Nutrient Cycling 

The essential elements or inorganic substances are provided by earth and are required by organisms for their body building and metabolism, so they are called biogenetic nutrients or bio-geochemicals.

All the three subdivisions of the earth contribute these elements as such or in the form of compounds.

However, as the earth is a closed system for matter, the supply of biogenetic nutrients is finite or limited.

It is estimated that amount of living matter (made up of biogenetic nutrients) contained in all the past and present organisms is several times more than the total mass of earth.

This is possible only if biogenetic nutrients do not remain locked up in the body of organisms but are released, during excretion and after death, back into the nonliving world so that they can be used again and again.

Circulation or exchange of biogenetic nutrients between the living and the nonliving components is called cycling of matter or biogeochemical cycling (at global scale).

However, the whole of biogenetic nutrients are not always in circulation.

The nutrients occur in two states-reservoir pool and cycling pool.

(i) Reservoir pool is the reservoir of biogenetic nutrients from which they are very slowly transferred to the cycling pool, e.g., metal phosphates, nitrogen gas of the atmosphere.

(ii) Cycling Pool is the pool of biogenetic nutrients which are repeatedly exchanged between the biotic and abiotic components of the biosphere.

Environmental factors, e.g., soil, moisture, pH, temperature etc. regulate the rate of release of nutrients into the atmosphere.

The function of the reservoir is to meet with the deficit which occurs due to imbalance in the rate of influx and efflux.

Bio-geochemical cycles: These are of two types (1) Gaseous cycle (2) Sedimentary cycle

1. In gaseous cycles, materials involved in circulation are gases. Four most abundant elements present in living organisms, i.e., C(CO2), H (water vapours), O and N have predominantly gaseous cycles. The main reservoir pool for gaseous cycles is atmosphere or hydrosphere (water). Gaseous cycles are quick and are relatively perfect systems as the elements remain in circulation more or less uniformly.

2. In sedimentary cycles, biogenetic materials involved in circulation are non-gaseous. The reservoir pool for these cycles is lithosphere, e.g., phosphorus, calcium, sulphur. Recycling of sulphur involves both gaseous and sedimentary phase (mixed type). Sedimentary cycles are very slow and are less perfect systems as the elements may get locked and go out of circulation for longer duration .

Different biogeochemical cycles are discussed below:

1. The Carbon Cycle:

Carbon is constituent of almost all organic compounds of the cell such as carbohydrates, proteins, lipids, enzymes, nucleic acids, hormones, etc., and thus may be considered basis of life.

Carbon constitutes 49% of dry weight of organisms and is next only to water.

Infact, 71 % of total global carbon is found dissolved in oceans.

This oceanic reservoir regulates the amount of CO2 in atmosphere.

According to an estimate 4 × 1013 kg of carbon is fixed in the biosphere through photosynthesis annually.

Carbon in the atmosphere is present as CO2, in hydrosphere as dissolved CO2 or carbonic acid or bicarbonates and in lithosphere as fossil fuels or carbonates and graphite in rocks.

Carbon present in lithosphere is not readily available, as it becomes available to the living world only when it is either burnt or changed chemically.

There is however, regular exchange of carbon between atmosphere and hydrosphere and is readily available to the living world in the form of free or dissolved carbon dioxide.

Carbon passes into living components mainly during photosynthesis in the form of CO2 by autotrophs, but atmosphere and hydrosphere do not get depleted of their carbon content because of its return through two major processes: (i) Biological-respiration by living organisms and decomposition of organic matter. (ii) Non-biological-combustion of carbon containing fuel that releases CO2 in the atmosphere.

Human activities have significantly influenced the carbon cycle.

Rapid deforestation and massive burning of fossil fuel for energy and transport have significantly increased the rate of release of CO2 into the atmosphere.

Fig. : Carbon cycle

2. The Oxygen Cycle :

Oxygen is constituent of important biomolecules in the cell and is also required for respiration in aerobes.

It is present in natural gaseous form (molecular O2) in atmosphere and constitutes about 21% of the air. In combined form it is component of CO2, water and number of oxidised salts.

Terrestrial organisms take oxygen directly from the air and aquatic organisms take oxygen either from water present in it, in diffused or dissolved state or directly from the atmosphere to be used in respiration.

It is returned to the atmosphere in the form of CO2 and H2O.

Fig. : Oxygen cycle

Carbon dioxide and water produced during respiration are used by plants during photosynthesis and molecular oxygen is released into the atmosphere for reuse in respiration.

The oxygen in the atmosphere thus remains in state of natural dynamic equilibrium.

Oxygen is also added to the atmosphere in terms of carbon dioxide, water, sulphur dioxide, nitrogen oxides, etc., during burning or combustion of wood, coal, petroleum, natural gases.

Oxides are also produced during microbial oxidation. Chemical or biological oxidation of oxides releases molecular oxygen.

3. The Nitrogen Cycle :

Nitrogen is component of amino acids, proteins, enzymes, nucleic acids and nucleotides, which are essential structural and functional components of living organisms.

Main source of nitrogen is air, as about 3/5 of the total air is molecular nitrogen (N2).

Sufficient required amount of nitrites and nitrates are made available to the plants through a number of processes involving microorganisms either from air or from organic nitrogen locked up in dead remains of living organisms.

Fig. Nitrogen Cycle

(a) Nitrogen fixation. The conversion of molecular nitrogen into nitrogenous compounds like ammonium salts or oxides of nitrogen is termed nitrogen fixation, which is of three types:

(i) Biological nitrogen fixation

(ii) Atmospheric nitrogen fixation

(iii) Industrial nitrogen fixation

(b) Decay and decomposition of dead remains of living organisms and of their excretory products.

Plants mainly absorb nitrogen in the form of nitrate ions which are reduced to NH3 in plant cells.

NH3 is used up in synthesis of amino acids.

Animals synthesize proteins and other nitrogenous compounds from amino acids obtained through digestion of plant proteins.

During protein metabolism, a number of nitrogenous waste products like ammonia, urea and uric acid are produced which are excreted and nitrogen present in them is recycled.

A bulk of organic nitrogen remains locked up in plants and animals.

The nitrogen present in dead remains of plants and animals in the form of organic compounds is converted into amino acids by a number of saprophytic micro-organisms.

Anaerobic breakdown of proteins is termed putrefaction.

Amino acids are converted into ammonium salts by ammonifying bacteria (Bacillus sp.) and some fungi.

Some ammonium salts are directly absorbed by plants.

However, most of these are converted into nitrites by nitrifying bacteria as plants prefer absorption of these in comparison to ammonium salts.

Ammonium salts are converted into nitrites by Nitrosomonas and Nitrococcus and nitrites are converted into nitrates by Nitrobacter, Nitrocystis and Penicillium species.

Some nitrates and nitrites in the soil are lost through leaching and precipitation and become constituent of rocks, going out of cycle.

This nitrogen is slowly released for recycling in nature after an interval of millions of years during weathering of these rocks only when these get exposed.

Some nitrogen present in nitrates is released to the atmosphere as molecular nitrogen by certain denitrifying bacteria (e.g., Thiobacillus denitrificans, Pseudomonas aeruginosa) in water logged soils or other areas having anaerobic environment.

These utilize O2 of the nitrates for their requirement and thus N2 is released in molecular form and escapes in the atmosphere in gaseous state.

4. The Water Cycle:

Water is the only substance on the surface of earth that exists in all the three states of matter.

Water is essential for photosynthesis and above all O2 of the atmosphere is derived from photolysis of water during photosynthesis, which is absolutely essential for the existence of animal life.

Water or hydrological cycle is made up of two overlapping cycles-the larger global cycle, not involving living organisms and the smaller local or biological water cycle, involving exchange between environment and living organisms.

(i) Global water cycle. Water is continuously lost from oceans, lakes, rivers and moist soil through evaporation. Clouds are formed by cooling and condensation of water vapours. The clouds move along the land over long distances, get cooled and precipitate as rain, snow or hail. Rain water may directly fall into ocean and may return to the atmosphere again through evaporation.

(ii) Local or biological cycle. It involves the entry of water into living organisms and its return to the atmosphere.

5. Phosphorus cycle:

Phosphorus has a natural reservoir in rocks as phosphates.

There is no respiratory release of phosphorus into atmosphere.

Atmospheric inputs of phosphorus through rainfall are much smaller than carbon inputs and gaseous exchange of phosphorus between organisms and environment are negligible.

Phosphorus present in insoluble form in soil is converted into soluble form by chemicals secretions of microorganisms and plant roots.

Dissolved phosphate is absorbed by plants and is used to built organic compounds like phospholipids, nucleotides, nucleic acids, etc.

Phosphorus moves from plants to animals of different trophic levels in ecosystem through food chains.

The phosphorus present in plants and animals (organic form) is returned to the soil (inorganic form) through decomposition of excreta of animals and dead remains of plants and animals by micro-organisms; to be reused by plants.

The phosphorus present in bones and teeth of animals is resistent to decay and thus remain out of the cycle for a long time.

Some amount of phosphorus washed down into sea, entering the food chain of aquatic ecosystem comes back to the soil.

Phosphorus in sea water is absorbed by sea weeds and finally passes into fish and seabirds through food chains.

Phosphorus rich faeces (guano) are deposited on land by sea-birds.

However, significant amount of phosphorus is lost in deep sediments, remaining out of the cycle for a long time.

It becomes available when deep sea strata is brought to surface through mining or some natural disturbances.

Some phosphate in soil combines with metals like aluminium, calcium and iron to form their insoluble salts which are not readily available to plants for absorption.

It thus appears that phosphorus is being lost from the available pool faster than it is returned to it.

6. Sulphur cycle:

Main source of sulphur to the plants are sulphates and to some extent elemental sulphur present in soil, water and rocks.

Sulphur absorbed from soil is incorporated into amino acids and subsequently into proteins by plants.

Organic sulphur is transferred to animals of different trophic levels through food chains.

Some animals may get sulphur from water as well.

Organic sulphur present in dead remains of plants and animals and animal excreta is converted into sulphates during decomposition by bacteria and fungi under aerobic conditions which are added to the soil and water for reuse by plants.

SO2 is also released into atmosphere through combustion of fossil fuels.

It gets dissolved in water to form H2SO4 during rainfall which combines with certain metals in soil to form sulphates.

Some bacteria (e.g., Beggiatoa) and fungi convert H2S and element sulphur into sulphates which are recycled through plants in the biosphere.

Rocks may also be erroded by wind and the materials including sulphur are blown into air as dust.

These materials are transferred to soil during rain.

Sulphates are also added to the soil and air by volcanic erruptions.

Sulphates from the rocks are also brought to the soil by rain water running over them.

Some sulphates seep into the soil and others reach ponds and lakes or carried by rivers to sea.

In sea, it may get deposited in sedimentary rocks and move out of the cycle.

It may get back into cycle through food chains or geological disturbances.