Agriculture Reference
In-Depth Information
diffusion in support of bacteroid respiration and thereby account for the greater O
2
limitation of
nodule metabolism in
NH
3
−
-inhibited nodules.
Excess N fertilizer may inhibit biological N fixation in legumes. However, a small amount of
mineral N is needed by clover until nodules are formed and N fixation commences (Frame and
Newbould, 1986). Hence, the use of starter fertilizer N is essential in soils of low fertility to improve
N
2
fixation. Bethlenfalvay et al. (1978), Streeter (1988), and Waterer and Vessey (1993) also reported
that both nodule growth and N
2
fixation of plants grown with a low N concentration in the root zone
were stimulated compared with those without mineral N. Frame and Newbould (1986) also reported
that the element needs of white clover and/or
Rhizobium,
especially for the N fixation process,
include Co, Cu, Mg, Mn, Mo, Ni, B, Zn, S, and possibly Se. The addition of lime to raise the pH to
5.5 and above can markedly affect the availability of some essential nutrients (Mengel et al., 2001).
In Australia and New Zealand, deficiencies of Co, Mo, S, and Zn are widespread, and the response
of legumes to applications of one or all of these elements can be dramatic (Frame and Newbould,
1986).
Phosphorus deficiency is widespread in the bean-producing regions of South America and is
perhaps the factor that most limits N
2
fixation on small farms (Graham, 1981). Graham and Rosas
(1979) and Graham (1981) reported that among essential plant nutrients, the availability of P in
adequate amounts is most important in biological N fixation. Plants engaged in symbiotic N
2
fix-
ation generally have a higher requirement for P than those grown with N fertilization (Robson,
1983; Jungk, 1998). Higher ATP requirements for nitrogenase function (Ribet and Drevon, 1996; Al
Niemi et al., 1997), plus P needs for signal transduction, membrane biosynthesis, and nodule devel-
opment and function, contribute to this requirement (Graham and Vance, 2000). In consequence,
the P concentration of pea and soybean nodules can be as high as 6 mg g
−1
dry weight, while that
of shoot P is only 2-3 mg g
−1
dry weight (Israel, 1987). In low-P environments, P fertilization of
N
2
-dependent species leads to an increase in nitrogenase activity, nodule number, and mass and
plant N accumulation (Robson et al., 1981; Israel, 1987). In clover and pea, the stimulation of N
2
fixation following P addition is through rapid enhancement of shoot growth, with a resultant influ-
ence on nodule parameters (Robson et al., 1981), whereas P deficiency in soybean and Stylosanthes
appears to impact nodule function more directly (Gates, 1974; Israel, 1993).
7.6.5 C
onCentratIon
of
o
xYGen
The two main reasons for lower oxygen levels in soils are waterlogging and poor structure, and
under both conditions, poor nodulation has been reported (Gibson, 1977). Gibson (1977) reviewed
the literature and reported poor nodulation of subterranean clover growing in soils with O
2
diffu-
sion rates of 8 × 10
−1
g cm
2
min
−1
compared to soils having O
2
diffusion rates of 20 × 10
−1
g cm
2
min
−1
.
7.6.6 d
rouGht
Drought is a serious problem worldwide in crop production. Drought influences many physiological
and biochemical processes in the plant, including legume nodulation. It is reported in the literature
that nitrogenase activity is much more sensitive than photosynthesis (Durand et al., 1987; Sinclair
et al., 1987; Layzell and Moloney, 1994). Water stress has been reported to be responsible for the
collapse of lenticels (Pankhurst and Sprent, 1975), decrease in the respiratory capacity of bacteroids
(Guerin et al., 1990), or decline in the leghemoglobin content of nodules (Guerin et al., 1990, 1991).
However, the report available in the literature suggests that there is a significant variation in N
2
fixa-
tion among soybean genotypes under water stress (Sall and Sinclair, 1991). Hence, genotype selec-
tion is an important strategy in improving biological N fixation in legumes under drought stress.
Sprent (1976) reported that both the nodule number and size were reduced by water deficit, with the
specific nodule activity reduced by almost 90% in stressed plants.
