The Nitrogen Cycle

• Living organisms require a source of nitrogen from which to manufacture proteins, nucleic acids and other nitrogenous compounds.
• Very few organisms can use nitrogen gas directly, so plants take up most of the nitrogen they require from the soil via active transport of nitrate ions (NO3^-). This occurs through the roots of the plant.
• Nitrate ions are very soluble and easily leach (wash) through the soil beyond the reach of plant roots.
• In natural ecosystems, the nitrate concentrations are restored largely by the recycling of nitrogen-containing compounds.
• In agricultural ecosystems, the concentration of soil nitrate can be further increased by the addition of fertilisers.
• When plants and animals die, the process of decomposition begins. This releases nitrate ions, which is very important because there are very few nitrate ions available from other sources.
• There a four main stages of the nitrogen cycle : ammonification, nitrification, denitrification and nitrogen fixation.