All living things depend on nitrogen, oxygen and carbon dioxide in the air. We also depend on water. However, these substances do not just get used up and disappear, nature recycles them!

Four main cycles to consider are:

1. The nitrogen cycle

Nitrogen circulates between air, the soil and living things.

2. The carbon cycle

Carbon dioxide circulates between the air, soil, and living things.

3. Photosynthesis

This process followed by respiration recycles oxygen.

4. The water cycle

Water circulates between the air, oceans and living things.