Biological Nitrogen Fixation:
Conversion of molecular nitrogen (N2) of the atmosphere into inorganic nitrogenous compounds such as nitrates or ammonia is called as nitrogen fixation. When this nitrogen fixation occurs through the agency of some living organisms, the process is called as biological nitrogen fixation in which atmospheric nitrogen is converted into ammonia.
An estimated 190 × 1012 gm per year of N2 is fixed by natural processes (both non-biological and biological). Of this, biological nitrogen fixation accounts for about 90%. Non-biological nitrogen fixation (by lightening (8%) and UV radiations (2%), contributes about 10% of the total N2 fixed by natural processes.
The Haber-Bosch industrial chemical process for ammonia production, which uses elevated temperatures (about 200°C) and high pressures (about 200 atmospheres) accounts for an estimated 80 × 1012 gm per year of N2 converted to ammonia on global basis and seldom meets agricultural demand. Significance of biological nitrogen fixation is quite obvious because it is carried out at ambient temperatures and one atmosphere by a variety of living bacteria, cyanobacteria (blue-green algae) and symbiotic bacteria.
Nitrogen Fixing Organisms:
Not all the organisms have capacity to fix molecular nitrogen (N2) of the atmosphere. Only certain prokaryotic micro organisms such as some free living bacteria, cyanobacteria (blue- green algae) and some of the prokaryotic micro-organisms in symbiotic association with other plants (mostly legumes) can fix atmospheric nitrogen.
They can be grouped as follows:
A. Free Living:
1. Autotrophic:
(a) Aerobic e.g., some cyanobacteria (blue-green algae). All those blue-green algae which can fix atmospheric nitrogen usually contain heterocyst’s such as Nostoc, Anabaena, Tolypothrix, Aulosira, Calothrix etc. But all the heterocyst’s bearing blue-green algae may not be atm. nitrogen fixers. A few non-heterocystous blue-green algae such as Gloeotheca are also known to fix atm. N2.
(b) Anaerobic e.g., certain bacteria such as Chromatium and Rhodospirillum.
2. Heterotrophic:
(a) Aerobic e.g., certain bacteria such as Azotobacter, Azospirillum, Derxia and Beijerinckia.
(b) Facultative e.g., certain bacteria such as Bacillus and Klebsiella.
(c) Anaerobic e.g., certain bacteria such as Clostridium and Methanococcus.
B. Symbiotic:
(a) Root Nodules of Leguminous Plants:
Various types of bacteria called rhizobia associated with root nodules of legumes can fix atm. N2. None of these two partners i.e., bacterium or legume alone can fix atm. N2. Rhizobia are aerobic bacteria that belong to different genera such as Rhizobium, Azorhizobium, Bradyrhizobium, Photorhizobium and Sinorhizobium.
A number of different species and varieties of these rhizobia infect specific leguminous host plants. Among these, Rhizobium leguminosarum is most common and well known. Different species of this bacterium specifically infect a number of leguminous crop plants such as clovers (Trifolium), pea (Pisum sativum) and bean (Phaseolus). Rhizobia remain free-living in the soil before infecting their respective host plant.
(b) Root Nodules of Parasponia (= Trema, family Ulmaceae):
Rhizobial symbiont of this host plant is Bradyrhizobium spp.
(c) Actinorhizal plants:
Many species of as many as 23 genera of non-leguminous-angiosperms which are all dicot and woody (trees or shrubs) and belonging to 8 different families, bear root nodules that can fix atmospheric nitrogen e.g., Myrica (bog myrtle), Casuarina, Alnus (Alder tree), Ceanothus (snow-brush), Eleagnus (autumn olive), Dastica etc. In all these plants, the bacterial symbiont is a filamentous soil-bacterium belonging to the genus Frankia of Actinomycetes.
(d) Other Symbiotic Associations:
A herbaceous plant, Gunnera (family Haloragaceae) and water fern Azolla form symbiotic association with blue-green algae Nostoc and Anabaena respectively which can fix atm. N2. Association of some blue-green algae with fungi in some lichens is also known to fix atmospheric nitrogen.