Editing Biogeochemical cycle (section)

== Reservoirs ==
The chemicals are sometimes held for long periods of time in one place. This place is called a ''reservoir'', which, for example, includes such things as [[coal]] deposits that are storing [[carbon]] for a long period of time.<ref name="carbon">{{cite web|last1=Baedke|first1=Steve J.|last2=Fichter|first2=Lynn S.|title=Biogeochemical Cycles: Carbon Cycle|url=http://csmgeo.csm.jmu.edu/geollab/idls/carboncycle.htm|website=Supplemental Lecture Notes for Geol 398|publisher=James Madison University|access-date=20 November 2017|archive-date=1 December 2017|archive-url=https://web.archive.org/web/20171201043948/http://csmgeo.csm.jmu.edu/geollab/idls/carboncycle.htm|url-status=live}}</ref> When chemicals are held for only short periods of time, they are being held in ''exchange pools''. Examples of exchange pools include plants and animals.<ref name="carbon" />

Plants and animals utilize carbon to produce carbohydrates, fats, and proteins, which can then be used to build their internal structures or to obtain energy.  Plants and animals temporarily use carbon in their systems and then release it back into the air or surrounding medium.  Generally, reservoirs are abiotic factors whereas exchange pools are biotic factors. Carbon is held for a relatively short time in plants and animals in comparison to coal deposits. The amount of time that a chemical is held in one place is called its [[residence time]] or [[turnover time]] (also called the renewal time or exit age).<ref name="carbon" />