Question

Nitrogen constitutes 78%78% of the atmospheric air and it is essential for the growth of plants. Which of the following is the reason for limited amount of nitrogen in the soil?

• 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 too fast.

Answer 1

Answer: (D)

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

Nitrogen fixation includes fixation of gaseous nitrogen by nitrogen-fixing bacteria, which give off ammonia. Ammonia is then converted to nitrite and nitrate by nitrifying bacteria in nitrification. Plants absorb nitrates. The nitrogenous macromolecules from wastes and dead organisms are converted to ammonia by ammonifying bacteria in ammonification. Ammonia can be either directly recycled into nitrates or returned to the atmospheric N${}_2$ form by denitrifying bacteria. This long process is carried out by microbes at lower rates limiting the nitrogen availability to plant. Thus, the correct answer is option D.