Part I: Nutrient Cycles

Nutrients flow from the living to the nonliving and back to the living parts of the ecosystem in a perpetual cycle. By means of these cycles, plants and animals obtain necessary nutrients. There are two kinds of biogeochemical cycles: the gaseous cycle (represented by oxygen, carbon, and nitrogen) and the sedimentary cycle (represented by phosphorus). The sulfur cycle is a combination of the two. The gaseous cycle involves two main reservoirs, the atmosphere and the oceans, and is pronouncedly global. The sedimentary cycle involves two phases, salt solution and rock. Minerals become available through the weathering of Earth's crust, enter the water cycle as salt solutions, take diverse pathways through the eco-system, and return to the sea or Earth's crust through sedimentation.

Nutrients move through the ecosystem by means of exchanges called fluxes between major components of reservoirs. These fluxes in terrestrial ecosystems move nutrients from atmosphere and soil to producers, then to consumers, and then to dead organic matter. Decomposers feeding on dead organic matter transform nutrients from organic to mineral form, transferring them to soil reservoir. Then they flow back to the primary producers. The movement of nutrients through the ecosystem is internal cycling. The rate of recycling is influenced by the rate of uptake of nutrients by primary producers and the rate of nutrient release by decomposers.