These are the cycles of important compounds such as carbon, nitrogen, sulphur or phosphorus. The compounds circulate in their large ecosystem and microorganisms play an important role in this. They bind nitrogen from the air, make it usable for plants, and convert it back to atmospheric nitrogen if there is an excess (see "Nitrogen fixation", "Nitrification" and "Denitrification"). Furthermore, microorganisms together with decomposing animals decompose dead material in the soil, some can process harmful sulphur or help us with digestion.
Explanation for children: In our large habitat, something happens all the time, which is why most compounds are always in motion. When we need to go to the toilet, we excrete a lot of nitrogen. After several steps it reaches the soil, where plants urgently need it. We have already learned about these processes under "nitrogen fixation", "ammonification" and "deammonification". Other compounds such as carbon, sulphur and phosphorus also work in a similar way. They are not at a specific location, but always circle around in our large system, wandering from station to station. Microorganisms are very important in these cycles because, for example, they can bind nitrogen from the air and convert it for plants or process dead material.