Environmental Engineering Reference
In-Depth Information
Table 22.8 Relative efficiencies of N
2
fixation
N fixed (kg ha
−1
yr
−1
)
Type of association
Organism
Nodules
Legumes
Tropical clover
900
Temperate lucerne
45-675
Non-legumes
Temperate alder
140
Tropical Casuarina
50
Root
Temperate rye grass
60
Temperate grassland
40
Blue-green algae associations
Tropical lichens
10-100
Tropical Azolla
80-125
Free-living
Blue-green crusts
15-50
Rice paddies blue-green
10-80
Azotobacter
<1
Clostridium
<1
22.8 shows the relative efficiencies of some N
2
-fixing systems. The relative inefficiency
of free-living N
2
fixers is clear, owing to their inability to obtain sufficient energy for the
fixation process. On the other hand, blue-green algae are phototrophic and can get energy
from photosynthesis, and are of great value in fixing N
2
. In rice cultivation up to 50 per
cent of the nitrogen requirement of the plant is met by N
2
-fixing blue-green algae such as
Anabaena
,
Calothrix
and
Nostoc
.
The most important N
2
fixation occurs through the legume-
Rhizobium
symbiosis. It is
estimated that legumes in agriculture fix 35 million tonnes of N
2
every year, 4 million
tonnes are fixed in the rice crop, and 100 million tonnes are fixed in remaining terrestrial
ecosystems. The importance of fixation by root nodule associations between
actinomycetes (especially
Frankia
) and a variety of perennial non-leguminous plants is
now being recognized. The plant genera which are known to form such nodules are
Casuarina
,
Hippophae
,
Myrica
,
Alnus
,
Dryas
and
Ceanothus
.
Other branches of the nitrogen cycle seem subsidiary, but can have important effects
at the local scale. Lightning can produce N oxides in the atmosphere which are brought to
the soil surface by precipitation. Significant quantities of N oxides are also produced by
the internal combustion engine, and such pollution increases nitrogen inputs to local
ecosystems. Human fixation of nitrogen is quantitatively much more important. Perhaps a