Question

Study the cycle shown above and select the option which gives correct words for all the four blanks A, B, C and D.

	A	B	C	D
1	Denitrification	Nitrificaiton	Plants	Animals
2	Nitrification	Ammonification	Animals	Plants
3	Denitrification	Ammonification	Plants	Animals
4	Nitrification	Denitrification	Animals	Plants

• A. 1
• B. 2
• C. 3
• D. 4

Answer 1

Answer: (C)

3

• Denitrification is the process by which the free nitrogen is returned back to the atmosphere by sequential degradation of fixed nitrogenous compounds.
• This process is accelerated by denitrifying bacteria like pseudomonas. Ammonification is performed by bacteria to convert the ammonia to ammonium.
• Nitrification can then occur to convert the ammonium to nitrite and nitrate. Microorganisms that can fix nitrogen are prokaryotes (both bacteria and archaea, distributed throughout their respective kingdoms) called diazotrophs.
• Some higher plants, and some animals (termites), have formed associations (symbiosis) with diazotrophs. Although nitrogen is very abundant in the atmosphere as dinitrogen gas (N${}_2$), it is largely inaccessible in this form to most organisms, making nitrogen a scarce resource and often limiting primary productivity in many ecosystems.
• Only when nitrogen is converted from dinitrogen gas into ammonia (NH${}_3$) does it become available to primary producers, such as plants.
  Hence, option C is correct.