Agriculture Reference
In-Depth Information
NitrogenFixingBio-fertilizers
The nitrogen fixing bacteria are of two types' i.e., biological nitrogen fixation (sym-
biotic) and non-symbiotic nitrogen fixation (free living). The former develops as
an association with crop plants through formation of nodules in their roots while
as free living bacteria can fix atmospheric nitrogen without association with plants.
Biological Nitrogen Fixation
Atmosphere contains approximately 70 % of N which is not readily available form
and therefore is not consumed by living organisms. It can be made available with
the help of chemical or biological processes, though chemical nitrogen fertilizers
are relatively expensive (Zilli et al.
2004
). Living organisms utilize nitrogen in the
form of ammonia to synthesize proteins, nucleic acids, amino acids, and other nitro-
gen-containing compounds for the maintenance of life. The process of conversion
of inert N
2
to biologically important NH
3
with the help of bacteria is called biologi-
cal Nitrogen Fixation. The nitrogen fixation is done by the bacteria, and the NH
3
produced is absorbed by the plant.
This biological reduction of nitrogen to ammonia is performed only by some
prokaryotes and is a highly oxygen-sensitive process. The Biological nitrogen fixa-
tion includes diverse range of diazotrophic soil microbes belonging to aerobes (
Azo-
tobacter, Beijerinckia, Drexia
), facultative anaerobes (
Clostridium, Pseudomonas,
Rhizobium
), heterotrophs (
Klebsiella, Enterobacter
), phototrophs (
Anabaena, Nos-
toc, Azosprillium
) The most competent nitrogen fixers establish a symbiosis with
higher plants in which the energy for nitrogen fixation and, in general, the oxygen
protection system in particular are provided by the plant partner. In these symbiotic
relationships prokaryotic partnership is provided by soil bacteria
Rhizobium
in legu-
minous plants and
Frankia
bacteria in actinorhizal symbiosis. Biological Nitrogen
Fixation confers tremendous amount of NH
3
to natural ecosystems.
Rhizobium
Rhizobium
is a gram-negative, free living organism present in soil, once it comes
in contact with specific legume crop, nitrogen fixation starts and this rhizobium-
legume association is of significant environmental and agricultural importance in
view of the fact that it accounts for an estimated 180 million tons biological ni-
trogen fixation per year (Postgate
1982
).
Rhizobium
invades the root hairs of the
legumes by forming nodules. First time, bacterium capable of fixing nitrogen was
isolated from nodules of a legume in 1888 by Beijirinck from Holland. Later on this
bacterium was reported in Bergey's Manual of Determinative Bacteriology under
the genus
Rhizobium.
Rhizobium
has the ability to fix the atmospheric N in symbi-
otic state only. Rhizobium also exists as an endosymbiotic N fixing microorganism
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