concept

Biogeochemical cycle

The circular route an element takes through a planet's reservoirs — atmosphere, living things, soil, ocean, rock — driven by biology, geology and chemistry at once, exactly as the compound name implies. Every cycle is described with the same grammar: reservoirs (where an element sits, in units of mass) connected by fluxes (how fast it moves). The four that matter most to life are carbon, nitrogen, phosphorus and sulfur, and the single largest difference between them is simply whether the element has a stable gas phase.