(N/A) The nitrogen cycle involves several key processes:
$1$. Nitrogen Fixation: Atmospheric $N_2$ is converted into ammonia or nitrates by nitrogen-fixing bacteria (e.g.,Rhizobium in root nodules of legumes) or through atmospheric and industrial processes.
$2$. Assimilation: Plants absorb these nitrogenous compounds to synthesize proteins and other organic molecules,which then enter the food chain as protoplasm in plants and animals.
$3$. Ammonification: Decomposers break down dead organic matter and waste products from plants and animals,releasing nitrogen back into the soil as ammonia.
$4$. Nitrification: Ammonia is converted into nitrites and then into nitrates by specific nitrifying bacteria in the soil.
$5$. Denitrification: Finally,certain bacteria convert nitrates back into atmospheric $N_2$ gas,completing the cycle.