The following points highlight the four main types of mineral cycle. The types are: 1. Oxygen Cycle 2. Water Cycle 3. Phosphorus Cycle 4. Sulphur Cycle.
Type # 1. Oxygen Cycle:
Oxygen, like carbon, is another essential element for the construction of living body and it is also needed for various life processes like respiration. Oxygen constitutes nearly 21 per cent of the atmospheric gases.
It exists in molecular form as O2 and also as a part of compounds like water and carbon dioxide. Oxygen, needed by plants and animals for respiration, enters directly in their body from the surrounding medium, i.e., from air in atmosphere or from water.
However, cycling of oxygen is very complex (Fig. 12.13). As a constituent of CO2, it circulates freely throughout the biosphere. Some carbon dioxide combines with calcium to form carbonates. Oxygen combines with nitrogen compounds to form nitrates, with iron to ferric oxides, and with many other materials to form various other oxides.
In these states oxygen is temporarily withdrawn from circulation. In photosynthesis the oxygen freed is split from the water molecule. This oxygen is then reconstituted into water during plant and animal respiration.
Some part of atmospheric oxygen that reaches the higher levels is reduced to ozone (O3) by high energy ultraviolet radiations. Thus, in this way oxygen level in the atmosphere is maintained at a constant level.
The earth’s atmosphere, after its formation, has undergone a slow but steady change due to various human activities. Human activity, however, has not had much effect on the oxygen content of the atmosphere because it is always replenished by photosynthesizers.
But the condition is altogether different for other atmospheric gases, collectively these are called greenhouse gases and the effect they produce is called the greenhouse effect.
The greenhouse gases allow the penetration of incoming solar radiations having short wave lengths (0.15 to 4 mm) but they prevent the escaping of long wave length radiations into the outer space and reflect back on the earth causing rise in temperature of the earth. This is referred to as greenhouse effect.
For example, carbon dioxide (a greenhouse gas) tends to prevent the long wave radiations (i.e., infra-red heat radiation) from earth from escaping into outer space and reflect it back to the earth.
This has caused an increase in earth’s temperature. Since, carbon dioxide content of the atmosphere is increasing with an alarming rate due to excessive use of fossil fuels liberating carbon dioxide into the earth’s atmosphere; it has a greenhouse effect (Fig. 12.14).
Type # 2. Water Cycle:
Water cycle basically forms a link between gaseous and sedimentary biogeochemical cycles. Hence, it needs a discussion at this stage before discussing the sedimentary cycles.
Water is indispensable for life constituting more than 80 per cent of living cells. The water cycle is represented by two overlapping cycles in nature.
(i) Global water cycle, being controlled by physical processes involving no life, and
(ii) Smaller water cycle, being controlled by biological processes involving life.
Global Water Cycle:
It involves evaporation of water from hydrosphere such as oceans, seas, rivers, lakes and ponds into the atmosphere as water vapours. The water vapours cool and condense to form clouds, water or snow. Water from clouds comes down to earth as rain or snow, it is called precipitation.
So, precipitation brings back water to the earth’s surface, some of it may drop directly into oceans or seas, some amount of water runs along the surface of ground and reaches to ponds, lakes, rivers and seas and some amount of water soaks into the ground to which we call underground water which may again flow through underground streams into bodies of surface water.
World’s precipitation rate is estimated to be 4.46 x 1020 g per year. At any given time, the water vapour present in atmosphere is estimated to be nearly 0.13 x 1020 g. This suggests that a rapid exchange of water vapour between the earth and atmosphere is essential.
It is important to note that a large amount of water remains underground and also in form of snow in the polar regions as well as on the mountain peaks above the snowline.
Smaller Water Cycle:
It involves the cycling of water from the environment into the living organisms and vice-versa. Aquatic animals obtain water from their surroundings and return it to the surrounding medium through their excretion and decomposition of their bodies after death.
The terrestrial animals obtain water either directly from fresh water sources or in form of food. They also return water into their surrounding through excretion and decomposition after death.
The plants absorb water from the soil by their roots. These retain some amount of water in their bodies and the rest is given out in the environment as water vapours during the process of transpiration and amount of water left in their bodies returns back to the environment after the death and decomposition of their bodies.
Evaporation of water from terrestrial plants cools the surrounding air and, thus, it helps in maintaining a microclimate around them.
The smaller water cycle is complicated one and differs considerably from the global water cycle but both of these operate simultaneously in nature to keep a water balance between the living and non-living world.
Type # 3. Phosphorus Cycle:
Plants and animals obtain phosphorus from the environment. Phosphorus is a component of nucleic acids, ADP, ATP, NADP, phospholipids, etc. It occurs in the soil as rock phosphate, apatite or calcium phosphate, fluorapatite Ca10(Fe2 (PO4)6 ironphosphate or aluminium phosphate. Soils derived from the rock beds rich in phosphates are rich in phosphorus.
Phosphorus Occurs in the Soil in Five Forms:
P1, (stable organic), P2 (labile organic), P3 (labile inorganic), P4 (soluble) and P5 (mineral form) and of these forms P3 and P4 are in equilibrium and entry of phosphorus in the green plants is considered to occur via labile inorganic pool.
The dissolved phosphorus is absorbed by plants and converted to organic form. From plants it travels to various trophic levels in the form of organic phosphates. When the plants and animals die, the decomposers attack them and liberate phosphorus to the environment. Thus this process proceeds in cyclic way.
Phosphorus along with many other mineral elements reaches the oceans and settles down as sediment. A good proportion of phosphorus leaches down to deep layers of soil. In this way, major proportion of phosphate becomes lost to this cycle by physical processes, such as sedimentation and leaching. Biological processes such as formation of teeth and bones also keep phosphorus locked up for some time.
Phosphorus occurs in soil solution mainly as orthophosphate (PO4 3−). Plants absorb phosphate from soil and incorporate it into organic compounds.
Phosphatising bacteria maintain supply of phosphate ions by converting organic phosphorus contained in detritus.
Absorption of phosphorus is generally promoted by the presence of mycorrhizae.
Sometimes a considerable amount of phosphates becomes separated from the phosphorus cycle; phosphates when form compounds with metals like aluminium, iron and calcium become unusable for plants unless they are changed chemically.
Phosphates are also locked up in bones and teeth, and become outside the cycle because these are very-very slowly acted upon by decomposers and, hence, take a long time to return to natural cycle.
Type # 4. Sulphur Cycle:
Sulphur, like nitrogen, is an essential part of protein and amino acids and is characteristic of organic compounds. The main source of sulphur for living organisms is the inorganic sulphur in form of sulphates.
The photosynthesizers (producers) obtain sulphur in form of sulphates from the soil or from water in case of aquatic plants and make use in their protein synthesis. This organic sulphur reaches to animals through food.
After death, the animals and plants are decomposed in the soil by microorganisms like Aspergillus, Neurospora and Escherichia under aerobic conditions as sulphates; these sulphates are mineralised in the soil. But under anaerobic conditions such as in marshes, it is reduced to sulphides. Sulphides, thus, produced are converted into soluble sulphates by sulphur bacteria, viz., Thiobacillus into sulphates.
Sulphur is added in the atmospheric air as sulphur dioxide from various industries. The fossil fuels also add sulphur dioxide to the environment when they are burnt. During rains, sulphur dioxide of the atmosphere forms H2SO4 (sulphuric acid) which later on decomposed to give rise to SO4 ions and then incorporated into living body.
Sulphur is removed from the organic phase in form of elemental sulphur which is insoluble and accumulates into reservoir pool in deep sediments.