2.3 Biogeochemical cycles

Objective notes

25 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

2.3.12(HL)—Lithosphere carbon stores

• The lithosphere contains carbon stores in fossil fuels and in rocks, such as limestone, that contain calcium carbonate

• The residence time for carbon in these stores can be hundreds of millions of years

2.3.13(HL)—Carbonate fossilization

• Reef-building corals and molluscs have hard parts that contain calcium carbonate that can become fossilized in limestone

• Limestone is the largest store of carbon in Earth systems

• Not all limestone is formed by fossilization of animal remains; it can also be formed by both biological and non-biological processes

• Details of these processes are not required

2.3.14(HL)—Fossil fuel formation

• In past geological eras, organic matter from partially decomposed plants became fossilized in coal, and partially decomposed marine organisms became fossilized in oil

• Formation of coal, oil and gas was greatest in specific geological eras when conditions were most suitable

2.3.15(HL)—Methane production

• Methane is produced from dead organic matter in anaerobic conditions by methanogenic bacteria

2.3.16(HL)—Methane cycle

• Residence time: ~10 years in atmosphere

• Oxidized to CO₂

• Potent greenhouse gas

2.3.17(HL)—Nitrogen cycle stores

• Organic: proteins in organisms and dead matter

• Inorganic: atmospheric N₂, ammonia, nitrites, nitrates

2.3.18(HL)—Bacterial roles in nitrogen cycle

• Nitrogen fixation: N₂ → ammonia

• Nitrification: ammonia → nitrates

• Denitrification: nitrates → N₂

• Decomposition: amino acids → ammonium

2.3.19(HL)—Denitrification conditions

• Only in anaerobic conditions (waterlogged soils)

• Insectivorous plants use insects as nitrogen source

2.3.20(HL)—Plant nitrogen fixation

• Plants cannot fix nitrogen

• Require mutualistic associations with nitrogen-fixing bacteria

2.3.21(HL)—Nitrogen cycle flows

• Mineral uptake, photosynthesis, consumption, excretion, death, decomposition, ammonification

2.3.22(HL)—Human impacts on nitrogen cycle

• Deforestation, agriculture, aquaculture, urbanization

2.3.23(HL)—Haber process

• Industrial production of ammonia for fertilizer

• From nitrogen and hydrogen

2.3.24(HL)—Nitrogen planetary boundary

• Exceeded due to human activities

• Nitrate increases in biosphere

• Major cause: inorganic fertilizers for crops

2.3.25(HL)—Global collaboration needed

• Address uncontrolled nitrogen use

• Bring nitrogen cycle within planetary boundaries