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
Search WWH ::




Custom Search