2.3 Biogeochemical cyclesIB Environmental Systems and Societies HL: Revision notes

Biogeochemical cycles ensure that chemical elements stay available to living organisms. They have stores, sinks and sources. Ecosystems can act as stores, sinks or sources of carbon.

Organic stores of carbon include organisms, crude oil and natural gas. Inorganic stores are in the atmosphere, soils and oceans.

Carbon flows between stores by photosynthesis, feeding, defecation, cellular respiration, death and decomposition. Carbon sequestration captures atmospheric carbon dioxide and stores it in a solid or liquid form.

Fossil fuels are stores with unlimited residence times; they formed when ecosystems were sinks in past eras and become sources when burned. Agricultural systems can be stores, sources or sinks depending on technique. Carbon dioxide dissolves into the oceans and is released as a gas when it comes out of solution; more dissolved carbon dioxide causes ocean acidification, harming marine animals. Measures are needed to alleviate these effects.

The lithosphere holds carbon in fossil fuels and in rocks such as limestone (calcium carbonate). Reef-building corals and molluscs have hard parts of calcium carbonate that can become fossilised in limestone; chalk is a type of limestone.

In past geological eras partially decomposed plants became coal, and partially decomposed marine organisms became oil and natural gas held in porous rocks.

Methane is produced from dead organic matter in anaerobic conditions by methanogenic bacteria, for example in flooded rice paddies. Methane has a residence time of about 10 years in the atmosphere and is then oxidised to carbon dioxide.

The nitrogen cycle has organic and inorganic stores. Flows include mineral uptake by producers, photosynthesis, consumption, excretion, death, decomposition and ammonification.

Bacteria play essential roles: nitrogen-fixing bacteria, decomposers, nitrifying bacteria and denitrifying bacteria. Plants cannot fix nitrogen, so atmospheric dinitrogen is unavailable to them unless they form mutualistic associations with nitrogen-fixing bacteria (as legumes do in root nodules). Denitrification happens only in anaerobic conditions such as waterlogged soil.

Deforestation, agriculture, aquaculture and urbanization change the nitrogen cycle. The Haber process produces ammonia from nitrogen and hydrogen for fertiliser. Rising nitrates in the biosphere mean the planetary boundary for the nitrogen cycle has been crossed, making irreversible changes to Earth systems likely.

Global collaboration is needed to control nitrogen use in industry and agriculture and return the cycle to within the boundary. Limits include differing national priorities, food security needs, enforcement and time lags.