines. Bound
amino
acids
are to
the
extent
of
20-40
%,
amino
sugars
(hexosamines)
5
to
10
%
purine
and
pyrimidine
derivatives
1 %
or
less
N
Transformations
in soil
:
Plants
absorb
most
of
the
N
in
the
NH4
+
and
NO3 -
forms.
N
i
t
r
a
t
e
i
s
t
h
e
d
o
m
i
n
a
n
t
s
o
u
r
c
e
a
s
i
t
s
c
o
n
c
e
n
t
r
a
t
i
o
n
i
s
h
i
g
h
e
r
t
h
a
n
NH4
+
and
it
is
free to move to
the
roots.
Potatoes, sugarbeet,
pine
apple,
prefer
both
the
forms;
Tomatoes,
celery,
bush beans,
prefer
NO3
-
,
Rice
and
blue
berries
prefer
NH4
+
.
NO3
-
-N
uptake
is
usually
high
and
is
favoured
by
low
pH
conditions.
NH4
+
-N
is
less subjected
to
losses
by
leaching
and
denitrification.
NH4
+
uptake
is
best
at
neutral
pH
values. When
the
plants
are
supplied
with
NH4
+
-N,
it
leads
to
acidity
in
the
soil.
Mineralization
of N
compounds:
N
m
i
n
e
r
a
li
z
a
t
i
o
n
i
s
s
i
m
p
l
y
t
h
e
c
o
n
v
e
r
s
i
o
n
o
f
o
r
g
a
n
i
c
n
i
t
r
o
gen
t
o
mineral
form
(NH4
+ ,
NO3
- ,
and
NO2 -
).
W
h
en
o
r
g
a
n
i
c
r
e
s
i
d
u
es
h
a
v
i
n
g
a
C
:
N
r
a
t
i
o
w
i
d
er
t
h
a
n
30
a
r
e
a
dd
e
d
t
o
t
h
e
s
o
il
,
i
m
m
o
b
i
li
s
a
t
i
o
n
o
f
n
i
t
r
o
gen
t
a
k
es
p
l
a
c
e.
I
f
C
:
N
(for
legume
residues),
r
a
t
i
o
i
s
n
a
rr
o
w
i
.
e
.
,
l
e
s
s
t
h
a
n
20
mineralisation
is the
result.
It
takes
place essentially
by
three
steps.
1.
Aminisation
2.
Ammonification
1.
Aminisation:
Heterotrophic
soil
microbes,
mostly,
bacteria
like
Pseudomonas
and
Bacillus
are
believed
to dominate
in
the
break
down
of
proteins
in
neutral
and
alkaline
soils.
Under
acidic
conditions fungi
prevail.
In
this
step
hydrolytic
decomposition
of
proteins
and
release
of
amines
and
amino
acids takes
place.
2.
Ammonification
:
The
amines
and
amino
acids
so
released
are
further
utilized
by
still
other
groups
of
heterotrophs
with
the
release
of
ammoniacal compounds. The
step
is
termed
as
ammonification.
R-NH2
+
HOH
NH3
+ R –
OH
+
Energy.
NH3
+
H2O
NH4
+ +
OHThe
ammonium
thus
formed
may
be
nitrified
to
nitrite
and
nitrate
which
are
used
by
plants.
Ammonium
fixation
The
presence
of
K+
(ionic
diameter
of
2.68
0A)
will
often
restrict
NH4
+
fixation
since
this
ion
can
also fill
fixation sites.
Consequently
it has
been
suggested
that
K
fertilization
prior
to NH4
+
application
is
a
practical
way
of
reducing
NH4
+
fixation.
In
agriculture soils
5-20%
of total
N
is
found
as
fixed
ammonium
ion
with
an
average
of
10%
Factors
affecting
ammonium
fixation
1.
Type
and
amount
of
clay:
NH4
+
fixation
increases
with
increase
inthe
content
particularly
2:1
type
of
clay
minerals
like
vermiculite,
fine
grained
mica
and
smectite.
2.
Moisture
content
of
the
soil
:
The
moisture
content
and
temperature
of
the soil
will
affect
the
fixation of
NH4
+.
Freezing
and
drying
increases
the
fixation.
Alternate
cycles
of
wetting
and
drying;
freezing
and
thawing
are
believed
to
contribute
to
the
stability
of
recently
fixed
NH4
+
3.
Amount
of
K+
:
The
presence
of
K+
will
restrict
NH4
+
fixation
since
K +
also
fills
the
fixation
sites
.
4..
Depth
of
the
soil
:
Fixation
of
NH4
+
is
generally
higher
in sub
soil than
surface
soil
due
to
higher
clay
content
and
lower rate
of
nitrification.
5.
Organic
matter
content
:
Higher
the
organic
matter content
more
is the
NH4
+
fixation.
6.
Population
of
nitrifying
bacteria
:
Nitrifying
bacteria
oxidize
NH4
+
to
nitrite
and
nitrates
thus
reducing
the
chances
for
NH4
+
fixation
Nitrification
The
biological
oxidation
of
NH4
+
released
by
the
process
of
ammonification
to
nitrate is
known
as
nitrification.
This
process
is
carried
out
by
nitrifying
bacteria
referred
to
as
nitrifiers.
It
is
a
two
step
process
in
which
NH4
+
is
first
converted
to
nitrite
(NO2
-
)
and
t
h
e
n
t
o
n
i
t
r
a
t
e
(
N
O
3
-
)
.
C
o
n
v
e
r
s
i
o
n
t
o
n
i
t
r
i
t
e
i
s
b
r
o
u
g
h
t
a
b
o
u
t
l
a
r
g
e
l
y
b
y
a
group
of
obligate
autotrophic bacteria
known
as
Nitrosomonas
as
The
conversion
from
nitrite
to
nitrate
is
affected
by
Nitrobacter
as
follows
:
Factors affecting
nitrification
Supply
of
the
ammonium
ion
:
Because
the
substrate
for
the
nitrifying
bacteria
is
the
ammonium
ion,
a
supply
of
this ion
is
the
first requirement
for
nitrification.
Population
of
nitrifying
organisms
:
Under
similar conditions
of
t
e
m
p
e
r
a
t
u
r
e
,
m
o
i
s
t
u
r
e
a
n
d
a
dd
e
d
amm
o
n
i
a
,
t
h
e
n
i
t
r
i
f
i
c
a
t
i
o
n
i
s
g
r
e
a
t
l
y
influenced
by
population
of
nitrifying
bacteria.
Soil
reaction
:
Nitrification takes place between
pH
of
5.5
to
10.0,
with
an
optimum
around
8.5.
S
o
i
l
a
e
r
a
t
i
o
n
:
T
h
e
n
i
t
r
i
f
y
i
n
g
b
a
c
t
e
r
i
a
a
r
e
a
n
a
e
r
o
b
e
s
a
n
d
h
e
n
c
e
r
e
q
u
i
r
e
sufficient
oxygen
supply.
Soil
moisture
:
The
rate
at
which
nitrification
proceeds
in
a
soil
is
governed
to
a
marked
extent
by
the water
content
being
retarded
by
both
very
low
or
very
high
moisture
content.
Temperature
:Very
low
nearer
freezing
and
increases
rapidly
up
to
35oC.
C : N
ratio
:
when
organic
residues
with
wide
C : N
ratio are added,
general
purpose
decay
organisms
are
dominant
and
nitrifiers
become
inactive.
Pesticides
:
Nitrifying
organisms
are
quite
sensitive
to
some
pesticides.
Denitrification
Denitrification
is
a
process
limited
to
anoxic soils
in
which
bacterial
reduction
of
NO3
-
and
NO2
-
takes
place
leading
to
the
release
of
NO,
N2O
and
N2
gases.
When soils
become
water
logged,
oxygen
is
excluded
and
anaerobic
decomposition
takes
place.
Some
anaerobic
organisms have
the ability
to
obtain
their
oxygen
from
nitrates
and
nitrites
with
the
accompanying
release
of
nitrous
oxide
and
nitrogen.
The
most
probable
biochemical
pathway
leading
to
these
losses
is
indicated
as
F
a
c
u
l
t
a
t
i
v
e
a
n
a
e
r
o
b
i
c
b
a
c
t
e
r
i
a
b
e
l
o
n
g
i
n
g
t
o
t
h
e
g
e
n
e
r
a
P
s
e
u
d
o
m
o
n
a
s
,
Bacillus
denitrificans
and
Paracoccus
are
responsible
for
this.
Autotrophs
like
Thiobacillus denitrificans also brings
about
denitrification.
Nitrification
inhibitors
Several products
have
been
developed
with
the
purpose
of
slowing
the
release
and/or
nitrification
of
applied
N
to
synchronise
the
supply
of
N
with
the
crop
demand
and
consequently decrease
nitrogen
losses
via
leaching
and
/
or
denitrification.
These
chemicals
restrict
the
growth
of
Nitrosomonas
and
keep
nitrogen
in
NH4
+
form.
Natural
nitrification
inhibitors
:
Neem
cake
(Azadiracta indica)
Karanj
cake
(Pongamia
glabra) Neem
oil
and
nimin
Chemical
nitrification
inhibitors
N
:
serve
or
nitrapyrin
Hydroquinone,Calcium
carbide
(CaC2)
Nitrogen
immobilization
Immobilisation
of
nitrogen
is
the
reverse
of
mineralisation
and
it
occurs
when
large
quantities
of
low
nitrogen crop residues
with
wide
C:N
ratio
(>30:1)
such
as
coconut
coir
or
cereal
straw
begin
decomposing
in
soil,
the
high
amounts
of
carbohydrates
in
such
residues cause
the
microbial
population
to
build
up
quickly.
As
new
cells
are
formed,
nitrogen
and
other essential
elements
from
soil
are
used
to
build
protoplasm.
This
leads
to
a
decrease
in
the
levels
of
inorganic
nitrogen
for
crops.
A
shortage
of
nitrogen
can
be
avoided
in
such
situations
by
supplying
enough
fertilizer
nitrogen to
compensate
for
immobilization
and
to
meet
the
crop
requirements.
This
lockup
of
inorganic
N
of
soil
is
temporary
and
slowly
will
be
released
after
Biological Nitrogen
Fixation
B
a
c
t
e
r
i
a
a
r
e
c
a
p
a
b
l
e
o
f
r
e
d
u
c
i
n
g
n
i
t
r
o
g
e
n
t
o
amm
o
n
i
a
w
i
t
h
t
h
e
h
e
l
p
o
f
a
n
e
n
z
y
m
e
c
a
l
l
e
d
nitrogenase.
The
process
is
known
as
biological
nitrogen
fixation
(BNF).
They
contribute
140-170
MT
N/yr
whereas
N
input
through
fertilizer
is
only
65
MT
N/yr.
Different
groups
of
N2
fixers
:
There
are
three
main
groups
of
N2
fixers
as
given
below
:
I.
Symbiotic
:
Symbiotic
nitrogen
fixers
reduce
nitrogen
in
association
with
plants
by
forming
some
specialized
structures
in
plants.
a)
Legume
–
rhizobium
symbiosis
:
Some
plants
of
leguminosae
family
form
a
symbiotic
association
with
bacteria
with
the
genus
Rhizobium
which
fix
atmospheric
nitrogen.
Rhizobia
infect
the
root
moving
to
the root
cortex through an
infection
thread
which results
in
the
formation
of
a
tiny
outgrowth
called
root
nodule.
Sesbania
rostrata
forms
nodules
on
the
stem
as
well.
This
association is
host
specific.
The
Rhizobia
turn
to
bacteroids which
have
nitrogenase
enzyme
embedded into
leghaemoglobin.
The
symbiotic
association
of
the
host
and
the
bacterium
is
mutually
beneficial
to
both
organisms.
Specific
species
of
bacterium
infects
the
roots
of
a
particular
group
of
legumes.
Crops
nodulated
by
single
Rhizobium
sp.
is
referred
to
as
‘cross
inoculation
group’.
b)
Symbiosis
with
non
legumes
–
Frankia
symbiosis
The
member
actinomycetes,
the
Frankia
forms
symbiosis
with
forest
trees
belonging
to
the
families
Casuarinaceae
and
Myricaceae.
Azolla
is
a
fresh
water
fern
found
floating
on
the
surface
of
water.
All
the
species
of
Azolla have
an
algal
symbiont
called
Anabaena
azollae
in
a
specialized
cavity
in
the
upper
leaf
surface.
In
rice
fields
the
symbiosis
can
fix
N2
upto
30-40
kg
N
ha-1
.
2.. Non
specific
associative
N2
fixers
:
Rhizobacteria
like
Azospirillum,
Acetobacter,
Azotobacter,
Flavobacterium,
Pseudomonas
etc.
These
bacteria
are
capable
of
using
exudates
of
roots
as
the
source
of
energy.
3.Free
living
nitrogen
fixers
:
Many
soil
bacteria
are
capable
of
reducing
molecular
N2
to
NH4
+
without
forming
any
association
with
plants.
They
supply
about
10-15
kg
N
ha-1.
They
also
function
as
plant
growth
promoting
rhizobacteria
(PGPR).
Understanding the microbial transformations of nitrogen in soil is crucial for optimizing plant nutrition. Factors such as climate, water supply, cultivation, soil texture, and depth influence the nitrogen content in soil. The different forms of soil nitrogen, including inorganic and organic compounds, play vital roles in soil fertility. Plants primarily absorb nitrogen in the forms of NH4+ and NO3-, with nitrate being the dominant source. The process of mineralization converts organic nitrogen into mineral forms, such as NH4+, NO3-, and NO2-. This knowledge is essential for efficient nutrient management in agricultural systems.
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Presentation Transcript
MICROBIAL TRANSFORMATIONS OF NITROGEN
Introduction Source of Nitrogen used by the plants is inert gas which constitutes about 78% of earthsatmosphere The ploughed layer of majority of cultivated soils contain 0.02% to 0.04% ofN Indian soils have very low Nitrogen contain because of low organic matter accumulation due to tropicalclimate. Soils developed under humid climate higher N content than those formed under dryclimate. On an average black soil(0.08%) than redsoils(0.05%)
Factors Effecting the content of N inSoil 1. Climate i. Temperature: Lower the temperature higher is the N content due to more organic matter addition and slow decomposition. ii. Water supply: Soil N content increases with water supply up to fieldcapacity. 1. Effectofcultivation: N content decreases with cultivation due to organic matter loss through decomposition. 3. Soiltexture: Finerthetexture,higheristheN content. 4. Depth in theprofile: Surface soilhas more N content than deepersoils. rate of
Forms of soilnitrogen: The total nitrogen content of soils ranges from less than 0.02 % in sub soils to more than 2.5 % in peat soils. The N present in soil can generally be classed as inorganic (around 2 %) and organic (around 98 %). 1. Inorganic nitrogen compounds: The inorganic forms of soil nitrogen include ammonium (NH4 + ), nitrate (NO3 -), nitrite (NO2 -), nitric oxide (NO) nitrous oxide (N2O) and elemental nitrogen. NH4 + ,NO3 -and NO2 -are important in soil fertility and represent 2 to 5 % of total nitrogen. 2. Organic nitrogen compounds : occur as consolidated amino acids or proteins, free amino acids, amino sugars and other unidentified compounds like materials that result from the reaction of NH4 + with lignin, polymerisation of quinones and nitrogen compounds, the condensation of sugars and amines. Bound amino acids are to the extent of 20-40 %,amino sugars (hexosamines) 5 to 10 % purine and pyrimidine derivatives 1 % or less
N Transformations in soil: Plants absorb most of the N in the NH4 + and NO3 -forms. Nitrate is the dominant source as its concentration is higher than NH4 + and it is free to move to theroots. Potatoes, sugarbeet, pine apple, prefer both theforms; Tomatoes, celery, bush beans, prefer NO3 -, Rice and blue berriespreferNH4+ . NO3 --N uptake is usually high and is favoured by low pH conditions. NH4 + -N is less subjected to losses by leaching and denitrification. NH4 + uptake is best at neutral pH values. When the plants are supplied with NH4 + -N,it leadsto acidityin thesoil.
Mineralization of N compounds: N mineralization is simply the conversion of organic nitrogen to mineral form (NH4 + , NO3 -, and NO2 -). When organic residues having a C: N ratio wider than 30 are added to the soil, immobilisation of nitrogen takes place.If C:N ratio is narrow i.e., less than 20 mineralisation is theresult. (for legume residues), It takes place essentially by threesteps. 1. Aminisation 2. Ammonification
1. Aminisation: Heterotrophic soil microbes,mostly,bacteria like Pseudomonas and Bacillus are believed to dominate in the break down of proteins in neutral and alkaline soils. Under acidic conditions fungi prevail. In this step hydrolytic decomposition of proteins and release of amines and amino acids takes place. 2. Ammonification : The amines and amino acids so released are further utilized by still other groups of heterotrophs with the release of ammoniacal compounds. The step is termed as ammonification. R-NH2 + HOH NH3 + R OH + Energy. NH3 + H2O NH4 + + OHThe ammonium thus formed may be nitrified to nitriteand nitrate which are used byplants.
Ammonium fixation The presenceofK+ (ionic diameterof2.680A)willoften restrictNH4 + fixation since this ion can also fill fixation sites.Consequently it has been suggested that K fertilization prior to NH4 + application is a practicalway ofreducing NH4+ fixation. In agriculture soils 5-20% of total N is found as fixed ammonium ion with an average of10% Factors affecting ammoniumfixation 1. Type and amount of clay: NH4 + fixation increases with increase inthe content particularly 2:1 vermiculite,finegrained mica andsmectite. 2. Moisture content of the soil: temperature of the soil will affect the fixation of NH4 +. Freezing and drying increases the fixation. Alternate cycles of wetting and drying; freezing and thawing are believed to contribute to the stabilityofrecentlyfixed NH4+ 3. Amount of K+ : The presence of K+ will restrict NH4 + fixation sinceK + also fills thefixationsites type of clay minerals like The moisture content and
. 4.. Depth of the soil:Fixation of NH4 + is generally higher in sub soil than surface soil due to higher clay content and lower rate ofnitrification. 5.Organic matter content : Higher the organic matter content moreis theNH4+ fixation. 6.Population of nitrifying bacteria : Nitrifying bacteria oxidize NH4+ to nitriteand nitrates thus reducing thechances for NH4 + fixation
Nitrification The biological oxidation of NH4 + released by the process of ammonification to nitrate is known asnitrification. This process is carried out by nitrifying bacteria referred to asnitrifiers. Itis a twostepprocess in which NH4 + is firstconverted tonitrite (NO2 -) and then to nitrate (NO3 - ). Conversion to nitrite is brought about largely by a group of obligate autotrophic bacteria known as Nitrosomonasas Theconversion from nitriteto nitrateis affected by Nitrobacter as follows:
Factors affecting nitrification Supply of the ammonium ion : Because the substrate for the nitrifying bacteria is the ammonium ion,a supply of this ion is the first requirement for nitrification. Population of nitrifying organisms : Undersimilar conditions of temperature, moisture and added ammonia, the nitrification is greatly influenced by population of nitrifyingbacteria. Soil reaction : Nitrification takes place between pH of 5.5 to 10.0, with an optimum around8.5. Soil aeration : The nitrifying bacteria are anaerobes and hence require sufficient oxygensupply.
Soil moisture : The rate at which nitrification proceeds in a soil is governed to a marked extent by the water content being retarded by both very low or veryhigh moisturecontent. Temperature :Verylow nearerfreezing andincreasesrapidlyupto 35oC. C : N ratio :when organic residues with wide C :N ratio are added,general purposedecay organisms are dominantand nitrifiers becomeinactive. Pesticides : Nitrifying organisms are quite sensitiveto somepesticides.
Denitrification Denitrification is a process limited to anoxic soils in which bacterialreduction of NO3 -and NO2 -takes place leading to the release of NO, N2O and N2 gases. When soils become water logged, oxygen is excluded decompositiontakesplace. Some anaerobic organisms have the ability to obtain their oxygen from nitrates and nitrites with the accompanyingrelease ofnitrousoxide and nitrogen. The most probable biochemical pathway leading to these losses is indicated as and anaerobic
Facultative anaerobic bacteria belonging to the genera Pseudomonas, Bacillus denitrificans and Paracoccus are responsible forthis. Autotrophs like Thiobacillus denitrificans also brings aboutdenitrification. Nitrification inhibitors Several products have been developed with the purpose of slowing the release and/or nitrification of applied N to synchronise the supply of N with the crop demand and consequently decrease nitrogen losses via leaching and / or denitrification. These chemicals restrict the growth of Nitrosomonas and keep nitrogen in NH4+ form. Natural nitrification inhibitors : Neem cake (Azadiracta indica) Karanj cake (Pongamia glabra)Neem oiland nimin Chemical nitrification inhibitors N : serve or nitrapyrin Hydroquinone,Calcium carbide(CaC2)
Nitrogen immobilization Immobilisation of nitrogen is the reverse of mineralisation and it occurs when large quantities of low nitrogen crop residues with wide C:N ratio (>30:1) such as coconut coir or cereal straw begin decomposing in soil,the high amounts of carbohydrates in such residues cause the microbialpopulation to build upquickly. As new cells are formed,nitrogen and other essential elements from soil are used to build protoplasm. This leads to a decrease in the levels of inorganic nitrogen for crops. A shortageofnitrogen can be avoided in such situations by supplying enough fertilizer nitrogen to compensate for immobilization and to meet the crop requirements. This lockup ofinorganic N ofsoilis temporaryand slowlywillbe released after
Biological Nitrogen Fixation Bacteria are capable of reducing nitrogen to ammonia with the help of an enzyme called nitrogenase. The process is known as biological nitrogen fixation(BNF). They contribute 140-170 MT N/yr whereas N input through fertilizer is only 65 MTN/yr. Different groups of N2 fixers: Therearethreemaingroups ofN2fixers as given below : I. Symbiotic : Symbiotic nitrogen fixers reduce nitrogen in association with plants by forming some specialized structures inplants. a) Legume rhizobium symbiosis : Some plants of leguminosae family form a symbiotic associationwithbacteria with thegenus Rhizobium whichfixatmospheric nitrogen. Rhizobia infect the root moving to the root cortex through an infection thread which results in the formation of a tiny outgrowth called rootnodule. Sesbania rostrata forms nodules on the stem as well. This association is host specific. TheRhizobia turn to bacteroids which have nitrogenase enzyme embedded intoleghaemoglobin.
The symbiotic association of the host and the bacterium is mutually beneficial to both organisms. Specific species of bacterium infects the roots of a particular group of legumes. Crops nodulatedbysingleRhizobium sp.is referredto as cross inoculationgroup .
b)Symbiosis with nonlegumes Frankia symbiosis The member actinomycetes,the Frankia forms symbiosis with forest trees belonging to the families Myricaceae. Casuarinaceae and Azolla is a fresh water fern found floating on the surface of water. All the species of Azolla have an algal symbiont called Anabaena azollaein aspecialized cavityin theupperleafsurface. In ricefields the symbiosis can fixN2 upto 30-40 kg N ha-1. 2.. Non specific associative N2 fixers: Rhizobacteria like Azospirillum, Pseudomonas etc. These bacteria are capable of using exudates of roots as the source ofenergy. Acetobacter, Azotobacter, Flavobacterium,
3.Free living nitrogen fixers reducing molecular N2 to NH4 + without forming any association with plants. They supply about 10-15 kg N ha-1. They also function as plant growthpromotingrhizobacteria(PGPR). : Many soil bacteria are capable of
