Nitrogen becomes fixed on land by certain animals. Then plants assimilate and use fixed nitrogen in the biosynthesis of amino acids, proteins, and nucleic acids. Pland tissues are the only source of nitrogen for animals. During decomposition and ammonification, fungi and bacteria will break down wastes containing nitrogen and the remains of organisms. The decomposers use some of the released proteins and amino acids for their metabolism.
Plants absorb the nitrogen that is left in decay products such as ammonia or ammonium.
Nitrifying bacteria also absorbs ammonia or ammonium. During nitrification, they will strip the ammonium or ammonia of their electrons. All of the compounds, ammonium, nitrite and nitrate are sensitive to leaching and runoff. Leaching is when soil water moves out of and area, and it loses the nutrients dissolved in it. When nitrogen is lost to the air, it is a process called denitrification.