Atmospheric Nitrogen (N 2 ) Assimilation Plants Denitrifying bacteria Nitrogen-fixing bacteria in legume root nodules Nitrogen fixation Decomposers Ammonium (NH 4 + ) Ammonium (NH 4 + ) Nitrite (NO 2 - ) Nitrate (NO 3 - ) Nitrifying bacteria Nitrification Aerobic and anaerobic bacteria Fungi Denitrification Ammonification Nitrogen fixation Nitrogen-fixing soil bacteria Nitrifying bacteria