Biological Nitrogen Fixation by Legumes - Nitrogen Management in Crop Production - page 292

arising from one of them (in this case N 2 ) can be calculated by using the following equation (Peoples
and Herridge, 1990):

xy
xb

−
−

P =

where x, y, and b are, respectively, the isotopic compositions of the plant-available soil N of a
N 2 -ixing plant growing in the soil, and of a legume fully dependent on N 2 fixation for growth. In
practice, x is obtained from the isotopic composition of a non-N 2 -ixing reference plant (a nonle-
gume, uninoculated legume, or nonnodulating legume genotype) that is totally dependent on soil N
(Peoples and Herridge, 1990).

7.5.7 15 n e nrIChment
The 15 N enrichment method is generally regarded as the standard method for estimating legume N 2
fixation (Herridge and Danso, 1995). Its main advantage is that it provides a time-averaged estimate
of Pfix, integrated for the period of plant growth to the time of harvest. The N 2 fixation is calculated
by using the following equation:

( atom %

15

N excesslegume N )

P fix =− 1

( atom %

15

N excesssoil − erived N )

where atom% 15 N excess = (atom% 15 N sample) - (atom% 15 N air N 2 ), and atom% 15 N of air
N 2 = 0.3663. The atom% 15 N of soil-derived N is generally estimated from the 15 N enrichment of
the non-N 2 -ixing reference plant grown in the same soil over the same period as the legume. The
choice of an appropriate non-N 2 -ixing reference plant is the single most important factor affecting
the accuracy of estimate P (Peoples and Herridge, 1990).
The major assumption of both the 15 N-enriched and natural 15 N abundance methods is that the
legume and nonlegume reference plants utilize soil N with the same isotopic composition (Herridge
and Danso, 1995). With the enrichment system, this translates into the legume and nonfixing ref-
erence plants utilizing the same relative amounts of N from added 15 N and endogenous soil N
(Herridge and Danso, 1995). This may not always occur and is the major drawback of this method
(Witty et al., 1988). In addition, the high cost of instrumentation to measure 15 N plus the expense of
the 15 N-labeled materials are real constraints to even greater use of the method.

7.5.8 15 n I sotope d IlutIon t eChnIque
Data on the 15 N enrichment of biomass can be used to calculate %Ndfa according to Fried and
Middleboe (1977) and Ruisi et al. (2012):

δ
δ

15

N
N

×

legume

%Ndfa =− 1 100

15

cereal

where δ 15 N legume is the atom% 15 N excess of legume tissue and δ 15 N cereal is the atom% 15 N excess
of cereal tissue. The 15 N natural abundance of the atmosphere (0.3663% 15 N) was used to calculate
the atom% 15 N excess of each species. The amount of N fixed by legume species can be estimated
as follows (Ruisi et al., 2012):

× %
100

Ndfa

NTotal legumeN

fixed =