Agriculture Reference
In-Depth Information
Stevenson and Van Kessel, 1996; Unkovich et al., 1997; Van Kessel and Hartley, 2000; Ruisi et al.,
2012). There is a wide range of values of dinitrogen fixation by crop and pasture legumes and by other
diazotrophic systems. For the legumes, the values recorded depend, inter alia, on the crop grown, the
effectiveness of the strains of rhizobia in fixing N with the host, and the environmental conditions
(Gibson and Jordan, 1983). However, legumes have historically been used to maintain soil N fertil-
ity (Fauci and Dick, 1994). The legume-
Rhizobium
symbiosis is estimated to account for 40% of the
world's fixed N (Ladha et al., 1992). Symbiotic N
2
fixation in legumes is determined by the formation
of effective nodules on the roots. Formation of effective nodules depends on plant, soil, and climatic
factors and their interactions. Hence, legumes have different N
2
-ixation capabilities depending on
the environmental conditions, management practices adopted, and type of legume species (Stute and
Posner, 1993; Fageria et al., 2005).
There has to be an upper limit of biological N fixation. Herridge and Bergersen (1988) postu-
lated a theoretical upper limit of 635 kg ha
−1
for soybean and more than 300 kg ha
−1
for pigeon
pea (
Cajanus cajan
L. Huth) and peanut (
Arachis hypogaea
L.). However, under field conditions
or farming systems, theoretical potential values are never achieved because of several limiting
factors. N supplied by hairy vetch (
Vicia villosa
Roth) and crimson clover (
Trifolium incarnatum
L.) in cover crop experiments ranged from 72 to 149 kg N ha
−1
(Hargrove, 1986; Ladha et al.,
1988; Holderbaun et al., 1990). Henzell (1968) summarized many field experiments with tropical
pasture legumes by concluding that they fixed 22-178 kg N
2
ha
−1
year
−1
under average conditions
in northern Australia, but that, under better conditions, this could rise to 290 kg N
2
ha
−1
year
−1
.
Soybean growing in the United States may have up to 400 kg N ha
−1
of which as little as 25%
or as much as 84% (Bezdicek et al., 1978) may have been fixed from the atmosphere, depending
largely on the available N status of the soil (Gibson and Jordan, 1983). Smith and Hume (1987)
reported that when soybean in symbiotic association with
Bradyrhizobium japonicum
can fix up
to 200 kg N ha year
−1
. In dry bean, the rate of N
2
fixation has been reported to vary from 25 to
71 kg ha
−1
in a crop cycle of about 90-100 days (Graham, 1981). Bliss (1993) reported that it has
been common to observe locally adapted bean cultivars able to fix at least 50 kg N ha
−1
and about
40-50% of the plant N from fixation.
The quantity of N fixed by legumes depends on the legume species and environmental factors
(Table 7.2). For good results, optimum environmental conditions are essential. These conditions are
discussed in the next section. Brazil is the largest soybean-producing country in the world. Brazilian
farmers do not apply N to soybean and its total N demand is met by inoculation by appropriate
N-fixing bacteria. N fixed by legumes may be utilized by the host plant. When the host plant resi-
dues are incorporated into the soil, some fixed N may be added to the soil and can be used by the
succeeding crop after decomposition of the residues. Some of the fixed N may be available to the
nonfixing plants growing in association with N-fixing plants. In addition, the root remains in the soil
may also add N to succeeding crops.
7.5 METHODS OF ASSESSING NITROGEN FIXATION
Various methods of N
2
fixation measurement have been proposed. These methods are acetylene
reduction assay, nodule evaluation, determining N balance, N difference method, N fertilizer equiv-
alence (NFE), and N isotopic technique. There are several techniques that have been used to mea-
sure the N
2
fixed by legumes in the cropping systems. These techniques are adopted to measure
dinitrogen fixation both under controlled and field conditions. Measurements under controlled as
well as field conditions are complementary to each other. However, more emphasis should be given
to field experimentation to have practical applicability of the results. According to Ladha et al.
(1988), the methods used have provided reasonable estimates of N
2
, none of which are entirely sat-
isfactory. However, N accumulation in the dry tissue of legume crops and
15
N dilution are widely
reported references for N
2
fixation. Techniques of N
2
measurements have been summarized in the
following section.
