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.