Biogeochemical cycles are the movement of matter through the biotic and the abiotic spheres of the ecosystem. These cycles follow from the following premises:
- Matter is neither created nor destroyed. There are processes by which mass-energy conversion takes place but these are not significant for the elements we are interested in.
- Except for meteorites that enter the earth from outer space (and these are negligible) earth does not receive matter from outside.
- This means that the elements in the earth exist in a closed system. They are not lost or replenished.
- Ninety different naturally occurring elements are known in the earth. Six of these – hydrogen, carbon, oxygen, nitrogen, sulphur and phosphorous – make up 95% of the mass of all living organisms. These elements are also present in various forms in the earth, oceans, and air. The presence of these elements keeps changing within each of these separate spheres. But because the total amount of each element is essentially constant, that is, the total number of atoms of each of these elements remain unchanged, these elements must move through a cycle by which they are transferred from, for example, living beings to earth, air and water and back to living beings. These cycles because they involve the biosphere, the geo-sphere and the chemical processes by which the atoms of these elements circulate are called biogeochemical cycles.
All the different elements present in living organisms can be seen parts of separate biogeochemical cycles. The living organisms of the planet are collectively called the biosphere, or the biotic sphere; The non-living, or the abiotc spheres of the planet can be divided into hydrosphere (water), lithosphere (land) and atmosphere (air). At any specific time the atoms of an element must be present in one of these.
Main Cycling Elements
The main cycling elements include the following:
- The macronutrients : that are present in relatively large amounts in living organisms. They include carbon, hydrogen, oxygen, nitrogen, phosphorous and sulphur – sometimes called the "big six". The other macronutrients are : potassium, calcium, iron and magnesium.
- The micronutrients that are required in very small amounts in living beings (but still necessary): boron (in green plants), copper (in some enzymes) and molybdenum (in nitrogen-fixing bacteria).
Reservoir and Residence
The elements can sometimes exist in one site for long periods of time; such a site is called a reservoir. For example, coal deposits that store carbon for a long period of time. When the elements are held in a place for only short periods of time, such places are called exchange pools. For example, plants and animals, which temporarily use carbon in their systems and release it back into the environment. Generally, reservoirs are abiotic factors while exchange pools are biotic factors. The amount of time that an element is held in one place is called its residence.
Biogeochemical cycles are complex systems. This is because, usually, a variety of pathways are available by which an element can move among hydrosphere, lithosphere, atmosphere, and biosphere. For instance, nitrogen can move from the lithosphere to the atmosphere by the direct decomposition of dead organisms or by the reduction of nitrates and nitrites in the soil.
All biogeochemical cycles are affected by human activities. For example, as fossil fuels are burned, carbon is transferred from a very old reservoir, the lithosphere, to a new one (the atmosphere, as carbon dioxide). Human activity can accelerate the biogeochemical cycles.
In addition to carbon cycle, humans have altered the nitrogen and phosphorous cycles by adding these elements to croplands as fertilizers, which has contributed to over-fertilization of aquatic ecosystems when excess amounts are carried by runoff into local waterways.
The long-term impact of this form of human activity on the biogeochemical cycles has only begun to be understood. Some scientists assert, however, that those affects can be profound, resulting in significant climate changes far into the future.
- Biogeochemical Cycles
- Life and Biogeochemical Cycles
- Biogeochemical Cycles