Biogeochemical Cycles

Biogeochemical Cycles

Biogeochemical cycles in the environment are the cyclic pathways in which regular and continuous transition of elements occur from the environment into the organism and from the organism into the environment.

Biosphere is that part of earth which has living organisms. It can be divided into hydrosphere, lithosphere and atmosphere. Hydrosphere includes water bodies, lithosphere is the solid part of the earth i.e., rocks, plains, mountains etc. Atmosphere includes the gases envelop around the hydrosphere and lithosphere.

There is a profound effect of biosphere on living organisms. Lithosphere supplies the minerals and its substratum for its support. Hydrosphere supplies water and atmosphere the gases. As a whole biosphere supplies means of living for living organism. The organisms grow and finally return the mineral or material to the biosphere through respiration, excretion and death. So, there is a cycling of material in the atmosphere.

The chemical elements including all the essential elements of protoplasm i.e., C, H, N, P, S, etc., however tend to circulate in the biosphere in characteristics path from the environment. These paths are generally termed as biogeochemical cycles.

Types of biogeochemical cycles

The biogeochemical cycles are basically of two types; sedimentary cycle and gaseous cycle. In sedimentary cycle main reservoir is soil and sedimentary rocks and other type of rocks. Examples are phosphorus and sulphur cycle. In case of gaseous cycle main reservoir of nutrient is the atmosphere and ocean. Examples are the carbon, nitrogen, hydrogen and oxygen cycles.

Both types involve biotic and abiotic agents. Both are driven by flow of energy and both are tied to hydrological or the water cycle.

About 1% of solar radiation  converts into energy by the process termed as photosynthesis. It is this energy which is responsible for driving the mineral as well as hydrological cycle and for maintaining temperature within a narrow range of tolerance by the protoplasm.

Hydrological cycle

Water determines the distribution, structure and function of organisms in the ecosystem. It is the universal solvent and hence important for cycling of other matter in the ecosystem. Water is essential for working of the macromolecules, solubilisation of chemicals, transport of materials and several chemical reactions. It is also essential for photosynthesis and is a source of hydrogen for photosynthesis in plants.

Plants use large amount of water to maintain their hydrostatic skeleton and to move chemicals about their bodies. Plants take elements in aqueous solution from the soil. The total quantity of water present on earth is estimated to be 266, 070 G (G= Geogram=1020 g). It is distributed as follows:

Rock= 250,000 G, Sedimentary rocks=2,100 G, Oceans=13,800 G

River and lakes= 0.25 G, Polar ice caps= 167 G, water vapour= 0.13 G

Annually some 4.46 G of water, falls as rain, of which more than 75% (i.e., 3.47 G) falls on oceans.  Less than 1 G of rain falls on land, of which very little is stored underground.

Water is found in liquid, solid and gaseous forms. Certain physical processes, forests and water bodies maintain the water cycle on the earth. The water cycle maintains a balance, but the recent activities of man ( i.e., industrialisation and urbanisation leading to deforestation) disrupts this balance.

Types of water cycle

It is consists of two overlapping cycles; larger global water cycle and smaller biological water cycle.

Larger global water cycle

All parts of hydrosere ( i.e.,oceans, lakes, rivers, ponds, moist soil) lose water through evaporation. At any time, atmosphere contains 1 G=10 20 gm of water vapours.

In higher altitudes, water vapours cool, condense and form clouds. The clouds move to long distances. They ultimately precipitate. Precipitation is of three main types-rain, snow and hail. The total world precipitation amounts to 4.46 × 10 20 G per year. This means that the atmosphere must be refilled with water vapours 34 times in a year.

The major store is in the oceans and the major flows are evaporation from them and precipitation upon them. However, there is a net flow of water vapour, driven by winds from the oceans to land where it falls as rain, hail or snow.

Precipitation in the form of snow deposits in the polar and alpine regions. In other colder areas it occurs during winter but the snow melts during summer. Rainfall occurs both over oceans and on land. Melting of snow and rainfall produces rivers. Some water seeps into the soil and becomes underground. Rivers take back the surplus water to oceans again. The figure below shows the components of the global hydrological cycle.


 Biogeochemical cycles (Hydrological Cycle)

Fig: Hydrological Cycle (Biogeochemical Cycles)

The earth locks a major part of water in it and releases it in small quantities during volcanic eruptions. Polar ice, alpine snow and underground water constitute a large proportion of water that remains on the land mass.  But the large store in the polar ice caps has little effect on the hydrological cycle in short term due to negligible evaporation from them.

The human beings pump underground water for their use and in agriculture. Plant life is directly dependent upon water present in the soil. Some of the underground water flows into oceans. Thus water flows from the land as surface run-off or movement of groundwater into rivers and back to the oceans  maintaining balance of the cycle.

Biological water cycle

The smaller biological water cycle involves entry of water into living beings and its return to the physical environment. Aquatic plants and animals absorb water from their surroundings. They excrete some water to the outside during their lifetime while the remaining releases after death through the process of decay.

Plant absorb water from underground source through their roots. The plants lose a major amount of water through transpiration. Transpiration keeps the surrounding air cool during summer and warmer during winter. It thus determines the micro climate of the area. It is also a cause of dew and local rain.

Land animals drink water from pool, ponds, lakes and rivers.  Animal loses part of this water through urine and sweat. The remaining passes back into the environment after their death and decay.