Biogeochemical Cycles

Biogeochemical Cycles

Intended Learning Outcomes

At the end of this Lecture, students will be able to

• Discuss different Biogeochemical cycles


Bio-Geo- Chemical Cycles

• Hydrological Cycle

• Carbon cycle

• Oxygen Cycle

• Nitrogen Cycle

• Sulfur Cycle

Bio-Geo- Chemical Cycles

Nutrients, unlike energy are recycled in the ecosystem. The Producers of an ecosystem take up several basic nutrients from their non-living ecosystem. Materials are in limited quantity in the earth’s system and to keep the system going continuously, these materials get transformed into biomass of the producers.

Thus they are utilized by the consumer population and are ultimately returned to the environment with the help of reducers or decomposers.

The unique method evolved in nature is recycling materials continuously is by linking them in cycle changes.

This cyclic exchange of nutrient material between the living organisms & their non-living environment is called Bio- Geochemical cycle.

As indicated by the name the nutrients are circulated through life (bio) and through earth (geo) repeatedly (cycle).

Hydrological Cycle

• The natural flow of water through various components of environment resulting in the global circulation is called water cycle.

Steps in Hydrological Cycle:

• Evaporation: Surface water is heated by sun and evaporates to become water vapour, water vapour floats in the air.

• Condensation: As water vapour rises into the air it gradually cools and condenses and become minute droplet of water.

• Clouds: Tiny droplets of water together forms clouds.

• Precipitation: The fall of water on earth surface in any form of water it may be in the form of dew, drizzle, rain is known as precipitation.

• Runoff: Precipitated rain water accumulates and flows on the surface and sub- surface towards rivers, streams, and underground stores and ultimately reaches to sea.

• Percolation & infiltration: The process of stored water flowing under earth, merge to the ground water source is called percolation and infiltration

• Transpiration: The water which directly evaporates from leaves of plants is called Transpiration.

• Completion of Cycle: All the water bodies continues its journey towards the natural slope and meet the sea where the cycle starts again.

Carbon Cycle

• Carbon dioxide gas is emitted from various natural and anthropogenic sources such as, volcanic eruptions, burning forests, decomposition of carbonates, factories & Automobile exhausts, during respiration by humans and plants.  The plants consume carbon dioxide during the process of photosynthesis.

• The major reservoir for carbon dioxide are in oceans, carbon dioxide dissolves readily in water. On land and in water, plants take up carbon dioxide and convert it into carbohydrates during the process of photosynthesis.

6CO2 + 6H20 ---------à C6 H12 O6 + 6 CO2

The plants are eaten by animals and when plants & animals die the carbon content in the organic matter again return to the soil with the help of decomposers

Phosphorus Cycle

The Phosphorus cycle is the simplest of all cycles. Phosphorus is generally available in the soluble form i.e. phosphate (PO4). Which is a phosphorous atom with 4 oxygen atoms. This heavy molecules never makes into the atmosphere, they are always present in dissolved form in water or in the rock. When phosphorous rock dissolves in water the phosphate goes into the solution. Producer take this phosphorous up and use it in various metabolic activities. Phosphate is considered to be a important constituent of cell membrane, DNA, ATP.

• Consumer obtain their phosphorus from plants they eat.

• Animals also use phosphorous as a component of bones & teeth, shells. When animal or plant die, phosphate returns to the soil or water by the decomposers.

• Nowadays the synthetic detergents & fertilizers have also become a source of Phosphates in a water course. Thus excess of phosphates due to these sources have resulted in a problem of Eutrophication. (I.e. excess of plant growth).

Nitrogen Cycle

Nitrogen is present in atmosphere as N2  in large amount (78 %) and it is fixed either by physical process of lightening or biologically by some bacteria like rhizobium, aezotobacter and cyanobacteria these microorganisms converts elemental nitrogen into nitrates. Nitrogen in the for of Nitrates is taken up by plants and used in metabolism for biosynthesis of amino acids, proteins, vitamins etc. and passes through the food chain.

After death of the plant and animals, the organic nitrogen in dead tissues is decomposed by several groups of nitrifying bacteria which convert them in to ammonia, nitrite & nitrates, which are again used by plants. Some bacteria (denitrifying bacteria) converts nitrates in to molecular Nitrogen or N2 under anaerobic condition, which is release back into atmosphere and the cycle goes on.

Sulphur Cycle

Sulphur occurs both in Free State as well as sulphides and sulphates as PBs, ZnS, BaSO4, etc. Sulphur is very essential element for the living organisms for synthesizing amino acids, enzymes, co-factors and certain vitamins. The burning of fossil fuels and volcanoes releases H2S and SO2 gas in atmosphere, which ultimately returns to the soil as sulphuric acid along with rain, forming sulphate compounds. Sulphur in the form of SO -2 is absorbed by plants. The  elemental  Sulphur  converted  to sulphates SO4-2 is  also  released  into  the  soil  through  weathering  of  some  sulphates containing rocks.

The sulphates SO4-2 are utilized by plants and passes onto heterotrophic organisms of food chain. After death the bodies of plants and animals as well as their waste are decomposed by microorganism releasing sulphur as organic sulphur bacteria. The sulphate SO4-2 is again utilized by plants and cycle goes on.

Oxygen Cycle

• Atmosphere contains about 21 % of oxygen. Almost all living organisms need oxygen. They use oxygen during the process of creating energy or process of photosynthesis.

• Just as water moves from sky to earth and back into hydrological cycle, oxygen is also cycled through the environment. Plants mark the beginning of oxygen cycle. Plant produce oxygen during the process of photosynthesis. Animal form the other half of the oxygen cycle, they breathe oxygen. So the oxygen created in plants are used up by animals.

• There is continuous exchange of O2 between the atmosphere and all water surface on the earth. The total amount of O2 in the biosphere is relatively constant, so that the oxygen cycle may get stable.


• Nutrients are circulated through life (bio) and through earth (geo) repeatedly (cycle).

• The different Bio-Geo- Chemical Cycles are

•Hydrological Cycle

•Carbon cycle

•Oxygen Cycle

•Nitrogen Cycle

•Sulfur Cycle


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