Carbon and Nitrogen are very important elements because they are required by all living things (carbon for respiration and nitrogen for proteins). They are constantly recycled through the environment.
The Carbon Cycle
Carbon exists in several forms as it moves through the carbon cycle. The non-living environment includes substances that never were alive as well as carbon-bearing materials that remain after organisms die. Carbon is found in the non-living part of the hydrosphere, atmosphere, and geosphere as:
- carbonate (CaCO3) rocks: limestone and coral
- dead organic matter, such as humus in soil
- fossil fuels from dead organic matter (coal, oil, natural gas)
- carbon dioxide (CO2) in the air
- carbon dioxide dissolved in water to form HCO3−
Carbon enters living matter through autotrophs, which are organisms capable of making their own nutrients from inorganic materials.
- Photoautotrophs are responsible for most of the conversion of carbon into organic nutrients. Photoautotrophs, primarily plants and algae, use light from the sun, carbon dioxide, and water to make organic carbon compounds (e.g., glucose).
- Chemoautotrophs are bacteria and archaea that convert carbon from carbon dioxide into an organic form, but they get the energy for the reaction through oxidation of molecules rather than from sunlight.
Carbon returns to the atmosphere and hydrosphere through:
- burning (as elemental carbon and several carbon compounds)
- respiration by plants and animals (as carbon dioxide, CO2)
- decay (as carbon dioxide if oxygen is present or as methane, CH4, if oxygen is not present)
- Carbon dioxide in the air is removed by plants through photosynthesis
- Eating of the plants by animals passes the carbons from plants to animals
- Both plant and animal respiration produce CO2 which return some of the carbons back into the atmosphere
- Decomposers release carbons in plant and animal remains and animal excretions back into the atmosphere
- Combustion of fossil fuels release carbons from dead plants/animals what were not decomposed
The Nitrogen Cycle
There are two main ways of nitrogen fixation:
- Fixation by Lightning - The energy from lightning causes nitrogen (N2) and water (H2O) to combine to form ammonia (NH3) and nitrates (NO3). Precipitation carries the ammonia and nitrates to the ground, where they can be assimilated by plants.
- Biological Fixation - About 90% of nitrogen fixation is done by bacteria. Cyanobacteria convert nitrogen into ammonia and ammonium. N2 + 3 H2 → 2 NH3 Ammonia can be used by plants directly. Ammonia and ammonium may be further reacted in the nitrification process.
Nitrification occurs by the following reactions:
2 NH3 + 3 O2 → 2 NO2 + 2 H+ + 2 H2O
2 NO2- + O2 → 2 NO3-
Aerobic bacteria use oxygen to convert ammonia and ammonium. Nitrosomonas bacteria convert nitrogen into nitrite (NO2-) and then nitrobacter convert nitrite to nitrate (NO3-). Some bacteria exist in a symbiotic relationship with plants (legumes and some root-nodule species). Plants utilize the nitrate as a nutrient. Animals obtain nitrogen by eating plants or plant-eating animals.
When plants and animals die, bacteria convert nitrogen nutrients back into ammonium salts and ammonia. This conversion process is called ammonification. Anaerobic bacteria can convert ammonia into nitrogen gas through the process of denitrification:
NO3- + CH2O + H+ → ½ N2O + CO2 + 1½ H2O
Denitrification returns nitrogen to the atmosphere, completing the cycle.
Summary of the cycle:
- Lightning fixation causes nitrogen gas (N2) in the air and water (H2O) to combine to form ammonia (NH3) and nitrates (NO3). Precipitation carries the ammonia and nitrates to the ground
- Nitrates in the soil are assimilated by plants in plant protein
- Eating of plants by animals passess the nitrogens into animal protein
- Dead animals return the nitrogen back into the soil for intake by the plants
- Some of the nitrogen is return to the atmosphere by denitrifying bacteria