Biogeochemical Cycles

The biogeochemical cycle definition is the flow of nutrients and other components through both the biotic and abiotic spectrums of the Earth. Basically, it is any of the natural pathways through which the essential elements of living matter are circulated. The term biogeochemical refers to the biological, geographic and chemical aspects of each cycle. The purpose of biogeochemical cycles or nutrient cycles is to maintain a hot equilibrium state that ensures the sustainability of life on the Earth’s surface. Depending on the state of matter, there are sedimentary and gaseous cycles. Gaseous cycles include the transfer of carbon, nitrogen, oxygen, and water. On the other hand, the phosphorus, sulphur and water cycles come under sedimentary cycles. 

Each cycle can be considered to have two parts, a reservoir pool which is a larger, slow-moving, usually abiotic portion that is full of nutrients, and a smaller but more active pool concerned with the quick exchange of an ecosystem's biotic and abiotic components. For an ecosystem to survive, all chemical aspects that comprise the living cells must be recycled continuously. 

Biogeochemical cycles can be classified as gaseous in which scenario the reservoir is in the air or in water, or sedimentary, in which case the reservoir is in the earth’s crust. Gaseous cycles can include nitrogen, carbon, and water cycles. While sedimentary cycles include iron, sulphur, phosphorus and more-soil bound elements. 

Gaseous cycles tend to move faster than sedimentary cycles and can adjust more readily to the changes in the biosphere because of the large atmospheric reservoir. For example, local accumulations of Carbon Dioxide are very easily dissipated by winds or are taken up by plants. Unnatural disturbances like global warming, do, however, affect the quality of self-adjustment. 

Sedimentary cycles can vary from element to element, but each cycle is fundamentally comprised of a solution phase and a rock phase. During the solution phase, minerals in the form of salt are released from rocks due to weathering, some of which dissolve in water and ultimately end up in the settlement in the seas, out of circulation. 

Types of Biogeochemical Cycles With Biogeochemical Cycle Diagrams

Carbon Cycle

The carbon cycle refers to the movement of carbon-based molecules among the atmosphere, lithosphere, and hydrosphere. Carbon is abundantly found in the atmosphere in the form of Carbon dioxide and Methane, which induce the greenhouse effect because of heat absorption. Carbon dioxide enters the terrestrial and marine ecosystems through photosynthesis in green plants. The carbon is then transferred through food chains. The combustion of fossil fuels, volcanic eruptions, and hotspots also release carbon dioxide back into the atmosphere. 

Oceans and large water bodies contain a high percentage of dissolved organic carbon obtained through the dissolution of carbon dioxide in the air. So formed carbonates are transferred from one organism to another through food chains. They are also stored as calcium carbonates. Thermohaline circulation refers to the exchange of carbon components between deep waters and surface waters.   

Nitrogen Cycle

The nitrogen cycle refers to the conversion of Nitrogen into different forms procured by biological and physical processes. Although Nitrogen accounts for 78% of the total atmospheric gases, biological use of nitrogen is scarce. The atmospheric nitrogen is transformed into Nitrogen compounds by Rhizobium bacteria present in the root nodules of legumes or by green-blue algae. 

Some physical factors also cause nitrogen fixation and produce ammonia like ultraviolet radiation, lightning, and the Haber-Bosch process. Assimilated Nitrates and Ammonia in legumes and algae are transferred through food chains. Decomposition of dead plants and animals further releases ammonium into the soil. This process is called ammonification.

Nitrification involves the transformation of ammonia into nitrates by denitrifying bacteria which later settles in plant tissues. The process of converting the so formed nitrates into inert nitrogen gas is known as denitrification. Pseudomonas and Clostridium also release Nitrogen gas. 

Oxygen Cycle

Oxygen accounts for 21% of the total atmospheric gases. The movement of oxygen among the Lithosphere, Hydrosphere, and atmosphere is termed as oxygen cycle. Oxygen-based minerals are found in the crust and mantle, and only 0.01% is released as free oxygen. Atmospheric carbon dioxide is consumed by green plants for photosynthesis which releases oxygen.

Another source for oxygen is photolysis wherein the ultraviolet radiation reduces the water and nitrous oxide into separate elements, thus producing oxygen molecules. Oxygen is used for respiration in living organisms. Oxygen tends to settle in the soil owing to decomposition and chemical weathering. In the marine ecosystem, calcium carbonate shells store oxygen which is converted into limestone after decomposition. The formed limestone is transformed into oxygen by plant and animal activities.

Phosphorus Cycle

The Phosphorus cycle is the movement of different forms of phosphorus through nature. Phosphorus is mostly available in solid matter.  It is essential for energy transfer. Weathering and mining release the phosphorus in rocks into the terrestrial and marine ecosystems. Some of it is hardened into layers and stored in the deep waters. Fish harvests and Guano collection usually transfers the phosphorus-based compounds from water bodies to land. 

As phosphorus is highly reactive, it combines with other elements. Microorganisms play a crucial role in converting insoluble phosphorus compounds into soluble phosphates that are incorporated into plants and algae. Food chains continuously transfer phosphate compounds from one organism to another, which finally settles in the soil after decay. Because of low levels of phosphorus in the soil, phosphorus-based fertilizers are added to the soil to enhance productivity.

Sulphur Cycle

The sulfur cycle usually refers to the flow of sulphur-based components among the lithosphere, hydrosphere, and atmosphere. Amino acids present in living organisms contain sulfur. Inorganic sulphur is abundant on the Earth's surface. This sulphur is reduced to sulfates through weathering and released into the atmosphere. The Sulphates are later transformed into organic compounds by microorganisms and plants. When animals consume plants, they incorporate organic sulphur compounds into their bodies. The decomposition of organisms releases sulphur back into the soil.

When Hydrogen Sulphide reacts with water, Sulphuric acid is formed, causing acid rains. 

Water Cycle

The hydrological cycle refers to the circulation of water in nature. Sunlight increases the temperature of water bodies and converts them into water vapour. Humidity and the flow of wind affect this process of evaporation. Sublimation also adds water vapour to the atmosphere from ice. Leaves tend to release water vapour through stomata, and this process is known as evapotranspiration. 

The water vapour in the atmosphere condenses and results in precipitation. The water then enters the terrestrial and marine ecosystems. Some amount of water penetrates the soil and is stored as groundwater. The rest either evaporates back into the atmosphere or adds up in the ocean as runoff. In the cooler regions, water is converted into glaciers which melt with rising temperatures, and the cycle continues.