In nature, elements are continuously recycled in various forms between different compartments of the environment (e.g. water, air, soil, organisms).
The 4 main activities responsible for the movement of carbon between different compartments of the environment are:
- Combustion: When fossil fuels are burnt, carbon dioxide is released into the air.
- Respiration: During aerobic respiration by living organisms, carbon dioxide is released into the air.
- Photosynthesis: Plants use carbon dioxide in the air during photosynthesis to produce glucose (C6H12O6). This is the only stage which decreases the concentration of carbon dioxide in air.
- Decomposition: Complex carbon compounds from dead plants and animals are broken down into simpler compounds by decomposers.
- Since nitrogen gas in the atmosphere (N2) is too unreactive to be used by plants, nitrogen must be converted into a soluble form that can be utilized by plants. This process is called nitrogen fixation.
- Nitrogen-fixing bacteria live in either the soil or the root nodules of legumes (e.g. peas). They convert nitrogen gas into nitrates (NO3-), which can be absorbed by roots of plants through active transport.
- Animals can obtain nitrogen by eating plants.
- Decomposers convert nitrogen from dead organisms and urea into ammonium (NH4+).
- Nitrifying bacteria converts ammonium to nitrites (NO2-) and nitrites into nitrates.
- Denitrifying bacteria converts nitrates back into nitrogen gas.