Molecular Soil Biology 2015, Vol.6, No.4, 1-12
2
consumption on lipid peroxidation may be another
possible explanation in the significant inverse relation
of the whole-grain intake to the risk of CAD. Thus,
these protective effects are likely due to multiple
mechanisms, such as fiber, antioxidants, and many
constituents of grain legumes.
Though grain legumes have several benefits, their
productivity is very low and far below the potential
production of the species (IFPRI, 2010). This low
productivity in grain legumes is often associated with
declining soil fertility of the farmland and reduced
N
2
-fixation. Yield reduction of grain legume can be
improved through inoculation of adaptable effective
rhizobia (Jida and Assefa, 2014; Desta et al.
,
2015).
Despite the fact that inoculating legumes with rhizobia
can achieve substantial increases in nodulation, grain
and biomass yield, nitrogen fixation and post-crop soil
nitrate levels, there is no doubt that specificity exists
between rhizobial strain and the legume variety, and
compatibility between the two is essential for
successful nodulation and nitrogen fixation (Emam
and Rady, 2014).
Biological N
2
Fixation (BNF) in Agriculture
The earth’s atmosphere contains the largest global
pool about 10
15
tons of dinitrogen (N
2
) gas, and the N
2
cycle involves the transformation of 3×10
9
tons of N
2
per year on a global basis (Postgate, 1982). Although
N
2
represents almost 80% of the earth’s atmosphere
(Abd-Alla et al.
,
2014), it is not useful to most
organisms unless it is converted into a reduced form
either biologically by bacteria or abiologically by
lighting or industrial processes. Nitrogen reduction is
a very complex mechanism not yet fully elucidated
(Franche et al
.,
2009). The result of net reduction of
molecular nitrogen to ammonia is generally accounted
for by the following equation:
N
2
+ 8H
+
+ 8 e
-
+ 16 Mg ATP → (nitrogenase) →
2NH
3
+ 2H
+
+ 16MgADP + 16Pi
Figure. 1 presents N
2
-fixation and N
2
fixing
agents in agriculture and terrestrial natural systems.
BNF is the process whereby a number of bacteria
species use the enzyme nitrogenase to convert
atmospheric N
2
into ammonia (NH
3
). It occurs in
almost every ecosystem, and accounts for ~1.4 x
10
11
kg N yr
-1
entering terrestrial systems, with
~76 % of the N added to natural ecosystems
while the remaining goes into agroecosystems
(Galloway et al.
,
2004). The world production of fixed
N from chemical fertilizer accounts for about 25%,
whereas BNF accounts for about 60% (Zahran, 1999).
Nitrogen (N) is an essential plant nutrient and one of
the key drivers of global agricultural production.
Between 150 and 200 million tons of mineral N are
required each year by plants in agricultural systems to
produce the world’s food, animal feed and industrial
products (Unkovich et al
.,
2008). To meet those
requirements, close to 100 million tons of N are fixed
annually via the industrial Haber Bosch process.
However, the use of nitrogenous fertilizers has
resulted in unacceptable levels of water pollution and
the eutrophication of lakes and rivers (Al-Sherif,
1998). BNF is considered to be more ecofriendly than
industrial N fixation because the NH
3
produced in the
former process is readily assimilated into organic form
by the plant (Valentine et al.
,
2011) and therefore
would be ideal for sustainable agriculture. It is an
efficient source of nitrogen for resource poor farmers
who are using little or no fertilizer, and plays a key
role in sustainable grain legumes production.
Approximately 2 tons of industrially-fixed N is
needed as fertilizer for crop production to equal the
effects of 1 ton of N biologically fixed (Abd-Alla et
al., 2014). Given the high cost of fertilizer in
developing countr ies and the limited market
infrastructure for farm inputs, current research and
extension efforts have been directed to integrated
nutrient management, in which legumes play a crucial
role (Chianu et al., 2008). More focus has been given
to the symbiotic associations as they have the greatest
quantitative impact on the nitrogen cycle.
Legumes are very important both ecologically and
agriculturally because they are responsible for a
substantial part of the global flux of N
2
to fixed forms.
Increased plant protein levels and reduced depletion of
soil N reserves are obvious consequences of legume N
2
fixation. Among the flowering plant families,
Leguminosae is the third largest represented by about
730 genera with more than 19,320 species (Lewis et al.,
2005). Most species of the Leguminosae form symbiotic
associations with rhizobia. Therefore, they are an
essential part of the terrestrial N cycle and used
to sustain ecosystem functioning (Sprent, 2001).
Legumes are grown for production of food and oil,
