“Rhizobia are soil bacteria that fix nitrogen once they found a base inside root nodules of legumes”. In other terms, they can be defined as legume root-nodule bacteria. Fixation of nitrogen cannot be done independently. That is rhizobia require a plant host. It represents a vital source of N input in agricultural soils including those in arid regions. That is they convert dinitrogen into ammonia. Ammonia is toxic in nature. Hence they are rapidly absorbed into organic compounds.


Nitrogen fixation helps in increasing productivity of low-N soils and soil fertility. The various behavioral factors such as drought stress, nutrient deficiency, salt stress, fertilizers, pesticides of nitrogen-fixing systems are reviewed.

Nitrogen – Fixing Bacteria

Organisms that is responsible for nitrogen fixation belong to the group of prokaryotes. Their main function involves the conversion of stable nitrogen gas in the atmosphere to a biologically useful form. Nitrogenase is an enzyme complex that reduces dinitrogen to ammonia.

A huge amount of energy is consumed during the nitrogen fixation and the nitrogenase enzymes are irreversibly inactivated by oxygen. Acetylene reduction assay is used to measure the Nitrogenase activity. A very less portion of species is capable of carrying out nitrogen fixation. That is around two genera of archaea, twenty genera of cyanobacteria and much more.

Role of Rhizobium in Agriculture

A symbolic amount of nitrogen remains after the harvesting of a grain even though a much of nitrogen is removed at the time of harvesting. It is mainly taken into account when there is no usage of nitrogen fertilizers. It is usually seen in less industrialized countries. Nitrogen is one of the most supplied plant nutrients as it is one of the common deficiency found in soils.  Several environmental concerns are raised regarding supply of nitrogen through soil.

Rhizobia infects the roots of the bacteria. They are usually found in the soil and after the infection modules are produced in the legume. As a result, nitrogen gas is fixed from the atmosphere. After this process, the nitrogen is used for the growth in the legume. Once legume dies there will be a breakdown of the nodule. as a result, rhizobia is released back to the cell where they can affect a new host.

For nitrogen fixation, the specific strains of rhizobia are required to make the functional nodules on the root to carry out the process. This is beneficial to legume as it results in the increase in crop yield. Legume inoculation has been an agricultural practice for several years and has constantly improved over time.

Salinity is one of the serious threat to agriculture in arid and semiarid regions. Most of these areas are restricted to the Mediterranean and tropical regions. There is a decrease in the productivity of most crop plants due to the increases in the water supplies used for irrigation or salinity of soils.

Practise This Question

Identify A in the nucleus.