Biogeochemical cycle refers to the flow of an elements in nature between the organisms and the environment. This flow is cyclic and consistent.
Elements within these cycles flow in various forms from the nonliving (abiotic) components of the biosphere to the living (biotic) components and back.
Biogeochemical cycle is also known as inorganic-organic cycle. The most important cycles are for the following elements: Carbon (C), Hydrogen (H), Oxygen (O), Phosphorus (P), Nitrogen (N), Sulfur (S), Potassium (K), Calcium (Ca), Iron (Fe), Magnesium (Mg), Barium (B), Zinc (Zn), Chlorine (CI), Molybdenum (Mo), Cobalt (Co), Iodine (I) and Fluorine (F).
The first six of these elements are used in a variety of ways by living organisms.
Importance of Biogeochemical Cycles
Biogeochemical cycles are helpful in explaining how planet earth conserves matter and uses energy. The cycles move elements through ecosystems, storing and recycling them.
A strong connection between living and non-living parts of the biosphere can be understood by biogeochemical cycle. Human activity is disturbing some of these cycles and hence affecting the ecosystems.
Hydrogen is the most abundant element. It occurs mainly in combination with oxygen forming water, and organic matter, such as plants and coal.
Carbon is the second most abundant element on earth. It is found in all organic molecules, whereas nitrogen is an important component of nucleic acids and proteins.
Phosphorus is used to make nucleic acids and phospholipids present in biological membranes.
Lastly, sulfur is essential for the formation of plant proteins, amino acids, vitamins, and enzymes.
The flow of these elements is interconnected. For example, the movement of water is very important for the leaching of nitrogen and phosphate into rivers, lakes, and oceans.
Thus, mineral nutrients are cycled, through the entire biosphere, from one living organism to another, and between the biotic and abiotic components of the ecosystem.
Hydrologic or Water Cycle
Hydrologic cycle (Figure 1) was defined by the National Research Council (NRC, 1982) as “the pathway of water as it moves in its various phases to the atmosphere, to the earth, over and through the land, to the ocean and back to the atmosphere”
Interchange of water between atmosphere, land and sea and between living organisms and their environment is accomplished through the water cycle. It involves the following processes:
Water reaches to the atmosphere from the earth (rivers, lakes, oceans, underground sources, plants and animals). This moisture gets condensed at higher levels and falls to the earth as precipitation in the form of rain, snow, hail, dew, sleet, frost etc.
After reaching the land surface, water infiltrates into the soil and the remaining water runs off the land surface to join streams. These streams finally discharge into the ocean.
Some of the infiltrated water percolates deep to join groundwater and some comes back to the streams or appears on the surface as springs.
The amount of water available for evaporation is determined by the amount supplied by precipitation and condensation. Since the total amount of moisture in this entire system remains constant, a balance is required. This balance is maintained by the water cycle.
Carbon is the basic constituent of all organic compounds. Since energy transfer occurs in the consumption and storage of carbohydrates and fats, carbon moves to the ecosystem with flow of energy.
The source of nearly all carbon found in the living organisms is CO2 which is found in free state in atmosphere and in dissolved state in the water on the earth (Figure 2).
Plants (producers) use CO2 through photosynthesis in the presence of sunlight and carbohydrate is formed. This process is called photosynthesis. Later on, complex fats and polysaccharides are formed in plants which are utilized by animals.
As carnivores feed on herbivores, carbon compounds are digested and converted into other forms. Carbon is released to the atmosphere directly as CO2 in respiration of both plants and animals.
Death of organisms, volcano eruption, fire, burning of fossil fuels are indirect methods of carbon release to the atmosphere.
Bacteria and fungi attack the dead remains of plants and animals. They degrade the complex organic compounds into simpler substances, which are then available for other cycles.
Part of the organic carbon is incorporated into the earth’s crust as coal, gas, petroleum, limestone and coral reef. Carbon from such deposits may be liberated after a long period of time (Figure 3).
Carbon is released back into the atmosphere when organisms die, volcanoes erupt, fires blaze, fossil fuels are burned, and through a variety of other mechanisms.
Carbon is stored for long periods in what are known as carbon reservoirs, which include the atmosphere, bodies of liquid water (mostly oceans), ocean sediment, soil, rocks (including fossil fuels), and Earth’s interior.
Nitrogen is a key element in the nucleic acids, DNA and RNA, which are the most important biomolecules for life. It is the most important element absorbed for plant growth. Nitrogen is also required for the synthesis of amino acid, proteins, enzymes, chlorophyll, nucleic acids, etc.
Green plants obtain nitrogen from the soil solution in the form of ammonium, nitrate and nitrite ions. The main source of all these nitrogen compounds is the atmospheric nitrogen.
The atmospheric nitrogen is not directly available to the organisms, with the exception of some prokaryotes such as blue green algae and nitrogen fixing bacteria.
Nitrogen cycle consists of the following steps (Figure 4):
In this stage, nitrogen moves from the atmosphere into the soil. Earth’s atmosphere contains a huge amount of nitrogen gas (N2).
To be used by plants, the N2 undergoes a process called nitrogen fixation. Fixation converts nitrogen in the atmosphere into forms that plants can absorb through their root systems.
Inorganic nitrogen in the form of nitrates, nitrites and ammonia is absorbed by the green plants and converted into nitrogenous compounds.
Nitrates are first converted into ammonia, which combines with organic acids to form amino acids. Animals derive their nitrogen requirement from the plant proteins.
Dead organic remains of plants and animals and excreta of animals are utilized by microorganisms. Organic compounds are utilized in their metabolism and ammonia is released.
Certain bacteria, such as Nitrosomonas, Nitrococcus, Nitrosogloea and Nitrospira convert ammonia present in the soils, nitrites, NO2−, and nitrates, NO3−.
Nitrobacter turns nitrites into nitrates, whereas nitrosomonas transform ammonia to nitrites. Nitrates can then be used by plants and animals that eat plants.
Nitrogen returns to the air as nitrates are converted to atmospheric nitrogen (N2) by bacteria. This process is called denitrification.
Pseudomonas aeruginosa are the most common denitrifying bacteria.
Nitrates of the soil are washed down to the sea or leached into the earth along with water.
Nitrates that are lost from the soil surface are locked up in the rocks. They are released later by weathering. This process is sedimentation of nitrogen.
Sulfur is a component of some vitamins, amino acids, and essential metabolites. Sulfur occurs in the soil and rocks as sulfides and crystalline sulfates.
In the atmosphere, sulfur occurs in the form of sulfur dioxide (SO2) and hydrogen sulfide (H2S). SO2 gas is formed during combustion of fossil fuels or by decomposition.
A small amount of sulfur occurs in dissolved state in rain water and through rains it reaches earth surface.
H2S or hydrogen sulfide gas is released to the atmosphere from soils, lakes and springs etc (Figure 5).
After reaching the earth, two unrelated groups of procaryotes oxidize H2S to S and S to SO4. It is carried our by two process:
- The first is the anoxygenic photosynthetic purple and green sulfur bacteria that oxidize H2S. H2S acts as a source of electrons for cyclic photophosphorylation.
- The second is the “colorless sulfur bacteria”, which oxidize H2S and S. In both the cases, the organisms can usually mediate the complete oxidation of H2S to SO4.
In the ecosystems, sulfur is transferred from plants to animals, then to decomposers and finally it returns to environment through the decay of dead organic matter.