. About 78% of the air around us is nitrogen but this gas is unreactive and needs to be processed into nitrates before it can be used by plants:
- a) Nitrogen gas in the air is converted into nitrates by lightning. Rain and other precipitation then bring the nitrates down into the soil. Although not shown on the diagram, the man-made Haber process also uses nitrogen in the air to produce plant fertiliser on an industrial scale.
- b) Nitrogen gas in the soil is converted into nitrates by bacteria in the soil and in nodules in plant roots.
2. Plants absorb nitrates through their roots and then use them to build proteins and grow.
3. Animals (including humans) eat the plants and absorb the nitrates by converting them into animal proteins that enable them to grow and function healthily.
4. Nitrogen returns to the soil as ammonia through:
- a) animal droppings and urea being broken down by decomposers, such as bacteria and fungi
- b) decomposers, such as bacteria and fungi, breaking down the dead bodies of animals.