I don't understand the carbon, nitrogen and phosphorus cycles.
Any nutrient or biogeochemical cycle involves the sources and sinks of that nutrient/element and also shows the transformations the nutrient/element undergoes. To understand any nutrient or biogeochemical cycle (whether it is carbon, nitrogen or phosphorus cycle), one should be able to identify these sources and sinks and the transformations. For example, for the carbon cycle, green plants are sinks, since they consume carbon dioxide during photosynthesis. The producers of carbon dioxide are anything that uses fossil fuels (such as factories, trains, vehicles, etc.), anything that carries out cellular respiration (such as plants, animals, human beings, etc.), etc. When plants and animals die, they may either be converted to fossil fuel (provided appropriate circumstances) or detrivores may consume them and release the carbon later on in soil, etc. All the other cycles can also be understood in similar terms.
Hope this helps.