Biogeo Chemical Cycling or Nutrient Cycling
Energy flow and nutrient circulation are the major functions of the ecosystem.
Energy is lost as heat forever in terms of the usefulness of the system. On the other hand, nutrients of food matter never get used up. They can be recycled again and again indefinitely.
Carbon, hydrogen, oxygen, nitrogen and phosphorus as elements and compounds makeup 97% of the mass of our bodies and are more than 95% of the mass of all living organisms.
In addition to these, about 15 to 25 other elements are needed in some form for the survival and good health of plants and animals.
These elements or mineral nutrients are always in circulation moving from non-living to living and then back to the non-living components of the ecosystem in a more or less circular fashion.
This circular fashion is known as biogeochemical cycling (bio for living; geo for atmosphere).
Among the most important nutrient cycles are the carbon nutrient cycle and the nitrogen nutrient cycle.
There are many other nutrient cycles that are important in ecology, including a large number of trace mineral nutrient cycles.
Types of Nutrient Cycles
A perfect nutrient cycle is one in which nutrients are replaced as fast as they are utilized.
Most gaseous cycles are generally considered as perfect cycles.
In contrast sedimentary cycles are considered relatively imperfect, as some nutrients are lost from the cycle and get locked into sediments and so become unavailable for immediate cycling.
Based on the nature of the reservoir, a nutrient cycle is referred to as Gaseous or Sedimentary cycle
• Gaseous Cycle: the reservoir is the atmosphere or the hydrosphere — water cycle, carbon cycle, nitrogen cycle, etc. and
• Sedimentary Cycle: the reservoir is the earth’s crust (soluble elements mostly found in earth’s crust) — phosphorous cycle, sulphur cycle, calcium cycle, magnesium cycle etc.
Carbon Cycle (Gaseous Cycle)
Carbon is a minor constituent of the atmosphere as compared to oxygen and nitrogen.
However, without carbon dioxide life could not exist because it is vital for the production of carbohydrates through photosynthesis by plants.
It is the element that anchors all organic substances from coal and oil to DNA (deoxyribonucleic acid: the compound that carries genetic information).
Carbon is present in the atmosphere, mainly in the form of carbon dioxide (CO2).
Carbon cycle involves a continuous exchange of carbon between the atmosphere and organisms.
Carbon from the atmosphere moves to green plants by the process of photosynthesis, and then to animals.
By process of respiration and decomposition of dead organic matter, it returns to the atmosphere. It is usually a short term cycle.
Some carbon also enters a long term cycle. It accumulates as un-decomposed organic matter in the peaty layers of marshy soil or as insoluble carbonates in bottom sediments of aquatic systems which take a long time to be released.
In deep oceans, such carbon can remain buried for millions of years till geological movement may lift these rocks above sea level.
These rocks may be exposed to erosion, releasing their carbon dioxide, carbonates and bicarbonates into streams and rivers.
Fossil fuels such as coals, oil and natural gas etc. are organic compounds that were buried before they could be decomposed and were subsequently transformed by time and geological processes into fossil fuels. When they are burned the carbon stored in them is released back into the atmosphere as carbon dioxide.
Nitrogen Cycle (Gaseous Cycle)
Nitrogen Fixing – Nitrogen to Ammonia (N2 to NH3)
There is an inexhaustible supply of nitrogen in the atmosphere, but the elemental form cannot be used directly by most of the living organisms.
Nitrogen needs to be ‘fixed’, that is, converted to ammonia, nitrites or nitrates, before it can be taken up by plants.
Nitrogen fixation on earth is accomplished in three different ways:
By microorganisms (bacteria and blue-green algae),
By man using industrial processes (fertiliser factories) and
To a limited extent by atmospheric phenomena such as thunder and lighting.
Certain microorganisms are capable of fixing atmospheric nitrogen into ammonia (NH3) and ammonium ions (NH4+).
Conversion of this ammonia to nitrate increases nitrogen leaching because nitrate is more water-soluble than ammonia.
Nitrification also plays an important role in the removal of nitrogen from municipal wastewater.
Ammonification – Urea, Uric Acid to Ammonia
This nitrogen escapes into the atmosphere, thus completing the cycle.
Step 1: N2 Fixing ==> Nitrogen → Ammonia or Ammonium Ions
Step 2: Nitrification ==> Ammonia or Ammonium Ions → Nitrite → Nitrate
Step 3: Ammonification ==> Dead Matter + Animal Waste (Urea, Uric Acid) → Ammonia or Ammonium Ions
Most of the ammonia escapes into the atmosphere. Rest is Nitrified (Step 2) to nitrates.
Some of the nitrates is available for plants. Rest is Denitrified (Step 4).
Step 4: Denitrification ==> Nitrate → Nitrogen
Phosphorus Cycle (Sedimentary cycle)
Phosphorus plays a central role in aquatic ecosystems and water quality.
Unlike carbon and nitrogen, which come primarily from the atmosphere, phosphorus occurs in large amounts as a mineral in phosphate rocks and enters the cycle from erosion and mining activities.
This is the nutrient considered to be the main cause of excessive growth of rooted and free-floating microscopic plants (phytoplankton) in lakes (leads to eutrophication).
The main storage for phosphorus is in the earth’s crust.
On land, phosphorus is usually found in the form of phosphates.
By the process of weathering and erosion, phosphates enter rivers, streams and finally oceans.
In the ocean, phosphorus accumulates on continental shelves in the form of insoluble deposits.
After millions of years, the crustal plates rise from the seafloor and expose the phosphates on land.
After more time, weathering will release them from rock, and the cycle’s geochemical phase begins again.
Sulphur Cycle (Sedimentary cycle)
The sulphur reservoir is in the soil and sediments where it is locked in organic (coal, oil and peat) and inorganic deposits (pyrite rock and sulphur rock) in the form of sulphates, sulphides and organic sulphur.
It is released by weathering of rocks, erosional runoff and decomposition of organic matter and is carried to terrestrial and aquatic ecosystems in salt solution.
The sulphur cycle is mostly sedimentary except two of its compounds, hydrogen sulphide (H2S) and sulphur dioxide (SO2), which add a gaseous component.
Sulphur enters the atmosphere from several sources like volcanic eruptions, combustion of fossil fuels (coal, diesel etc.), from the surface of the ocean and gases released by decomposition.
Atmospheric hydrogen sulphide also gets oxidised into sulphur dioxide.
Atmospheric sulphur dioxide is carried back to the earth after being dissolved in rainwater as weak sulphuric acid (acid rain).
Whatever the source, sulphur in the form of sulphates is taken up by plants and incorporated through a series of metabolic processes into sulphur bearing amino acid which is incorporated in the proteins of autotroph tissues. It then passes through the grazing food chain.
Sulphur bound in a living organism is carried back to the soil, to the bottom of ponds and lakes and seas through excretion and decomposition of dead organic material.
1.) Consider the following:
3. Decay of organic matter
4. Volcanic action
Which of the above add carbon dioxide to the carbon cycle on Earth?
a) 1 and 4 only
b) 2 and 3 only
c) 2,3 and 4 only
d) 1, 2, 3 and 4
Answer: c) 2,3 and 4 only
Photosynthesis takes out CO2 from the carbon cycle. Rest all add CO2.
2.) Which of the following adds/add nitrogen to the soil?
1. Excretion of urea by animals
2. Burning of coal by man
3. Death of vegetation
Select the correct answer using the codes given below.
a) 1 only
b) 2 and 3 only
c) 1 and 3 only
d) 1, 2 and 3
Answer: c) 1 and 3 only.
All the above three adds to the nitrogen cycle.
Burning coal releases CO, CO2, sulphur dioxide and oxides of nitrogen – air pollutants.
Oxides of nitrogen fall on earth as acid rain. Acidic rain is a complex mixture of nitrous, nitric, sulphurous and sulfuric acids which all combine to lower the pH.
But, the question asks, “Which of the following adds/add nitrogen to the soil?”
Animal waste like urea, uric acid and death of vegetation add nitrogen in the form of nitrates directly into the soil.
3.) Consider the following:
1. Carbon dioxide
2. Oxides of Nitrogen
3. Oxides of Sulphur
Which of the above is/are the emission/emissions from coal combustion at thermal power plants?
a) 1 only
b) 2 and 3 only
c) 1 and 3 only
d) 1, 2 and 3
Answer: d) 1, 2 and 3.
Burning coal releases CO, CO2, sulphur dioxide and oxides of nitrogen.
4.) What can be the impact of excessive/inappropriate use of nitrogenous fertilisers in agriculture?
1. Proliferation of nitrogen-fixing microorganisms in soil can occur.
2. Increase in the acidity of soil can take place.
3. Leaching of nitrate to the ground-water can occur.
Select the correct answer using the code given below.
a) 1 and 3 only
b) 2 only
c) 2 and 3 only
d) 1,2 and 3
Answer: c) 2 and 3 only
Nitrification is important in agricultural systems, where fertiliser is often applied as ammonia. Conversion of this ammonia to nitrate increases nitrogen leaching because nitrate is more water-soluble.
Agricultural fertilisation and the use of nitrogen-fixing plants also contribute to atmospheric NOx, by promoting nitrogen fixation by microorganisms. Excess NOx leads to acid rain. Acid rain lowers pH of the soil (increase in acidity of soil)
The legume-rhizobium symbiosis is a classic example of mutualism — rhizobia supply ammonia or amino acids to the plant and in return receive organic acids as a carbon and energy source.
So, excessive/inappropriate use of nitrogenous fertilisers can make the plants independent of both symbiotic and free-living nitrogen fixers. Fixers don’t get the food from the plants due to a broken relationship and other factors. So, their population decreases.
5.) With reference to agricultural soils, consider the following statements:
1. A high content of organic matter in soil drastically reduces its water holding capacity.
2. Soil does not play any role in the Sulphur cycle.
3. Irrigation over a period of time can contribute to the salinization of some agricultural lands.
Which of the statements given above is/are correct?
a) 1 and 2 only
b) 3 only
c) 1 and 3 only
d) 1, 2 and 3
Answer: b) 3 only
A high content of organic matter (humus) in soil increases its water holding capacity.