Biogeochemical Cycle (Nutrient Cycle)

The continuous exchange of nutrients between organisms and their environment — needed for every organism's growth, development, maintenance and reproduction — is called a biogeochemical cycle, or the cycling of materials. Two basic types exist. Gaseous cycles move their element mainly through the atmosphere, and include the oxygen, carbon and nitrogen cycles. Sedimentary cycles instead move their element mainly through solid earth deposits, and include the phosphorus, sulphur and calcium cycles. Since oxygen and nitrogen cycling were covered in earlier study, this chapter details two representative cycles in full. The carbon cycle is the circulation of carbon between organisms and their environment, driven by the two reciprocal processes of photosynthesis (which pulls atmospheric CO2 into living tissue) and respiration (which releases it back); additional carbon returns to the atmosphere through the burning of fossil fuels, deforestation, forest fires, volcanic eruptions and the decomposition of dead organic matter, while some carbon is instead locked away for long periods as peat, coal, oil, gas and buried sediments. The phosphorus cycle is a sedimentary cycle: phosphorus, a component of key biomolecules such as DNA, RNA, ATP, NADP and phospholipids, is not naturally abundant in the biosphere but occurs in bulk in rock deposits, marine sediments and guano (accumulated seabird/bat excrement), from which it is released by weathering and mining before circulating through the lithosphere and hydrosphere; producers take it up as inorganic phosphate ions, it passes along the food chain through each trophic level, and it is eventually returned to the lithosphere and hydrosphere when organisms die and decomposers break their tissue down …