Question

Nitrogen constituting about 78% of the atmospheric air acts as a limiting factor in soil because

• A. The nitrogen is quickly removed from the soil and tied up in plants.
• B. The nitrogen is not being returned to the atmosphere quickly enough.
• C. The nitrogen compounds are water soluble and quickly washed away.
• D. The nitrogen-fixing bacteria fix nitrogen in the soil and water at a slow rate.
• E. The denitrifying bacteria remove nitrogen from the soil at a faster rate.

Answer 1

Answer: (D)

The nitrogen-fixing bacteria fix nitrogen in the soil and water at a slow rate.

The percentage of nitrogen in the atmosphere is 78%. Nitrogen forms an integral component of proteins which is involved in the formation of structural framework of living organism. However, living organisms, except nitrogen fixing bacteria, algae, and fungi cannot use N2 directly and need different forms of fixed nitrogen for their requirement of protein synthesis. Further, some forms of nitrogen in exceeded quantities is poisonous for plants and animals. Nitrogen is therefore fixed in nature in several ways and passed to the living organism. The nitrogen cycle includes biological processes such as ammonification, nitrification, denitrification, nitrogen fixation, nitrogen assimilatory reduction as well as non- biological processes. Nitrification which is the oxidation of ammonium nitrogen to nitrites and nitrates is the result of metabolism by nitrogen-fixing bacteria. However, these nitrifying bacteria have a low activity level and take nearly an hour to produce twice its weight in NO_2.