Question

Your advice is sought to improve the nitrogen content of the soil to be used for the cultivation of a non-leguminous terrestrial crop.
A. Recommend two microbes that can enrich the soil with nitrogen
B. Why do leguminous crops not require such enrichment of the soil?

Answer 1

Hint: Nitrogen is a macronutrient that is required by the plant for the synthesis of nucleic acids, amino acids, and proteins. Plants absorb nitrogen from the soil in the form of nitrates. Atmospheric nitrogen cannot be assimilated by the plant because of its triple bond nature. Thus atmospheric nitrogen is metabolized by soil bacteria to form water-soluble nitrates. This process is called nitrogen fixation.

Complete answer:
A. Certain bacteria can enrich the soil with nitrogen. These are nitrogen-fixing bacteria. For example, Azospirillum, Anabaena, Nostoc, Azotobacter, and Oscillatoria. These microorganisms can be added to the soil in the form of biofertilizers such that the nitrogen content of the soil can be increased.
Nitrogen fixation can either be a non-biological or a biological process. Lightning produces enough energy to break the triple bond allowing the atoms to react with an oxygen atom to form oxides. Oxides combine with rain to form acids. This seeps into the soil enriching it. Diazotrophs utilize nitrogenase enzymes under anaerobic conditions to fix atmospheric nitrogen.

B. Leguminous crops do not require enrichment of the soil as they are in symbiotic association with the Rhizobium. Rhizobia occupy the root nodules of the leguminous plants. They help the plant to absorb nitrogen from the soil and in turn, the plant shelters it. When the plant dies the fixed nitrogen is sent back to the soil for the utilization of other plants.

Note: Nitrogenases are iron-containing enzymes that require Molybdenum or Vanadium as cofactors. Nitrogen fixation is an anaerobic process. Certain plants contain a protein leghaemoglobin that sequesters oxygen atoms.