(i) Energy enters into the biosphere through the process of photosynthesis carried out by plants in the presence of sunlight. Then, it is transferred from one tropic level to next.
(ii) Natural Process Respiration.
Man-made Process Burning of fossil fuel, (coal and petroleum), i.e. combustion.
(iii) X-Nitrification It is the process of conversion of ammonia into nitrites and then into nitrates by nitrifying bacteria (Nitrosomonas and Nitrobacter).
Y-Ammonification It is the process of ammonia formation through decomposition of dead, complex organic matter. This can be done by microorganisms living in soil, e.g. fungi, putrefying bacteria, etc. Z-Denitrification It is the process of reducing nitrates back into atmospheric nitrogen gas. It is performed by Pseudomonas.