While the abiotic and biotic factors of an ecosystem can be thought of as separate things, the reality is that they are connected in complex ways. Biogeochemical cycles describe the movement of matter across the surface of the planet, from the geosphere to the atmosphere to the biosphere to the hydrosphere and back again. The cycles include reservoirs (places where a compound or chemical resides for a time), and the processes that move material between reservoirs. Movement within Earth's biogeochemical cycles may be rapid or may take millennia.

The results of all the cycles create an equilibrium in the global environment. Local disturbances occur but have little effect on the larger systems. However, an accumulation of many local changes perturb (alter) the smooth operation of a biogeochemical cycle. Particular reservoirs may then expand or contract, and processes may slow or speed up. This can have a profound effect on the organisms that rely on the availability of certain compounds for survival.

There are as many biogeochemical cycles as there are elements on the periodic table, but four are of particular importance (about one of which you've already learned):

1. Water
2. Nitrogen
3. Phosphorous
   
4. Carbon

You've already learned about the water cycle. This week will focus on the nitrogen and phosphorous cycles, while next week will cover the carbon cycle.