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 Nitrification Plants Animals 2 Nitrification Ammonification Animals Plants 3 Denitrification Ammonification Plants Animals 4 Nitrification Denitrification Animals Plants

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 (N2), 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 (NH3) does it become available to primary producers, such as plants. Hence, option 3 is correct.