The
nitrogen cycle is the process by which
nitrogen is converted between its various chemical forms. This transformation can be carried out through both biological and physical processes. Important processes in the nitrogen cycle include
fixation,
ammonification,
nitrification, and
denitrification. The majority of
Earth's atmosphere (78%) is
nitrogen, making it the largest pool of nitrogen. However, atmospheric nitrogen has limited availability for biological use, leading to a scarcity of usable nitrogen in many types of ecosystems. The nitrogen cycle is of particular interest to
ecologists because nitrogen availability can affect the rate of key ecosystem processes, including
primary production and
decomposition. Human activities such as fossil fuel combustion, use of artificial nitrogen fertilizers, and release of nitrogen in wastewater have dramatically
altered the global nitrogen cycle.