In this article, we will cover three major gaseous cycles:
- Carbon cycle
- Nitrogen cycle
- Hydrological cycle (Water cycle)
A typical Carbon Cycle
The major source of carbon are:
- Bio-molecules in organic form
- Carbon oxides in the atmosphere
- The organic form of carbon in soil.
- Fossil fuels and sedimentary rocks
- Dissolved compounds in oceans
Atmospheric carbon in the form of carbon dioxide is the critical source of carbon in an ecosystem. The Ecosystems gain most of the carbon from the atmosphere by the process of photosynthesis. In the process of photosynthesis carbon dioxide from the atmosphere, together with solar energy and water is converted into food and carbon gets trapped in the biomolecules.
The carbon trapped in the food is used by the living organisms to fulfill their energy and nutrient requirements. After fulfilling their requirement the excess carbon is released by organisms either in gaseous form as a byproduct of the process of respiration or is organic form through excretion.
The gaseous form directly combines to the nutrient pool, i.e. the atmosphere but the organic form undergoes further decomposition to convert the organic form of carbon into either gaseous carbon oxides or in inorganic form to become the part of the carbon cycle.
Atmospheric carbon can directly dissolve in the ocean water through the process of diffusion and can remain dissolved in oceans. The Carbon dioxide dissolved in oceans may convert in inorganic compounds such as carbonate salts or may be used by living organisms in their biochemical cycle. When such organisms die, their shells and body parts sink to the ocean floor where they accumulate as carbonate-rich deposits. It is to be noted that ocean deposits are by far the biggest sink of carbon on the planet.
The lithosphere is again a major sink of carbon. The carbon in organic and inorganic forms gets deposited in the lithosphere and remains inside the earth crust trapped for millions of years. These carbon are trapped in the form of fossil fuels and also sedimentary rocks such as limestone, dolomite etc. The trapped becomes the part of the carbon cycle when the fossil fuel is used by humans to fulfil their energy needs or when the rocks undergo melting due to tecto-volcanic activities.
A typical Nitrogen Cycle
Nitrogen is an essential constituent of protein and is a basic block of all living tissue. It constitutes nearly 16% by weight of all proteins. Almost 2/3 rd of the atmosphere is composed of Nitrogen so the availability of Nitrogen is inexhaustible but the elemental Nitrogen is useless for living organism. The elemental nitrogen is needed to be converted into organic compounds which can be absorbed by the living organisms.
Nitrogen fixation on earth is accomplished in three different ways:
- By microorganisms- bacteria and blue-green algae.
- By man using industrial processes- e.g., fertilizer factories.
- To a limited extent by atmospheric phenomena such as thunder and lighting.
Certain microorganisms are capable of fixing atmospheric nitrogen into ammonium ions. These include free-living nitrifying bacteria, specialized- Azotobacter and anaerobic Clostridium and symbiotic nitrifying bacteria living in association with leguminous plants and symbiotic bacteria living in non-leguminous root nodule plants, eg-Rhizobium as well as blue-green algae, e.g., Anabaena, Spirulina, etc.
Ammonium ions can directly be taken up as a source of nitrogen by some plants, or are oxidized to nitrates or nitrites by two groups of specialised bacteria: Nitrosomonas bacteria promote the transformation of ammonia into nitrite. Nitrite is then further transformed into nitrate by the bacteria Nitrobacter.
The nitrates converted by bacteria is assimilated by plants to form amino acids. The amino acids are transferred into the food chain and reach to the highest trophic level of an ecosystem. This amino acid is the source of nitrogen for other living organisms. The nitrogen trapped in living organisms is released when they excrete or when detritus bacteria decompose the living organisms, thus completing the cycle.
The certain quantity of soil nitrates, being highly soluble in water, is lost to the system by being transported away by surface runoff or groundwater. In the soil as well as oceans there are special denitrifying bacteria, eg-Pseudomonas, which convert the nitrates/nitrites to elemental nitrogen. This nitrogen escapes into the atmosphere, thus completing the cycle.
The periodic thunderstorms convert the gaseous nitrogen in the atmosphere to ammonia and nitrates which eventually reach the earth's surface through precipitation and then into the soil to be utilised by plants.
Hydrological Cycle(Water Cycle)
A typical Water Cycle
The hydrological cycle is a theoretical model that describes the exchange of water among the atmosphere, lithosphere, biosphere, and hydrosphere. All these also act as a reservoir of water. The various process such as evaporation, condensation, diffusion etc. help in the exchange of water among the atmosphere, lithosphere, biosphere, and hydrosphere.
The driving forces for water cycle are- solar radiation, gravity. Evaporation and precipitation are two main processes involved in the water cycle. These two processes alternate with each other. Water from oceans, lakes, ponds, rivers and streams evaporates by sun's heat energy. Plants also transpire huge amounts of water. Water remains in the vapour state in air and form clouds which drift with the wind. Clouds meet with cold air in the mountainous regions above the forests and condense to form rain precipitate which comes down to gravity.
The ocean supply most of the evaporated water found in the atmosphere. On an average 84% of the water is lost from oceans by evaporation. While 77% is gained by it from precipitation. Water runoff from lands through rivers to oceans makes up 7% which balances the evaporation deficit of the ocean. On land, evaporation is 16% and precipitation is 23%.