Approximately 80% of the atmosphere is nitrogen gas (N2). Unfortunately, N2 is unusable by
most living organisms. Plants, animals and microorganisms can die of N deficiency,
surrounded by N2 they cannot use. All organisms use the ammonia (NH3) form of N to
manufacture amino acids, proteins, nucleic acids and other N-containing components
necessary for life. BNF is the process that changes inert N2 to biologically useful NH3. This
process is mediated in nature only by bacteria. Other plants benefit from N fixing bacteria
when the bacteria die and release N to the environment or when the bacteria live in close
association with the plant. In legumes and a few other plants, the bacteria live in the nodules.
N fixation by legumes is a partnership between a bacterium and a plant. BNF can take many
forms in nature, including blue green algae, lichens and free-living soil bacteria. These types
of N fixation contribute significant quantities of NH3 to natural ecosystems but not to most
For sustainability to be achieved in any copping system, the replacement of soil mineral
nutrients that are removed or lost is of paramount importance. Sustainability is the successful
management of resources to satisfy changing human needs while maintaining or enhancing
the quality of the environment and conserving resources. The removal of plant material and
its constituent minerals at harvest is generally one of the largest single factor contributing to
the decline in soil fertility (Okalebo et al., 2006). A large proportion of the N accumulated
during the growth of legume crops are removed with the harvested seed, and it is commonly
concluded that the net return of fixed N to the soil is likely to be small when the amounts of
N fixed by the legumes have been compared with the amounts removed in the seed. For
instance, approximately 80 kilogramme N ha-1 is removed in the grain of maize grown in the
USA (Hauck, 1990), and between 100 and 160 kilogramme N ha-1 is removed in the grain of
winter wheat in Netherlands (Dilz, 1988). A continued supply of N is therefore fundamental
to the long-term productivity of any cropping system.
The economic and environmental costs of the heavy use of chemical N fertilizers in
agriculture are a global concern. Sustainability considerations mandate that alternatives to N
fertilizers must be urgently sought. Modern agriculture should be based on maximum output
in the short term, with adequate concern for input efficiency or stock maintenance. N
fertilizer ranks first among the external inputs to maximize output in agriculture. However,
input efficiency on N fertilizer is one of the lowest among the plant nutrients and, in turn,
contributes substantially to environmental pollution. The continued and unabated use of N
fertilizers would further accelerate depletion of stocks of non-renewable energy resources
used in P fertilizerroduction.
Chemical fertilizers have had a substantial impact on food production in the recent past. For
example (Russel, Beech & Jone, 1989) estimate that in 1985, the use of 38.8 million metric
million metric tons, more than half the total cereal production in that year. However, the trend
of the 1970‟s of increasing yields from N fertilizer addition has recently been slowed down
both in the developed (Plucknett & Smith, 1986) and in the developing countries (Barker &
Chapman, 1988).
Biological Nitrogen Fixation (BNF), a microbiological process which converts atmospheric
N2 into a plant-usable form, offers an alternative to N fertilizers (Mugwe et al., 2007). N
fixing systems offer an economically attractive and ecologically sound means of reducing
external inputs and improving internal resources. Symbiotic systems such as that of legumes
and Rhizobium are major sources of N in most cropping systems and that of Azolla and
Anabaena can be of particular value to flooded rice crop. Though N fixation by associative
and free-living microorganisms is important, both scientific and socio-cultural constraints
limit the utilization of BNF systems in agriculture. Production level problems and socio-
cultural factors also limit the integration of BNF systems into actual farming situations.
Maximum benefit can therefore be realized only through analysis and resolution of
constraints to BNF performance in the field and adoption and use of the technology by
farmers.
When symbiotic N fixing systems are used, the amount of N input is reported to be as high as
320-360 kilogramme N ha-1 (Ladha, Pareek, So & Becker, 1999). Among symbiotic N fixing
systems, nodulated legumes have been used in cropping systems for centuries. They can
serve several purposes in sustainable agriculture. They are used as primary sources of food,
fuel, fiber and fertilizer, or secondarily, to enrich the soil, preserve moisture and prevent soil
erosion.
Symbiotic N fixation is the primary pathway by which inorganic N is made available for
the nodules are formed by efficient and effective Rhizobia (Sprent, 2001; Giller, 2001; and
Shah & Emerich, 2006). The term symbiotic effectiveness is used to describe the ability of a
nodulated legume to fix N, and this can be expressed qualitatively (as high, moderate or
ineffective) or quantitatively (total N, shoot or nodule dry weight) (Simms & Taylor, 2002).
Quantitative symbiotic effectiveness is measured by comparison with the performance of
standard Rhizobia strains, with reference to the legume receiving adequate mineral N, or with
non-inoculated legumes.