Biogeochemical cycles
Biogeochemical cycles


In this article, we will discuss about the biogeochemical cycles. Biogeochemical cycles are the pathways through which nutrient and chemicals transported throughout the ecosystem. There are different types of nutrient and chemical cycles, such as carbon cycle, oxygen cycle, etc. Majority of the cycles have two phases: atmospheric phase and aqueous phase. But some cycles have only one phase. Nutrients are continuously exchanged between organism and their physical environment and it is unlike to the energy flow that flows in one direction. The nutrient cycles involve storage and transfer of nutrients through the various components of the ecosystem so that they are repeatedly used. We will also provide relevant references to understand the concept deeply.


Any of the natural pathways by which essential elements of living matter circulated. A biochemical cycle is the transport and transformation of chemicals in ecosystems. Biogeochemical cycles mainly refer to the movement of nutrients and other elements between biotic and abiotic factors.

Energy flows directionally through ecosystems, entering as sunlight (or inorganic molecules for
chemoautotrophs) and leaving as heat during the transfers between trophic levels. Rather than
flowing through an ecosystem, the matter that makes up living organisms conserved and
recycled. The six most common elements associated with organic molecules—carbon, nitrogen,
hydrogen, oxygen, phosphorus, and sulfur—take a variety of chemical forms and may exist for
long periods in the atmosphere, on land, in water, or beneath Earth’s surface. Geologic processes,
such as weathering, erosion, water drainage, and the subduction of the continental plates, all play
a role in the cycling of elements on Earth. Because geology and chemistry have major roles in the
study of this process, the recycling of inorganic matter between living organisms and their
nonliving environment called a biogeochemical cycle.



By far the largest reservoir of Earth’s oxygen is in the minerals of the crust and mantle (99.5%).
Oxygen is also cycled between the biosphere and lithosphere. Marine organisms in the biosphere
create calcium carbonate shell material (CaCO3) that is rich in oxygen. When the organism dies
its shell is deposited on the shallow sea floor and buried over time to create the limestone rock of
the lithosphere. Weathering processes initiated by organisms can also free oxygen from the
lithosphere. Plants and animals extract nutrient minerals from rocks and release oxygen in the

Stage-1: All green plants during the process of photosynthesis, release oxygen back into
the atmosphere as a by-product.
Stage-2: All aerobic organisms use free oxygen for respiration.
Stage-3: Animals exhale Carbon dioxide back into the atmosphere which is again used
by the plants during photosynthesis. Now oxygen is balanced within the atmosphere.

Image of oxygen cycle
Image of oxygen cycle


Oxygen is an important element required for life. However, it can be toxic to some anaerobic bacteria (especially obligate anaerobes). The oxygen cycle mainly involved in maintaining the level of oxygen in the atmosphere. The entire cycle summarized as, the oxygen cycle begins with the process of photosynthesis in the presence of sunlight, releases oxygen back into the atmosphere, which humans and animals breathe in oxygen and breathe out carbon dioxide, and again linking back to the plants. This also proves that both the oxygen and carbon cycle occur independently and interconnected to each other.


Carbon is the fundamental building block virtually in all the molecules that make up living creatures in the biosphere; existing in rocks such as coal or limestone or in the atmosphere as a part of carbon dioxide gas. It constantly cycles from one reservoir to another. To manufacture organic materials carbon must fixed. It is the fourth most abundant element in living organisms. Carbon is present in all organic molecules, and its role in the structure of macromolecules is of primary importance to living organisms. Carbon compounds contain energy, and many of these compounds from plants and algae have remained stored as fossilized carbon, which humans use as fuel. Since the 1800s, the use of fossil fuels has accelerated.

Carbon present in the atmosphere absorbed by plants for photosynthesis. These plants then consumed by animals and carbon gets accumulated into their bodies. These animals and plants eventually die, and upon decomposing, carbon released back into the atmosphere. Some of the carbon that not released back into the atmosphere eventually become fossil fuels. These fossil fuels then used for synthetic activities, which pump more carbon back into the atmosphere.

Image of carbon cycle
Image of carbon cycle


Even though carbon dioxide found in small traces in the atmosphere, it plays a vital role in balancing the energy and traps the long-wave radiations from the sun. Therefore, it acts like a blanket over the planet. If the carbon cycle disturbed it will result in serious consequences such as climatic changes and global warming. Carbon is an integral component of every life form on earth. From proteins and lipids to even our DNA. Furthermore, all known life on earth is based on carbon. Hence, the carbon cycle, along with the nitrogen cycle and oxygen cycle, plays a vital role in the existence of life on earth.


Nitrogen Cycle is a biogeochemical process which transforms the inert nitrogen present in the atmosphere to a more usable form for living organisms. Getting nitrogen into the living world is difficult. Plants and phytoplankton are not equipped to incorporate nitrogen from the atmosphere (which exists as tightly bonded, triple covalent N2) even though this molecule comprises approximately 78 percent of the atmosphere. Nitrogen enters the living world via free-living and symbiotic bacteria, which incorporate nitrogen into their macromolecules through nitrogen fixation (conversion of N2). Cyanobacteria live in most aquatic ecosystems where sunlight is present; they play a key role in nitrogen fixation. Cyanobacteria are able to use inorganic sources of nitrogen to “fix” nitrogen. Rhizobium bacteria live symbiotically in the root nodules of legumes (such as peas, beans, and peanuts) and provide them with the organic nitrogen they need. Free-living bacteria, such as Azotobacter, are also important nitrogen fixers.


Process of the Nitrogen Cycle consists of the following steps – Nitrogen fixation, Nitrification, Assimilation, Ammonification and De-nitrification. Atmospheric nitrogen (N2) which is primarily available in an inert form, converted into the usable form -ammonia (NH3). During the process of Nitrogen fixation, the inert form of nitrogen gas deposited into soils from the atmosphere and surface waters, mainly through precipitation. The entire process of Nitrogen fixation completed by symbiotic bacteria, known as Diazotrophs. Azotobacter and Rhizobium also have a major role in this process. The ammonia converted into nitrate by the presence of bacteria in the soil. Nitrites formed by the oxidation of ammonia with the help of Nitrosomonas bacteria species. Later, the produced nitrites converted into nitrates by Nitrobacter. This conversion is very important as ammonia gas is toxic for plants.

Primary producers – plants take in the nitrogen compounds from the soil with the help of their roots, which are available in the form of ammonia, nitrite ions, nitrate ions or ammonium ions and used in the formation of the plant and animal proteins. When plants or animals die, the nitrogen present in the organic matter released back into the soil. The decomposers, namely bacteria or fungi present in the soil, convert the organic matter back into ammonium. De-nitrification is the process in which the nitrogen compounds make their way back into the atmosphere by converting nitrate (NO3-) into gaseous nitrogen (N). This process of the nitrogen cycle is the final stage and occurs in the absence of oxygen. De-nitrification carried out by the denitrifying bacterial species- Clostridium and Pseudomonas, which will process nitrate to gain oxygen and gives out free nitrogen gas as a byproduct.

Image of nitrogen cycle
Image of nitrogen cycle


Helps plants to synthesize chlorophyll from the nitrogen compounds. Helps in converting inert nitrogen gas into a usable form for the plants through the biochemical process. In the process of ammonification, the bacteria help in decomposing the animal and plant matter, which indirectly helps to clean up the environment. Nitrates and nitrites are released into the soil, which helps in enriching the soil with the necessary nutrients required for cultivation. Nitrogen is an integral component of the cell and it forms many crucial compounds and important biomolecules.


Sulfur is one of the most abundant elements on the earth. It is a yellow, brittle, tasteless, odorless non-metal. Sulfur is present in all kinds of proteins. Plants directly absorb sulfur-containing amino acids such as methionine, cystine, and cysteine. Sulfur released into the atmosphere by the burning of fossil fuels, volcanic activities, and decomposition of organic molecules. On land, sulfur stored in underground rocks and minerals. It released by precipitation, weathering of rocks and geothermal vents.

The sulfur released by the weathering of rocks. Sulfur comes in contact with air and converted into sulfates. Sulfates taken up by plants and microbes and converted into organic
forms. The organic form of sulfur then consumed by the animals through their food
and thus, sulfur moves in the food chain. When the animals die, some of the sulfur released by decomposition while some enter the tissues of microbes. There are several natural sources such as volcanic eruptions, evaporation of water, and breakdown of organic matter in swamps, that release sulfur directly into the atmosphere. This sulfur falls on earth with rainfall.

Image of sulfur cycle


The sulfur cycle is of paramount importance as it provides an insight into the understanding of how the different biomolecules functions. The sulfur cycle provides a balance in the sulfur concentration among the various reservoirs as which is necessary for the ecological balance on earth and its continuity as a hospitable place. The availability of several other elements also affected by the sulfur cycle as sulfur often found in a combined state with other elements such as iron, phosphorus, nitrogen, etc., in nature. The terrestrial part of the sulfur cycle comprises a series of biological processes that play a significant role in increasing the sulfur available for microbial life and plants. The chemoautotrophic sulfur bacteria in the food chain convert the chemical energy into several other forms which lead to a biomass increase on the planet.


The nutrient cycles are of two types: the gaseous and the sedimentary. The reservoirs of gaseous type of nutrient cycle are generally located in the atmosphere or the hydrosphere. For example: the nitrogen reservoir in the atmosphere. Whereas the sedimentary type, the reservoir exists in the earth crust. For example: phosphorus in the lithosphere. The nutrient cycles involve the input of nutrients, output of nutrients and the internal nutrient cycling. Nutrients enter in the ecosystem from the external sources as wet deposition or dry deposition. Nutrients gained from the rainfall in dissolved state called wet deposition, and nutrients gained from dust known as dry deposition.


Adair, E. C., Parton, W. J., Del Grosso, S. J., Silver, W. L., Harmon, M. E., Hall, S. A., . . . Hart, S. C. (2008).
Simple three-pool model accurately describes patterns of long-term litter decomposition in
diverse climates. Global Change Biology, 14(11), 2636-2660. doi:

Alin, S. R., & Johnson, T. C. (2007). Carbon cycling in large lakes of the world: A synthesis of production,
burial, and lake-atmosphere exchange estimates. Global Biogeochemical Cycles, 21(3).

Agren, A., Jansson, M., Ivarsson, H., Bishop, K., & Seibert, J. (2008). Seasonal and runoff-related changes in total organic carbon concentrations in the River Ore, Northern Sweden. Aquatic Sciences, 70(1),

Brimblecombe P (2005) The global sulfur cycle. In: Holland H, Turekian KK (eds) Treatise of geochemistry, vol 8. Elsevier, Amsterdam

Sundquist ET, Visser K (2003) The geologic history of the carbon cycle. In: Holland H, Turekian KK (eds) Treatise of geochemistry, vol 8. Elsevier, Amsterdam…8..425S/abstract

Leave a Reply