2.3 Biogeochemical cycles

Objective notes

11 learning objectives

2.3.1Biogeochemical cycles purpose

• Ensure chemical elements remain available to living organisms

• Human impact affects ecosystem sustainability

2.3.2Cycle components

• Stores: equilibrium with environment

• Sinks: net accumulation

• Sources: net release

2.3.3Carbon stores

• Organic: organisms, crude oil, natural gas

• Inorganic: atmosphere, soils, oceans

2.3.4Carbon flows

• Photosynthesis, feeding, defecation, respiration, death, decomposition

2.3.5Carbon sequestration

• Capturing atmospheric CO₂ and storing as solid/liquid

• Natural: trees absorb CO₂ → biomass

• Fossilization into coal, oil, natural gas

2.3.6Ecosystems as carbon stores/sinks/sources

• Sink: photosynthesis > respiration (e.g., young forest)

• Store: balanced (e.g., mature forest)

• Source: respiration > photosynthesis (e.g., forest fire)

2.3.7Fossil fuels as carbon stores

• Fossil fuels are stores of carbon with unlimited residence times

• They were formed when ecosystems acted as carbon sinks in past eras and become carbon sources when burned

• Consider: the concept of fossil fuels but not the detail of how and when coal, oil and natural gas were formed

2.3.8Agriculture as carbon store/source/sink

• Agricultural systems can act as carbon stores, sources and sinks, depending on the techniques used

• Regenerative agricultural methods, such as crop rotation, cover crops and no till, will promote the role of soil as a carbon sink whereas drainage of wetland, monoculture

• Cropping over a longer timescale (e.g., timber production) and the subsequent use of harvested products will also affect these roles

2.3.9Ocean carbon exchange

• Carbon dioxide is absorbed into the oceans by dissolving and is released as a gas when it comes out of a solution

• While oceans act as a carbon sink, the human use of fossil fuels releases inorganic carbon at a faster rate than oceans can absorb it

2.3.10Ocean acidification

• Increases in concentrations of dissolved carbon dioxide cause ocean acidification, harming marine animals

• Small decreases in pH can interfere with calcium carbonate deposition in mollusc shells and coral skeletons

2.3.11Alleviating carbon cycle impacts

• Measures are required to alleviate the effects of human activities on the carbon cycle

• Consider at least three required measures

• These include low-carbon technologies, reduction in fossil-fuel burning/soil disruption/deforestation, carbon capture through reforestation and artificial sequestration