14. ECOSYSTEM
4. NUTRIENT CYCLING
Nutrients are never lost from the ecosystems. They are recycled again and again.
The movement of nutrient elements through various components of an ecosystem is called nutrient cycling (biogeochemical cycles).
Nutrient cycles are 2 types:
- Gaseous cycle: For this, the reservoir exists in the atmosphere. E.g. Nitrogen & Carbon cycles.
- Sedimentary cycle: For this, the reservoir is located in Earth’s crust. E.g. Sulphur & Phosphorus cycles.
Carbon Cycle
Reservoir of carbon: Atmosphere (about 1%), organisms (49% of
dry weight), oceans (71% dissolved carbon. It regulates the amount of
atmospheric CO2), fossil fuel etc.
Carbon cycling occurs through atmosphere, ocean and through living and dead organisms.
4×10^13 kg of carbon is fixed in the biosphere through photosynthesis annually.
A major amount of carbon returns to the atmosphere as CO2 through respiration.
Processing of wastes & dead organic matter by decomposers also release CO2.
Some amount of the fixed carbon is lost to sediments and removed from circulation.
Burning of wood, forest fire and combustion of organic matter, fossil fuel and volcanic activity are other sources for releasing CO2 in the atmosphere.
Role of human activities in carbon cycle: Deforestation, burning of fossil fuel etc. has increased the rate of release of CO2 into the atmosphere.
Phosphorus is a constituent of biological membranes, nucleic acids & cellular energy transfer systems. Many animals use phosphorus to make shells, bones and teeth.
The natural reservoir of phosphorus is rock (in the form of phosphates).
When rocks are weathered, minute amounts of phosphates dissolve in soil solution and are absorbed by the plants. Herbivores and other animals obtain this from plants. The waste products and the dead organisms are decomposed by phosphate-solubilising bacteria releasing phosphorus.
Carbon cycling occurs through atmosphere, ocean and through living and dead organisms.
4×10^13 kg of carbon is fixed in the biosphere through photosynthesis annually.
A major amount of carbon returns to the atmosphere as CO2 through respiration.
Processing of wastes & dead organic matter by decomposers also release CO2.
Some amount of the fixed carbon is lost to sediments and removed from circulation.
Burning of wood, forest fire and combustion of organic matter, fossil fuel and volcanic activity are other sources for releasing CO2 in the atmosphere.
Role of human activities in carbon cycle: Deforestation, burning of fossil fuel etc. has increased the rate of release of CO2 into the atmosphere.
Phosphorus Cycle
Phosphorus is a constituent of biological membranes, nucleic acids & cellular energy transfer systems. Many animals use phosphorus to make shells, bones and teeth.
The natural reservoir of phosphorus is rock (in the form of phosphates).
When rocks are weathered, minute amounts of phosphates dissolve in soil solution and are absorbed by the plants. Herbivores and other animals obtain this from plants. The waste products and the dead organisms are decomposed by phosphate-solubilising bacteria releasing phosphorus.
Differences between carbon & phosphorous cycles
|
Carbon cycle |
Phosphorous
cycle |
|
Atmospheric input is higher |
Much smaller |
|
There is gaseous exchange between organism & environment |
Gaseous exchange is negligible |
ECOSYSTEM SERVICES
The products of ecosystem processes are called ecosystem services.
E.g. forest ecosystems purify air and water, mitigate droughts and floods, cycle nutrients, generate fertile soils, provide wildlife habitat, maintain biodiversity, pollinate crops, provide storage site for carbon and provide aesthetic, cultural & spiritual values.
Robert Costanza and his colleagues have tried to put price tags on nature’s life-support services.
Researchers have put an average price tag of US $ 33 trillion a year on fundamental ecosystems services. This is nearly twice the value of the global gross national product GNP (US $ 18 trillion).
Out of this total cost, soil formation accounts for about 50%.
Contributions of other services like recreation & nutrient cycling are less than 10% each.
The cost of climate regulation and habitat for wildlife are about 6 % each.
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