Agriculture Reference
In-Depth Information
The amount of N derived from the soil (Ndfa) can be calculated by subtracting the amount of
N fixed and the amount of N derived from the fertilizer (the latter can be calculated according to
Giambalvo et al., 2010) from the total N in the aboveground biomass. The N balance, which rep-
resents the net potential contribution of N
2
fixation to the system, can be calculated by subtracting
the amount of N removed in grains from the amount of N fixed, according to Evans et al. (2001).
Ruisi et al. (2012) calculated the N fixed by chickpea (
Cicer arietinum
L.), faba bean (
Vicia
faba
L.), lentil (
Lens culinaris
Medik), and pea (
Pisum sativum
L.), and wheat was used as a cereal
in the experiment. The percentage of N fixed differed by species in the order faba bean > chick-
pea > pea > lentil. On an average, the faba bean accumulated more N from the atmosphere and left
more residual N in the soil than did the other three species. Ruisi et al. (2012) concluded that the
faba bean makes the greatest contribution to the N balance, a result that is in agreement with Lopez-
Belliodo et al. (2006), Walley et al. (2007), and Hauggard-Nielsen et al. (2009) who compared several
grain legumes (chickpea, dry bean, faba bean, lentil, lupin, and pea). Ruisi et al. (2012) reported that
it is important to note that when computing the amount of Ndfa on an aboveground biomass basis,
one ignores fixed N in roots and nodules and from rhizodeposition, which can lead to a substantial
underestimation of N
2
fixation. Research has shown that belowground contributions of fixed N may
represent between 30% and 60% of the total N accumulated by legume crops (Peoples et al., 2009).
7.5.9 n
atural
15
n a
BundanCe
Almost all N transformations in the soil result in isotopic fractionation. The net effect is often a
small increase in the
15
N abundance of soil N compared with atmospheric N
2
(Shearer and Kohl,
1986). In looking at such small differences in
15
N concentration, data are commonly expressed in
terms of parts per thousand (δ
15
N or 0/00). The equation used to calculate δ
15
N is written as follows
(Peoples and Herridge, 1990; Herridge and Danso, 1995):
(
atom
%
15
Nsample tomNstandard
atom
)(
−
%
15
)
δ
15
N
=×
100
(
%
15
Nstand
ard)
where the standard is usually atmospheric N
2
(0.3663 atom%). By definition, the δ
15
N of air N
2
is
zero. The natural abundance method gives an integrated estimate of P over time as with
15
N enrich-
ment studies, but it can be applied to established experiments (provided nonfixing reference mate-
rial is available) because no pretreatment, that is,
15
N application, is necessary. An estimate of P is
obtained by using the following equation (Peoples and Herridge, 1990; Herridge and Danso, 1995):
(
δ
15
NsoilN NlegumeN
NsoilN
)
−
(
δ
15
)
P
=
(
δ
15
)
−
B
The δ
15
N value of B is a measure of isotopic fractionation during N
2
fixation and is determined
by analysis of the δ
15
N of total plant N of the nodulated legume grown in N-free media. Isotopic
fractionation during N
2
fixation is minimal but not zero and should be taken into account when
calculating Pfix (Peoples and Herridge, 1990). Although the principles of the natural abundance
technique are the same as those underlying
15
N enrichment, the main limitations are quite different
and have been reviewed by Shearer and Kohl (1986), Ledgard and Peoples (1988), and Peoples and
Herridge (1990).
7.6 FACTORS AFFECTING DINITROGEN FIXATION
The environment of a plant may be defined as the sum of all external forces and substances affect-
ing the growth, structure, and reproduction of that plant (Fageria et al., 2011). Crop environment
