David & Matan
The Nitrogen Cycle
The nitrogen cycle is the global circulation of nitrogen from the environment to
living organisms and back to the environment. It is the process by which nitrogen is
converted between its chemical forms. There are five steps in the nitrogen cycle, in
which nitrogen cycles between the abiotic environment and organisms: nitrogen
fixation, nitrification, assimilation, ammonification, and denitrification. Bacteria are
involved in all of these steps except for assimilation.
Nitrogen fixation – conversion of gaseous nitrogen to ammonia
Nitrification – soil bacteria perform this in two steps: 1. Soil bacteria convert
ammonium to nitrate 2. Soil bacteria oxidize nitrite to nitrate.
Assimilation – Plant roots absorb nitrate ammonia, or ammonium and assimilate the
nitrogen of these molecules into plant proteins and nucleic acids.
Ammonification – conversion of biological nitrogen compounds into ammonia and
ammonium ions
Denitrification – reduction of nitrate to gaseous nitrogen. Denitrifying bacteria
reverse the action of nitrogen-fixing and nitrifying bacteria by returning nitrogen to
the atmosphere.