The nitrogen cycle

In short: Nitrogen is a building block of all proteins and of genetic material. It moves in a cycle between air, soil, plants, animals and microorganisms.

In more detail: Air is about 78 per cent nitrogen, but plants and animals cannot use it in this form. Only bacteria, lightning or industrial plants convert it into compounds such as ammonium and nitrate that plants can take up.

In detail

• Nitrogen fixation: Certain bacteria can capture nitrogen from the air. Some live in nodules on the roots of legumes such as clover, peas and beans and supply the plant with nitrogen.
• Lightning: The enormous heat of a lightning bolt splits nitrogen molecules. Nitrogen oxides form and reach the soil with the rain.
• Nitrification: Other soil bacteria convert ammonium step by step via nitrite into nitrate. Nitrate is the form many plants prefer to absorb.
• Food chain: Plants build nitrogen into proteins. Animals take it in with their food and excrete the surplus, for example as urea.
• Decomposition: When plants and animals die, fungi and bacteria break down their remains and release ammonium again. In this way part of the cycle closes within the soil.
• Denitrification: In oxygen-poor soils and waters, bacteria convert nitrate back into atmospheric nitrogen. Nitrous oxide can also form, a powerful greenhouse gas.
• Artificial fertiliser: Since the early 20th century the Haber-Bosch process has made ammonia from atmospheric nitrogen. Mineral fertiliser makes high yields possible and today feeds a large part of humanity.
• Too much of a good thing: Excess fertiliser and slurry reach groundwater and rivers as nitrate. There they encourage algal growth and can affect drinking-water quality.
• Related cycles: Material cycles and soil are closely linked (kohlenstoffkreislauf, bodenaufbau-und-humus, kompost-anlegen).