Biogeochemical cycles

Carbon cycle
1. Processes
  1. Photosynthesis
  2. Respiration
    1. Aerobic: break down organic compounds to carbon dioxide, releases more energy.
    2. Anaerobic: allows some organisms to survive and use food sources in oxygen-deficient environments, releasing methane.
  3. Food chains
    1. Pass organic compounds between organisms.
  4. Fossilisation
    1. Incomplete decomposition of organisms under anaerobic conditions, leads to deposition of fossil fuels. Some marine organisms absorb carbon dioxide and store it in their skeletons, later form sediments of limestone and chalk.
  5. Combustion
  6. Volcanoes
    1. Release carbon dioxide from fossil fuels or carbonate rocks.
2. Dynamic equilibria
  1. Inputs and outputs balance. A change in the cycle results in negative feedback mechanisms, rebalance the cycle allow the planet to remain stable.
  2. Global homeostatis
    1. Natural negative feedback mechanisms combine to produce a self-regulating system resisting change.
    2. Gaia hypothesis: considers Earth as single interacting system made up of biotic and abiotic parts maintaining stability. Some consider it as a single organism. Impact of human activities disguised by feedback mechanisms that counteract them, humans may cause changes that stop feedback (deforestation) or cause changes that are too rapid for feedback mechanisms to act quick enough.
3. Human impacts
  1. Combustion increases atmospheric carbon dioxide.
  2. Coal mines, gas and oil processing release methane.
  3. Combustion of wood reduces stored carbon, increases atmospheric carbon dioxide. Reduced photosynthesis and fixation of carbon in biomass.
  4. Anaerobic gut bacteria release methane, subsequently oxidised to carbon dioxide.
  5. Anaerobic bacteria in rice padi fields release methane.
  6. Soil disturbance by ploughing increases rate of decomposition, more carbon dioxide.
  7. Global climate change increases rate of decomposition, releasing more carbon dioxide. Increases rate of photosynthesis, absorbing more carbon dioxide. Methane released from melting permafrost or methane hydrate in marine sediments.
4. Basis of life, forms long life chains of atoms producing a large variety of compounds.
Phosphorus cycle
1. Important component in DNA, RNA, ATP, bones and proteins.
2. Atmosphere not involved, phosphates not soluable, limiting factor on plant growth (particularly in oceans where there are no rocks to release phosphates during weathering.)
3. Dissolved phosphates are absorbed by plants, passed along food chains. Phosphates excreted or found in dead organic matter may be available for other plants or be lost to sediments. Humans increase crop growth y manufacturing soluable phosphate fertilisers from phosphate rocks. Causes eutrophication if leached from fields because of their low solubility but are present in sewage effluent.
Nitrogen cycle
1. Essential component of proteins (enzymes, cell membranes, hormones), DNA, RNA and chlorophyll.
2. Most immobile in rocks or in the atmosphere, no plants or animals can assimilate it directly or reuse nitrogenous wastes they release.
3. Processes
  1. Ionising phenomena
    1. Provide energy for atmospheric nitrogen and oxygen to react and produce oxides of nitrogen (lightning, forest fires.) Oxides of nitrogen can dissolve in rainwater and produce dilute nitric acid, when washed into soil it reacts with minerals in soil to form nitrates.
  2. Reduction of nitrogen to ammonia
    1. Carried out by micro-organisms during nitrogen fixation, some live freely in soil others symbiotically in root nodules of leguminous plants.
  3. Food chains
    1. Pass nitrogen between organisms as amino acids and proteins.
  4. Nitrification
    1. Oxidation of ammonium ions to nitrites then to nitrates by nitrifying bacteria.
  5. Denitrification
    1. Chemical reduction of nitrates in soil to nitrogen and nitrogen oxide gases by anaerobic denitrifying bacteria, released into atmosphere.
  6. Leaching
    1. Loss of soluable substances such as nitrates from surface layers of soil, carried away by water.
  7. Absorption of soluable ions of nitrates, nitrites and ammonium from the soil by roots.
4. Human impacts
  1. Haber process involves industrial fixation of atmospheric nitrogen by converting it to ammonia with large amounts of energy.
  2. Nitrate fertilisers may increase problems of leaching (particularly if followed by rain.)
  3. Drainage increases aerobic nitrifying bacteria and reduces anaerobic denitrifying bacteria.
  4. Soil disturbance by ploughing increases rate of decomposition releases more ammonium into soil.
  5. Legumes (peas, beans, clover) may be grown to increase levels of nitrogen compounds in soil, other crops can subsequently use.
  6. NO are released into atmosphere by combustion processes, subsequently increase nitrates washed into soil by rain.

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