Question

Name the microorganism which can fix nitrogen in the soil.
A. Rhizobium
B. Azotobacter
C. Anabaena
D. All of the above

Answer 1

Bacteria that fix nitrogen, microorganisms that are capable of turning atmospheric nitrogen into fixed nitrogen (inorganic compounds usable by plants). These species are responsible for more than 90 per cent of all nitrogen fixation, and thus play a significant role in the nitrogen cycle.

Complete Answer:
There are two types of nitrogen-fixing bacteria. The cyanobacteria (or blue-green algae) Anabaena and Nostoc and genera such as Azotobacter, Beijerinckia, and Clostridium are included in the first type, the free-living (nonsymbiotic) bacteria.

Mutualistic (symbiotic) bacteria form the second type; examples include Rhizobium, associated with leguminous plants (e.g. separate pea family members); Frankia, associated with certain dicotyledonous species (actinorhizal plants); and certain species of Azospirillum, associated with cereal grasses.

The root hairs of host plants are invaded by symbiotic nitrogen-fixing bacteria, where they multiply and promote the development of root nodules, plant cell enlargements and intimately associated bacteria.Inside the nodules, the bacteria transform free nitrogen to ammonia, which is used for production by the host plant.

Nitrifying bacteria are one of a small group of aerobic bacteria that use inorganic chemicals as an energy source. They are microorganisms that are important in the nitrogen cycle as converters of soil ammonia to nitrates, compounds that can be used by plants.

Two separate classes of bacteria that convert ammonia into nitrites (Nitrosomonas, Nitrosospira, Nitrosococcus and Nitrosolobus) and bacteria that convert nitrites into nitrates (toxic to plants) are mediated by the nitrification method (Nitrobacter, Nitrospina, and Nitrococcus). Through the action of nitrifying bacteria, irrigation with diluted ammonia solutions results in a rise of soil nitrates in agriculture.

The correct Answer is option (D) All of the above.

Note: In order to ensure adequate nodule formation and optimum growth of legumes (e.g. alfalfa, beans, cloves, peas, soybeans), seeds are typically inoculated with commercial crops of appropriate Rhizobium species especially in soils with or without the necessary bacterium.