Another major focus of ecosystem ecology is understanding how the chemical elements necessary for life persist and translocate in pools and fluxes within the ecosphere. The biosphere actively interacts with the three abiotic spheres (hydrosphere, atmosphere, and lithosphere) to provide the available concentration of each for life. This action has a significant impact on the relative distribution of these elements. The simple sugar products of photosynthesis, C6H12O6, are the base for organic matter, so carbon, hydrogen, and oxygen dominate the composition of life, and while oxygen is available in the lithosphere, and hydrogen in the hydrosphere, carbon is actually quite scarce in the environment, making the disproportionate amount of carbon in biomass a hallmark of life. In fact, there are about 20 elements used regularly in living organisms, of which nine called the macronutrients are the major constituents of organic matter: hydrogen, oxygen, carbon, nitrogen, calcium, potassium, silicon, magnesium, and phosphorus. Some of these elements are readily available in the abiotic environment, in which case conservation through cycling of the elements is not paramount; however, those in scarce supply, such as nitrogen and phosphorus (Table 4), are reused many times before being released from the system. These biogeochemical cycles provide the foundation to understand how human modification leads to eutrophica-tion (N and P cycles) and global climate change (C cycle).
Therefore, much effort has been made to study and understand these cycles, particularly the carbon, nitrogen, and phosphorus cycles, details of which are addressed elsewhere in this encyclopedia.
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