Biological nitrogen fixation (BNF) is the process, whereby atmospheric nitrogen is reduced to ammonia in the presence of nitrogenase. Phosphorus is needed for plant growth, nodule formation and development, and ATP synthesis, each process being vital for nitrogen fixation. Nitrogen fixation, which involves the chemical reduction of N2​ to NH3​ or NH4​, requires a source of electrons. Sources of electrons for the nitrogenase activity vary with the organism. They are all small proteins and highly reductive molecules such as flavodoxin, ferredoxin, nicotinamide, or adenine dinucleotide

Introduction:
Nitrogen fixation is the process by which atmospheric nitrogen (N2) is converted into ammonia (NH3) by certain microorganisms called nitrogen-fixing bacteria. One such nitrogen-fixing bacteria is Rhizobium, which forms a symbiotic relationship with leguminous plants. This symbiotic association between Rhizobium and leguminous plants is known as rhizobium-legume symbiosis. In this relationship, Rhizobium fixes nitrogen and provides it to the plant, while the plant provides carbohydrates and a suitable environment for the bacteria to thrive.

Explanation:
The question asks about the factors that increase nitrogen fixation by Rhizobium. Among the given options, the correct answer is option 'B' - Nitrogen.

Importance of Nitrogen:
Nitrogen is an essential nutrient for plants, and it is required in large quantities for their growth and development. However, atmospheric nitrogen (N2) cannot be directly utilized by plants. It needs to be converted into a usable form, such as ammonia (NH3) or nitrate (NO3-). This conversion is carried out by nitrogen-fixing bacteria like Rhizobium.

Nitrogen Fixation Process:
During nitrogen fixation, Rhizobium bacteria colonize the root nodules of leguminous plants. Inside the nodules, the bacteria convert atmospheric nitrogen into ammonia through the enzyme nitrogenase. This ammonia is then utilized by the plant for the synthesis of amino acids, proteins, and other nitrogen-containing compounds.

Role of Nitrogen:
Nitrogen is a crucial component of amino acids, which are the building blocks of proteins. Proteins play a vital role in various metabolic processes and are involved in the growth and development of plants. Therefore, an adequate supply of nitrogen is necessary for the efficient functioning of the plant.

Increase in Nitrogen Fixation:
The presence of nitrogen in the environment plays a significant role in increasing nitrogen fixation by Rhizobium. When there is a deficiency of nitrogen in the soil, leguminous plants release specific chemical signals called flavonoids from their roots. These flavonoids attract Rhizobium bacteria and initiate the process of nodulation.

When the Rhizobium bacteria come into contact with the plant's roots, they recognize the flavonoids and respond by producing a series of chemical signals called Nod factors. These Nod factors further stimulate the formation of root nodules and the colonization of Rhizobium bacteria within them.

The presence of nitrogen in the environment acts as a positive signal for the plants to reduce the production of flavonoids and Nod factors. This reduction in flavonoids and Nod factors helps in the efficient colonization of Rhizobium bacteria and enhances nitrogen fixation.

Conclusion:
In conclusion, the presence of nitrogen in the environment increases nitrogen fixation by Rhizobium bacteria. Nitrogen is an essential nutrient for plants, and its deficiency triggers the release of signals that attract Rhizobium bacteria. These bacteria fix atmospheric nitrogen into ammonia, which is utilized by the plant for growth and development. Therefore, nitrogen is a crucial factor in enhancing the symbiotic relationship between Rhizobium and leguminous plants.