Agriculture Reference
In-Depth Information
7.3 BIOCHEMISTRY OF NITROGEN FIXATION
Biological N by legumes is fixed mainly by two types of bacteria,
Rhizobium
and
Bradyrhizobium
.
However, the key element in the process of N fixation is the enzyme
nitrogenase
. Biological N fixa-
tion is similar to industrial N fixation in that it produces ammonia from molecular N and hydrogen.
N-fixing prokaryotes, in contrast to industrial processes, conduct this reaction at ambient tempera-
tures and pressures. The reduction of N
2
to 2NH
3
, a six-electron transfer, is coupled with the reduc-
tion of two protons to evolve H
2
(Epstein and Bloom, 2005). The enzyme nitrogenase catalyzes
the reduction of dinitrogen gas to ammonia according to the following equation (Brady and Weil,
2002):
N
2
+ 8H
+
+ 6e
−
(nitrogenase) ↔ 2NH
3
+ H
2
The ammonia formed in the above reaction combines with organic acids to form amino acids and
finally proteins as shown below:
NH
3
+ organic acids → amino acids → proteins
The nitrogenase enzyme is a complex of two proteins, the smaller of which contains iron while
the larger contains iron and molybdenum. To convert N
2
into NH
3
by nitrogenase requires energy,
and this energy is supplied by plants through photosynthesis. The amount of energy required is about
355 kJ mol
−1
NH
3
produced (Marschner, 1995). Calculations based on the carbohydrate metabolism
of legumes show that a plant consumes 12 g of organic carbon per g of N
2
fixed (Heytler et al., 1984).
Nitrogenase is unique to N
2
-ixation microorganisms and has been found, for example, in aero-
bic and anaerobic bacteria, blue-green algae, and root nodules of legumes. Nitrogenase enzyme
is destroyed by free O
2
, and it must be protected from exposure to O
2
. When N fixation takes
place in root nodules, one mechanism of protecting the enzyme from free oxygen is the formation
of
leghemoglobin
. Leghemoglobin is virtually the same molecule as the
hemoglobin
that gives
human blood its red color when oxygenated. This compound, which gives active nodules a red
interior color, binds oxygen in such a way as to protect the nitrogenase while making oxygen avail-
able for respiration in other parts of the nodule tissue (Brady and Weil, 2002). The concentration
of leghemoglobin is closely but not linearly correlated with the N
2
-ixing capacity of root nodules
(Werner et al., 1981).
For the nitrogenase reaction, energy in the form of a reductant and as ATP is essential. Energy
from ATP and electrons from the electron carrier (usually ferredoxin) induce a conformational
change in the iron protein and convert it into a powerful reductant capable of transferring electrons
to the molybdenum-ion protein, which in turn reduces N
2
(Marschner, 1995). Reduction of N
2
to
NH
3
requires 15-30 ATP molecules per N
2
molecule reduced (Shanmugam et al., 1978). The ATP
energy is mainly produced during respiration (Evans and Barber, 1977). In
Rhizobiua
, between
30% and 60% of the energy supplied to the nitrogenase is released as H
2
(Schubert et al., 1978).
Rhizobium strains are also capable of splitting the H
2
by hydrogenase, however, thus recycling the
electrons for subsequent N
2
reduction (Marschner, 1995). Selection of
Rhizobium
strains with a gen-
erally higher recycling of electrons from H
2
may be important for higher efficiency of the N
2
fixation
(Schubert et al., 1978; Marschner, 1995).
7.4 QUANTITY OF NITROGEN FIXATION BY LEGUMES
Introducing legumes into cereal-dominated cropping systems can provide many advantages, such as
fixing great amounts of atmospheric N, which will then be partly available to the subsequent crop,
reducing the occurrence of pests and weeds, and improving the quality of the soil (Peoples et al., 1995b;
