Water cycles continuously from the atmosphere to the earth to the oceans and back to the atmosphere again Water is indispensable to the functioning of living organisms which live or die on the basis of their ability to capture water and incorporate it into their bodies. Water evaporates from the oceans which cover ¾ of the earth’s surface into the atmosphere a process powered by energy from the sun. Over land areas, approximately 90% of the water that reaches the atmosphere comes from plants via transpiration. Following condensation, most precipitation from the atmosphere falls clean and fresh directly into the oceans but, some fall on land where it passes into the surface or subsurface bodies of fresh water. Only about 2% of all the water on earth is fixed in any form – frozen, held in the soil or incorporated into the body of organisms. This cycle of evaporation/transpiration and precipitation goes on over and over again.
THE CARBON CYCLE
The carbon cycle is based on carbon dioxide which makes up only about 0.03% of the atmosphere. Carbon is used to make carbohydrates, fats and proteins, the major sources of food energy. These compounds are oxidized to release carbon dioxide, which can be captured by plants, algae and photosynthetic bacterial to make organic compounds. The chemical reaction is powered by the light energy of the sun. This results in the fixation of about 10% of the roughly 700 billion metric tons of carbon dioxide in the atmosphere each year.
All heterotrophic organisms including non-photosynthetic bacteria, fungi, animals and few plants that have lost the ability to photosynthesize, obtain their carbon indirectly from the organisms that fix it. When their bodies decompose, organisms release carbon dioxide to the atmosphere. Once there, it can be reincorporated into the bodies of other animals.
The carbon cycle is unusual among nutrient cycles because it need not involve decomposers. Most of the organic compounds that are formed as a result of carbon dioxide fixation in the bodies of photosynthetic organisms are ultimately broken done and released back into the atmosphere or water. However, certain carbon containing compounds like cellulose are more resistant to breakdown.
The carbon in this cellulose may eventually be incorporated into fossil fuels, such as oil or coal.
In global terms, photosynthesis and respiration are approximately balanced, but the balance has been shifted recently because of the consumption of fossil fuels by man. The combustion of coal, oil and gas has released large stores of carbon into the atmosphere as carbon dioxide. This increase of carbon dioxide in the atmosphere appears to be changing global climate making it warmer.
THE NITROGEN CYCLE
The primary inorganic reservoir of Nitrogen is the atmosphere. Nitrogen gas constitutes 78% of the earths atmosphere but the total amount of fixed nitrogen in the soil, oceans and the bodies of organisms is only 0.03% of that figure.
Nitrogen cycles between organisms and reservoirs via the nitrogen cycle. This cycle can be broken down into a number of stages.
- Ammonification: When organisms excrete nitrogenous waste or die, their nitrogen is converted to ammonium ions by the action of saprotrophic fungi and bacteria. This process is known as ammonification.
- Nitrification: In warm, moist soils with a pH near 7, ammonium ions are oxidized within a few days of their formation or their addition as fertilizer (Salisbury and Ross, 1985). The oxidation benefits the bacteria performing the reactions by releasing energy which the bacteria can use for synthesis of ATP. The process takes place in two stages. First, ammonium is oxidised to nitrite, by bacteria of the genera
Nitrosomonas, Nitrosospira, Nitrosococcus and Nitrosolobus (Hamilton, 1988). Next, nitrite is oxidised to nitrate by bacteria of the genera Nitrobacter, Nitrospira and Nitrococus.
- Uptake of Nitrogen by plants: Most plants absorb the majority of their nitrogen as nitrate. However many plants also absorb ammonium like in forests on acidic soils where conversion of ammonium to nitrate is slow
- Nitrogen Fixation: Nitrogen fixation is the reduction of atmospheric nitrogen to Ammonium ion. It is of great importance to organisms. Together with Lightning, it is the natural way in which organisms gain access to the huge reserves of nitrogen in the atmosphere. Nitrogen fixation can only be carried out by certain species of bacteria and Cyanobacteria (Postgate, 1988). Some of these species are free-living, occurring in soil or in water. Others exist in symbiotic relationships with higher plants. The most well-known of the nitrogen fixing bacteria are in the genus Rhizobium. These bacteria form symbiotic associations in root nodules of many plants in the family Leguminosae which includes some important crops as peas, groundnuts, beans and clovers.
- Denitrification: Certain proportion of the fixed nitrogen in the soil is steadily lost. Under anaerobic conditions, nitrate is often converted to nitrogen gas and nitrous oxide both of which return to the atmosphere. This process which several genera of anaerobic bacteria carry out is called Denitrification. In its absence, all nitrogen would eventually become fixed, converted into nitrate and washed into the oceans. Life would thus be possible only in marine and littoral habitats as all living organisms depend on the results of nitrogen fixation to synthesize proteins, nucleic acid and other necessary nitrogen – containing compounds. Denitrification and nitrogen fixation together constitute the mechanism for returning nitrogen from the oceans to the land.
The phosphorus cycle unlike those of carbon and nitrogen lacks an atmospheric component. Phosphorus enters ecosystems through the weathering of rocks. Plants obtain their phosphorus from the soil either as dihydrogen phosphate or more slowly as hydrogen phosphate. Once in an organism, though, phosphorus does not undergo reduction; it remains as phosphate. In this form it is found in a number of compounds including nucleic acids, phosphorylated carbohydrates and fats. Herbivores obtain their phosphorus from plants while carnivores obtain theirs from herbivores. Decomposers return phosphorus to the soil as phosphate ion. In most soils and waters, phosphorus is in short supply and limits plant growth. This is because they are relatively insoluble and are present only in certain kinds of rock. Crushed phosphate-rich rocks found in certain regions are used as fertilizer and added to agricultural lands in form of superphosphate in the belief that it becomes fixed to and enriches the soil. The absorption of phosphate along with ammonium, nitrate and the potassium ion is though, greatly aided by the presence of mycorrhizae (fungus plants).
INTERACTIONS BETWEEN THE NUTRIENT CYCLES
The various nutrient cycles are interconnected and depend on one another to a great extent. The burning of fossil fuels for example not only puts large amount of carbon into the atmosphere, it also increases the amount of atmospheric nitrogen, phosphorus and
sulphur. The interdependence of the nutrient cycles is obvious when one considers nutrient cycling through organisms. When a herbivore eats a plant or a carnivore an animal, it ingests at one go not just carbon, nitrogen and phosphorus but oxygen, calcium, potassium, chlorine and all other elements which are found in organisms.
SELF ASSESSMENT EXERCISE
What is nitrogen fixation? Explain the role played by some species of bacteria and leguminous plants in nitrogen fixation.
Explain how the phosphorous cycle differs from other nutrient cycles.
In an ecosystem which is an assemblage of organisms along with the non-living factors of the environment, nutrients circulate over and over again among organisms within the system.
- Important nutrients which are critical to the lives of organisms within an ecosystem include, water, carbon, nitrogen, oxygen and phosphorus.
- Unlike the flow of energy among the living organisms of an ecosystem which occurs on an open system, all the nutrients used in an ecosystem by living organisms operate on a closed system. That is they are recycled within organisms in the system over and over again.
- Carbon, nitrogen and oxygen have gaseous or liquid reservoirs as does water. All the other nutrients, such as phosphorus have solid reservoirs.
- The carbon cycle is unusual among nutrient cycles because it need not involve decomposers.
- The nitrogen cycle is a complex process involving the activities of many genera of bacteria.
- Phosphates are relatively insoluble and are present in most soils only in small amounts. They often are so scarce that their absence limits plant growth, hence the use of fertilizer like superphosphate in agriculture to boost crop production.
- There is an interconnection between the various types of nutrient cycles.
What are biogeochemical cycles? What are the primary reservoirs for the chemicals in these cycles?
From what source does most of the water over land reach the atmosphere? How does transpiration differ from evaporation?