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Crop productivity from biological nitrogen
By Dr S.M. Alam
Nitrogen is the most abundant element in our atmosphere and is
a major constituent of all living organisms. It is an
important element supplied from the soil for production.
Compounds of nitrogen useful for the nutrition of living
things are made available by several different processes
varying greatly in magnitude and importance.
 During
thunderstorms, small amounts of atmospheric nitrogen gas are
converted into nitric acid by electrical discharges and same
ammonium nitrogen is reported to be derived from a few types
of rocks during the weathering process.
An increasing quantity of the nitrogen from air is
industrially reduced to ammonia each by the Haber Process.
Some hundred million tons of the N is fixed per year. In
many agricultural situations, availability of a suitable
source of nitrogen is the major factor limiting crop
productivity.
Inputs of nitrogen into agricultural systems are primarily
from chemical fertilizers and nitrogen derived from
atmospheric dinitrogen by the process of biological nitrogen
fixation, which is the microbial conversion of atmospheric
dinitrogen gas into plant usable ammonia. Indeed, the
provision of biologically fixed nitrogen plays a key role in
crop production in world agriculture.
Presently, there is an increasing awareness in many areas
that the development of ecologically sustainable
agricultural systems is essential for maintaining
agricultural productivity at sufficient levels to meet the
increasing demands of world population.
One of the key factors for sustained agricultural
productivity is effective management of nitrogen in the
environment. Indeed, the successful manipulation of nitrogen
inputs through the application of biologically fixed
nitrogen of 10 results in farming practices which are
economically liable and environmentally prudent.
The symbiotic association between leguminous plant and roots
nodule bacteria have been estimated at approximately 80 per
cent of the biologically-fixed nitrogen in agricultural
areas with the remainder being contributed by a diversity of
other symbiotic systems, non-symbiotic associations between
nitrogen fixing bacteria and roots and their free living
micro-organisms.
Historically, biological N-fixation has been an essential
component of many farming systems. During the past 50 years,
the widespread use of chemical fertilizers to supply
nitrogen to plant species has had a substantial impact on
food production, and thus nitrogen fertilizer has become a
major input in crop production around the world.
Many important objectives have been proposed by research
workers on the need to increase the contribution from the
biological nitrogen fixation to N inputs in agricultural
systems. These objectives include the need for maximization
of the N-fixing potential of the present agricultural
systems, the introduction and expansion of the capacity to
fix N to new farming system.
The precise quantities of nitrogen supplied to the biosphere
by all the various processes is not accurately known, but it
is generally agreed that the biological fixation of nitrogen
gas is of major importance accounting for some hundred
million tons of nitrogen fixed per year.
The major agronomic benefits from symbiotic biological
N-fixation are the direct input of atmospheric nitrogen to
the agricultural system and the control of crop diseases as
a consequence of involving legumes in crop rotations. The
environmental benefits from using biological N-fixation are
seen to be associated with the replacement of chemical based
technologies with a biological system. These possible
advantages include the decreased inputs of fertilizer N
resulting in decreased levels of ground water pollution by
nitrates and reduced outputs of greenhouse gas production
because the process of fixing nitrogen biologically does not
depend on fossil fuel.
Biological N-fixation is a process carried out by
micro-organisms. Currently, about 50 genera of bacteria are
known to fix atmospheric nitrogen. Many N-fixing bacteria
can achieve N-fixation on their own pre-living heterotrophic
and autotrophic organisms.
The three major groups of microbes are the root nodule
bacteria (Rhizobium, Bradyrhizobium; Sinorhizobium,
Azorhizobium), actinonycetes (Frankia); and cyanobacteria
and certain species of blue green algae (Anabaena,
Calothrix, Tolypothrix Tolypothrix, Nostoc). The major
amount of fixed N is contributed by legume symbioses and
thus providing significant amounts of fixed N to
agricultural systems. Nitrogen fixing legumes are
significant components of many agricultural systems.
Oilseed legumes occupy about six per cent of land currently
under cultivation and the pulse legumes are sown on about
five per cent of the cultivated land.
While, the pastures and fodder crops occupy about 14 per
cent of the world agricultural land. Biological N-fixation
has been demonstrated to have many agronomic, economic and
environmental benefits for agriculture. These benefits
generally include the direct supply of atmospheric N to the
legume crops and pastures.
Symbiotic legumes play a key role in the maintenance of
world crop production through the inputs of biologically
fixed nitrogen. The development of the symbiosis between
root nodule bacteria and their specific host legume is a
continuous process.
The root nodule bacteria are common soil bacteria and they
live and survive in the soil as free-living heterotrophic
organisms. Forage legumes are also good source of N-fixer.
The contribution of the legume component to the pasture
system is important for the inputs of fixed N. In many
agricultural systems, farmers mostly depend on legume
nitrogen fixation by forage legumes to sustain soil N
fertility.
Considerable progress is being made in this area recently
for the fertility of soils. The temperate grain legumes also
play an important role in N-fixation. These include broad
beans faba beans, common bean, garden beans, field peas,
vetch, chickpea red clover, white clover, lentils, cathyrus
and lupine. These crops play a significant role in the
fixation of N and thus improvement in soil fertility.
Soybean is the most widely grown crop legume with 60 million
hectares sown in 1995, producing with an estimated 120
million tons of grain. This is one of the major sources of
food and animal feed, which are necessary for human health.
The crop can be grown in both temperate and tropical climate
and it is capable of producing high yields of grain that
contains large amounts of protein and oil. It is an
important component of local diets.
As a nodulating legume, soybean forms a nitrogen fixing
symbiosis with Bradyrhizobium japonium, B. elkarii, B.
liaoningense and sinorhizobium fredii. The N requirement of
soybean can be met by both assimilation of mineral nitrogen
from the soil and symbiotic nitrogen fixation. Recent
approaches to improving biological nitrogen fixation by
soybean crops include attempts to optimize the numbers the
members and effectiveness of rhizobia in the rooting zone,
selection of elite strains of bradyzhizobia, improved
maculation technique, and improvement in breeding programme
for nitrate tolerance in soybean.
Legume pulses and oilseeds are important components of many
agricultural systems in tropical regions. Biological
nitrogen by these legume crops has a key role in sustaining
productivity in tropical agriculture.
The important legume crops grown in tropical areas spread
throughout the world with comprises of chickpea, black gram,
cluster bean, common bean, coarpea, groundnut, lentil, green
gram, navy beans, mungbean and snake beans. The main benefit
obtained from these crops is their capabilities to fix
atmospheric nitrogen resulting in the crop productivity.
Flooded rice-based systems also play an important role in
the regimen of N-fixation. It has been estimated that
greater than 50 per cent of the world population is
dependent on rice grain as the principal component of diet.
About 75 per cent of the rice is grown in wetlands, where
the rice grows in flooded fields.
Under these conditions, the flooding phenomenon of the rice
fields provides for the establishment of a diverse range of
nitrogen fixing organisms that make substantial inputs of
fixed nitrogen into the rice-based systems.
The most important nitrogen fixing organisms present
abundantly in the flooded rice-based systems include
heterotrophic and autotrophic free-living bacteria
photosynthetic bacteria and cyanobacteria, symbiotic
cyanobacteria that associate with Azolla a fern and
symbiotic bacteria that form root and stem nodules one
legume crops.
The amounts of nitrogen fixed by these diverse and powerful
organisms make substantial contributions to the
sustainability of wetland rice cultivation. The genus
Anabaena plays an important role in the fixation of
atmospheric nitrogen. The fast growing aquatic legumes
(Sesbania nostrate) play important role in the N-fixation.
Many countries and mostly the far-eastern countries use
Azolla as a green manure for the cultivation of rice. The
use of nitrogen fixing green manures has been documented to
improve rice grain yield through effects on physical and
chemical soil parameters and reduced losses of nitrogen from
the system.
There are many challenges facing research workers in their
quest to enhance the role of biological nitrogen fixation in
crop production. A considerable international effort will be
needed to provide the strong support required to overcome
these challenges.
The increasing awareness that legumes are an essential
component of many farming systems is based on improvements
in our understanding of the roles for biological nitrogen
fixation in developing sustainable agricultural systems.
Considerable progress has been made in recent years in
understanding symbiotic nitrogen fixation. Despite the
complexities of the symbiosis between Rhizobium species and
legumes, it is now possible to macerate nodules and separate
the mixture into a bacteroid preparation, soluble proteins
and plant cellular material.
Courtesy: The Dawn
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