Flow Of Materials And Energy Within The Ecosystem – 19.4.4 Cycling of Materials in Nature
The chemicals found in living organisms are derived originally from the biotic components of ecosystems such as soil, water, air. These materials are cycled through the ecosystems and are eventually returned to the environment when waste products and dead bodies of organisms are decomposed by bacteria and fungi. They are available for reuse by organisms.
Thus, unlike energy, matter moves through the ecosystem in cycles. The amount of matter that enters of leaves each cycle is relatively small. This fact is illustrated by carbon, water, and nitrogen cycles.
The carbon cycle
Carbon is an essential constituent of organisms. Carbon from carbon dioxide is utilized by plants in photosynthesis. This carbon becomes a part of the food made by them. The green plants are eaten by the herbivores, which in turn are eaten by the carnivores. Thus carbon is transferred from the plants to the animals. During respiration by plants and animals, CO2 is released into the atmosphere. Carbon dioxide is also released when dead organisms are decomposed by the decomposers. From the atmosphere it is again returned to the plants and cycle continues (Fig. 19.16)
Fig. 19.16The carbon cycle. Naturally the amount of carbon dioxide used in photosynthesis and released in respiration and decomposition is balanced. However, greater amount of carbon dioxide has been added to the atmosphere by human activities. Do you know how? What may be its consequences?
Materials And Energy – The nitrogen cycle
Nitrogen is important for organism because it is an essential part of proteins, nucleic acids and chlorophyll. Although earth’s atmosphere is about 78% nitrogen gas (N2) but organisms are unable to use this molecular nitrogen unless it is converted to certain compounds (such as nitrates)
The nitrogen cycle, in which nitrogen cycles between the abiotic environment and organisms have four steps. The first step in the nitrogen cycle, which is called biological nitrogen fixation, involves the conversion of gaseous nitrogen (N2) to ammonia. Biological fixation is carried out by the activity of nitrogen fixing bacteria in the soil and aquatic environment. Some nitrogen fixation to nitrates also occurs by combustion, volcanic action, lightening discharges and industrial means.
The second step of the nitrogen cycle is nitrification. It involves the conversion of ammonia (NH3) to nitrates. Nitrification is also accomplished by bacteria.
In the third step, called assimilation, plants through the roots absorb ammonia or nitrates and incorporate the nitrogen into proteins, nucleic acids and chlorophyll. When animals consume plant tissues, plant nitrogen compounds are converted to animal’s nitrogen compounds forming the bodies of animals.
The fourth and final step of the nitrogen cycle is gentrification during this stage nitrogen containing wastes and dead bodies of organisms are decomposed to ammonia and nitrates. The produced escapes to the atmosphere (Fig. 19.17)
Fig. 19.17The nitrogen cycle. All nitrogen originally comes from the atmosphere
The Water Cycle
Water continuously circulates from the oceans to the atmosphere, to the land and back to the ocean. Water evaporates from the surface and from soil, streams, rivers, and lakes in the form or water vapours.
In addition, transpiration by plants adds a considerable amount of water vapour to the atmosphere. The water vapour in the atmosphere condenses and forms clouds. It eventually comes down to the land and oceans in the form of precipitation (rain, dew, snow, or hail). Water may evaporate from land and re-enter the atmosphere directly. Alternately, it may flow in rivers and streams as run off water, reaching the ocean. Some water percolates downward in the soil to become ground water. Ground water remains in the ground for hundreds to many thousands of years, but eventually it supplies water to the soil, to streams and rivers, to plants, and to the ocean (Fig 19.18).
Fig. 19.18The water cycle. Most of the earth’s surface water is stored in the oceans. Only about 2 percent present in the fresh lakes, ponds, rivers and streams, and in the form of precipitation (rain and snow) is available for our everyday and industrial use.