Biogeochemical Cycles / Nutrient Cycles

Biogeochemical cycles / Nutrient cycles

The cycling of chemicals between the biological and the geological world is called Biogeochemical cycle.

The Biotic and Abiotic components of the biosphere constantly interact through biogeochemical cycles. During these interactions, there is a transfer of nutrients between living organisms and the non-living environment.

The  important biogeochemical cycles are water cycle, Nitrogen cycle, Carbon cycle and Oxygen cycle, Phosphorus cycle and Sulphur cycle.

Biogeochemical Cycles are classified into :

1. Atmospheric cycles - Ex: Carbon, Oxygen and Nitrogen cycles
2. Hydrological cycle  - Ex: Water cycle
3. Sedimentary cycle   - Ex: Phosphorus and Sulphur cycles

Water cycle

The water cycle involves various steps like evaporation, transpiration, condensation and precipitation.

    •  When the water bodies are heated during the day, water enters the atmosphere as water vapour by the process of evaporation.

    •  There is another way in which water evaporates into the atmosphere. This happens through transpiration.

    •  The water vapours in the atmosphere changes to water droplets and collects to form clouds. This process is called condensation.

    •  Air currents move the clouds formed by condensation and carry them over the land, where they break into rain, snow or fog. This is called precipitation.

Nitrogen cycle

The sequence in which nitrogen passes from the atmosphere to the soil and organisms, and then is eventually released back into the atmosphere, is called the Nitrogen cycle.

 • Nitrogen makes up 78 percent of the earth’s atmosphere. The percentage of nitrogen in the atmosphere is maintained by nitrogen cycle.

  • Nitrogen is an essential constituent of proteins, nucleic acids like DNA and RNA, vitamins, and chlorophyll.

 • Plants and animals cannot utilise atmospheric nitrogen readily. It has to be fixed by some organisms called as nitrogen fixers.

 • Nitrogen-fixing bacteria live in the root nodules of certain leguminous plants.. These bacteria convert atmospheric nitrogen into ammonia, which is utilised readily by plant called Nitrogen fixation.

  •  Nitrogen-fixing bacteria along with free living bacteria in the soil achieve 90 percent of nitrogen fixation.

 • Lightning plays an important role in nitrogen fixation. When lightning occurs, the high temperature and pressure convert nitrogen and water into nitrates and nitrites.Nitrates and nitrites dissolve in water and are readily used by aquatic plants and animals.

   • Ammonification is the process by which soil bacteria decompose dead organic matter and release ammonia into the soil.

   •  Nitrification is the process by which ammonia is converted into nitrites and nitrates.

   •  Denitrification is the process by which nitrates are converted into atmospheric nitrogen.

              

Carbon cycle

Carbon is cycled repeatedly through different forms by the various physical and biological activities constituting the carbon cycle.

Carbon cycle maintains the balance of the element carbon in the atmosphere. Carbon is found in various forms on the Earth. 

*Diamond and graphite found in the soil are made up of an element called carbon. 

*Carbon is present in the atmosphere as carbon dioxide. 

*Carbon can also occur as carbonates and bicarbonate salts in minerals. The endoskeletons and exoskeletons of various aquatic animals are also formed from carbonate salts.

*Carbon is an essential part of nutrients like carbohydrates, fats, proteins, nucleic acids and vitamins.

Carbon cycle maintains the amount of carbon in the atmosphere. The carbon cycle starts in plants. 

Step - 1 : Plants, use carbon dioxide in the atmosphere, convert it into glucose in the presence of sunlight by the process of photosynthesis. Plants and animals break these carbohydrates for energy and release carbon dioxide through respiration.
Step - 2 : When the plants and animals die, fungi and bacteria decompose the dead remains.  This releases the carbon in the remains as carbon dioxide. 
Step - 3 : Some plants and animals which get burried in the soil under certain temperature and pressure over millions of years get transformed into fossil fuels. Coal and petroleum are some of the fossil fuels. On burning these fuels, carbon dioxide is released into the atmosphere. 

                    

Oxygen cycle

The sequence in which oxygen from the atmosphere is used by organisms and eventually released back into the atmosphere through photosynthesis is called as oxygen cycle.

    •  Oxygen makes up 21 percent of the air. It is an essential constituent of carbohydrates, proteins, fats and nucleic acids. 

    •  Oxygen is found in air, in combined form as carbon dioxide, and in the earth’s crust as carbonates, sulphates and nitrates. 

    •  Plants and animals use atmospheric oxygen during respiration and release the same during photosynthesis. 

    •  Fossil fuels require oxygen for combustion. 

    •  The ozone layer is present in stratosphere, one of the layers of the atmosphere. Each molecule of ozone is made up of three oxygen atoms. The ozone layer prevents harmful radiations from reaching the earth’s surface, where they might damage life forms.

                                  
Phosphorus cycle

 Phosphorous is an essential nutrient found in the macromolecules of humans and other organisms, including DNA

·    The phosphorous cycle is slow. Most phosphorous in nature exists in the form of phosphate ion.  Phosphorus is often the limiting nutrient, or nutrient that is most scarce and thus limits growth, in aquatic ecosystems.

·     When nitrogen and phosphorous from fertilizer are carried in runoff to lakes and oceans, they can cause eutrophication, the overgrowth of algae. The algae may deplete oxygen from the water and create a dead zone.

In nature, phosphorous is found mostly in the form of phosphate ions. Phosphate compounds are found in sedimentary rocks, and as the rocks weather wear down over long time periods the phosphorous they contain slowly leaches into surface water and soils. Volcanic ash, aerosols, and mineral dust can also be significant phosphate sources.

Phosphate compounds in the soil can be taken up by plants and, from there, transferred to animals that eat the plants. When plants and animals excrete wastes or die, phosphates are returned to the soil. Phosphorous-containing compounds may also be carried in surface runoff to rivers, lakes, and oceans, where they are taken up by aquatic organisms.

When phosphorous-containing compounds from the bodies or wastes of marine organisms sink to the floor of the ocean, they form new sedimentary layers. Over long periods of time, phosphorous-containing sedimentary rock may be moved from the ocean to the land by a geological process called uplift. However, this process is very slow, and the average phosphate ion has an oceanic residence time in the ocean of 20,000 to 100,000 years.

Sulphur Cycle:

Sulphur is one of the components that make up proteins and vitamins. Proteins consist of amino acids that contain sulphur atoms. Sulphur is important for the functioning of proteins and enzymes in plants, and in animals that depend upon plants for sulphur.

It enters the atmosphere through both natural and human sources. Natural recourses can be for instance volcanic eruptions, bacterial processes, evaporation from water, or decaying organisms. When sulphur enters the atmosphere through human activity, this is mainly a consequence of industrial processes where sulphur dioxide (SO₂) and hydrogen sulphide (H₂S) gases are emitted on a wide scale.

When sulphur dioxide enters the atmosphere it will react with oxygen to produce sulphur trioxide gas (SO₃), or with other chemicals in the atmosphere, to produce sulphur salts. Sulphur dioxide may also react with water to produce sulphuric acid (H₂SO₄). Sulphuric acid may also be produced from demethyl-sulphide, which is emitted to the atmosphere by plankton species.

All these particles will settle back onto earth, or react with rain and fall back onto earth as acid deposition. The particles will then be absorbed by plants again and are released back into the atmosphere, so that the sulphur cycle will start over again.