Molecular Soil Biology 2015, Vol.6, No.4, 1-12
8
for plant growth and development (Shu-Jie et al
.,
2007). The majority of soils in Africa have low levels
of nitrogen and phosphorus and hence the capacity to
support plant growth such as leguminous crops is
limited (Tairo and Ndakidemi, 2013). The supply of
these mineral nutrients is vital in enhancing legume
growth and development. Nitrogen is required by
plant for proper growth and development as it is
necessary for the formation of amino acids which are
building blocks of protein. Besides, N is a necessary
component of several vitamins in legumes (Uchida,
2000). Phosphorus is considered to be an essential
component in plant bioenergetics of the living cell
(Hüttemann et al
.,
2007). It is also vital for development of
new tissue and the transfer of the genetic information
within the plant (Rausch et al.
,
2001).
Rhizobia are known to have a constructive influence
on the real chemistry of soil nutrients and thus
promote nutrients uptake (Lugtenberg and Kamilova,
2009). Makoi et al. (2013) revealed that improved
uptake of N and P following inoculation with efficient
strains of
Rhizobium.
Nyoki and Ndakidemi (2014)
found that
B.
japonicum
inoculants supplemented with
phosphorus in cowpea improved the uptake of N and P.
Similarly, Jida and Assefa (2014); Desta et al
.
(2015)
revealed inoculation improve nodulation and nutrient
uptake of faba bean. Microbial inoculants have
become promising solution to some of the problems
associated with intensive agriculture by enhancing
nutrient availability and uptake, and ultimately
enhanced yield.
Grain legume species also have mechanisms to allow
recovery of phosphorus from unavailable forms. One
mechanism is the exudation of organic acids from
legume roots which decr eases the pH in the soil
surrounding the roots and releases phosphorus.
Several organic acids are exuded with malate and
citrate by faba bean (Nuruzzaman et al
.,
2005a) and
soybean (Nwoke et al
.,
2008), respectively. Grain
legumes can also release phosphatase enzymes into
the soil to breakdown organic material that contains
phosphorus (Gilbert et al
.,
1999). The third mechanism is
a contact reaction between the root surface and the
insoluble phosphorus adjacent to the root (Ae and
Shen, 2002). Faba bean appears to be one of the most
promising of several legumes to express the advantage
in phosphorus recovery (Nuruzaman et al
.,
2005b).
However, the extent of the benefit of legume phosphorus
acquisition to the cropping system is highly dependent
on the soil type and the soil environment (Jones et al
.,
2003)
Residual N
Part of the symbiotically fixed N in a legume crop is
available to subsequent crops through the decomposition
and mineralization of the legume residues. The
legume residues can supply more mineral N to
succeeding crops than cereal residues due to their
relatively high N contents and relatively low C:N ratio
as compared to cereal residues. Cereals cropped in
sequence with legumes derive N benefits compared
with cereal monoculture. Nitrogen benefits in
legume-cereal rotation have been attributed entirely to
the transfer of biologically fixed N (Munyinda
et al.,
1998). Others have expressed the view that N benefits
may be due to a combination of legume N-sparing and
the transfer of fixed N (Keatinge
et al.,
1998).
Grain legumes cause significant and positive yield
effects on subsequent non-legume crop when
compared with rotations with non-legumes (Adeleke
and Haruna, 2012). Maize yields increase when grown
in crop rotations with soybeans compared to
maize grown after maize (Carsky et al
.,
1997).
Agro-economic studies of mung bean-wheat and
fallow-wheat cropping systems revealed that wheat
growth, development and yield differ significantly
when followed after mung bean crop as compared to
fallow (Asim et al
.,
2006). Many factors have been
hypothesized to explain these results including
enhanced N availability following grain legume and
other rotational effects such as reduction of disease
and pest, and higher mycorrhizal colonization rate and
diversity. However, to determine N contribution of
legume to subsequent or associated crop reliable
estimates of N
2
fixation and residual soil N are
required.
Crop rotations involving legumes were reported to
reduce the rate of applied nitrogen fertilizer in
succeeding crop. N derived from legume rhizodeposits
contributed to an increase of 35 – 44% in residual N
content in soil and constituted 79 – 85% of the
below-ground N of plants at maturity (Mayer
et al.,
2003). Jensen (1996) reported that 47% of the total
below-ground N derived from plants originated from
