natural resources and their management environmental cycles

Seen Environmental Learning Information Sheet No 2 1

The importance of environmental cycles If we consider the Earth as a ‘closed’ ecosystem, apart from energy in the form of sunlight, all the materials that we need for life must be contained within it. How is it then that after millions of years the essential resources for life, oxygen, carbon, water and nitrogen have not all been used up? The answer is that these elements are constantly being recycled and re-used naturally so that they are always available. Today, the activities of people are having a detrimental effect on these natural cycles and we can no longer take it for granted that they will continue to operate normally or continue to supply the Earth with the essential ingredients of life. Technological change and industrialisation, population growth and the need for more food and water are severely disrupting the cycles. As a result problems such as global warming, acid rain, lake eutrophication, ozone thinning and water shortages are arising. Solving these problems requires an understanding of the cycles and how they work, so that we can take measures to restore nature’s natural balance.

Cycle 1: Carbon Did you know that carbon is the most common element found on the earth? Oil, trees and diamonds are all made of it. In fact the carbon cycle has two parts:

• The biological cycle

• The geological cycle The biological carbon cycle Lets take a closer look at the former first because it is its role as a gas, carbon dioxide, in the air during photosynthesis whereby plants make food that is the most important environmentally.

Without it living things on Earth could not survive.

Plants make their food (glucose) from carbon dioxide, water and sunlight. During this process called photosynthesis, carbon dioxide is removed from the atmosphere to make the glucose and oxygen is released into it. Plants use glucose for energy and to make tissues such as cellulose and lignin. The equation describing photosynthesis is:

Since animals can’t make their own food, they must eat plants or other animals that eat plants. In this way the carbon in the air is used to make the bodies of animals. The carbon dioxide that is removed from the air by photosynthesis is returned to it in two main ways:

• The carbon that makes up the food glucose is used as energy by plants and animals, when it combines with oxygen in a process called respiration. Respiration takes place in every cell. One waste product of this is

Environmental Cycles Seen Environmental Learning Information Sheet no 2

Natural Resources and their Management

energy from sun Carbon dioxide + waterglucose + oxygen chlorophyll

Theme: Natural Resources and their Management Topic No 2: Environmental Cycles


carbon dioxide which is then returned to the atmosphere.

• Carbon dioxide is also returned to the air by microbes that decompose (break down) the bodies of living things after their death in the presence of oxygen. This can only happen when the dead remains are open to the atmosphere.

The carbon cycle The carbon cycle has been in a natural balance for millions of years. Around the same amount of CO2 is removed from the air by photosynthesis as is returned to it through respiration and the decay of dead plants and animals. The geological carbon cycle From the diagram it looks as thought the cycle is complete. But it is not. A small but important leak can occur if dead plants or animals get ‘locked up’ within rocks before they decay and release their carbon dioxide back to the air. Over millions of years, this small leak has resulted in most of the carbon in the cycle being locked into rocks (97%) or fossil fuels (2.9%). Fossil fuels such as coal, oil and natural gas are formed when decay cannot occur because oxygen is absent. Coal formed about 250m years ago

when many parts of the world were covered by sub-tropical swamps. Some of the dead vegetation became covered by sand and mud and was compressed into coal. Oil and natural gas resulted about 150m years ago from the remains of dead sea creatures falling to the sea bed and being covered by sand and mud that was compressed into rock. As a result of the temperature rising in these underground rocks and the action of anaerobic bacteria, the sea creatures were converted into oil and gas. This locking of carbon happened slowly over millions of years. Some carbon dioxide is also removed from the air by dissolving in sea water. The world’s oceans (just like the forests) are called ‘carbon sinks’ because they can take carbon dioxide out of the air. Cold water can dissolve more carbon dioxide than warm water. Most carbon is locked up in sedimentary rocks such as chalk and limestone that were made from the shells of tiny sea creatures.. These sea creatures took the carbon dioxide out of the sea water to make their shells. When they dies they fell to the sea bed and were compressed by the weight of sand and mud deposited on top of them. Uplift has since raised these rocks so that many of them now form hills on land. Throughout human history until now the carbon cycle has been in balance so that the proportions of carbon in each part of the cycle have remained the same. Human activities are now changing this balance, increasing the amount of carbon dioxide in the air with devastating effects on the environment including acid rain (see Information Sheet on Acid Rain) and global warming (see Information Sheet on Global Warming)

Glucose + oxygen energy + CO2 + water



Human activity and the carbon cycle The spread of industrialisation, agriculture and the increased use of fossil fuels have all interfered with the natural carbon cycle. When coal, oil or natural gas are burnt carbon dioxide is released back into the atmosphere. For the last 200 years this has been happening at a much faster rate than it was originally removed when the fuels were formed. Burning fossil fuels can also result in acid rain ‘acid rain’. Acid rain is really dilute sulphuric acid and if this falls on limestone can cause a chemical reaction that releases even more carbon dioxide into the air. Forest clearance also has an impact. Because research shows that in mature forests, photosynthesis, respiration and decay balance out, if trees are cut or clear-felled, then there is less change to the overall carbon balance. But most forests are cleared for agriculture and are burnt. Burning releases vast amounts of carbon dioxide very quickly. Trees and oceans act as carbon sinks. This means that they can take carbon dioxide out of the air. Planting new forests is an effective way of removing carbon dioxide from the air since when

a tree is growing it is needing a lot of carbon for energy and tissue growth. Oceans can also remove carbon from the air but this amount has an upper limit. Eventually the seawater becomes saturated and can dissolve no more. Because cold water absorbs more carbon dioxide than warm water, as the oceans warm through global warming, the ocean becomes saturated and begins to release carbon dioxide back into the air. This in turn may cause air temperatures to rise and cause a ‘runaway’ temperature increase. Today, levels of carbon dioxide in the air are at their highest ever. Only about a half of new releases of carbon dioxide into the air are being ‘mopped up’ by the oceans. So the concentration of atmospheric carbon dioxide is increasing. (For more about the measure taken to control this and its impact on our environment see the Information Sheet on Global Warming)



Cycle 2: Water Just as there is a constant cycling of carbon in a ‘biological’ and ‘geological’ cycle, the same is true for water. Water is essential for all living things. Without it there could be no life as it forms an essential part of photosynthesis the process by which plants capture the sun’s energy and use it for living and growth. While water is ’cycled’ through living things it also becomes trapped or locked into rocks. Aquifers hold 99% of the world’s fresh water and while some are recharged by fresh water, many hold water that seeped into them millions of years ago. Namibia relies for nearly a third of its water on aquifers and is using this water faster today than it is being replenished.

The water cycle In any one year, heat from the sun is enough to evaporate about one a quarter metres of seawater. Two thirds of this vapour that escapes the sea falls back into it as rain, but the rest drops from

clouds that move over land. This runs off as rivers, soaks into aquifers, is trapped in dams or is taken up by plants and then released into the air through evapo-transpiration. Most of it is returned to the oceans within ten days, which means that often the same water is used over and over again. For millions of years, people made little impact on this natural cycle as they made use of perennial rivers, natural springs and lakes. Gradually however with population growth, industrialisation and the needs of urban dwellers for food and water, more and more water has been extracted from this natural cycle. Because global consumption is doubling every 20 years and new sources are becoming scarcer, within 30

years there will not be enough water to go round. At the rate the world is developing and population is growing, water according to the UN will become the most pressing environmental and developmental issue this century.



Human activity and the water cycle

• Agriculture uses 87% of all water globally with rice and cotton requiring the most water.

• Industry uses the next

• Domestic consumption Some environmental consequences of interrupting the cycle include:

• Decrease in groundwater supplies: 99% of all the world’s fresh water is found in aquifers. These are waterlogged seams of rock or gravel.. The deeper they are, the more costly it is to exploit them. While some aquifers are still recharged by rainfall, others contain water that became trapped in them thousands of years ago and cannot be replenished. As aquifers run dry, water has to be sourced further away from where it is required, increasing its cost.

• Increased pollution: Overpumping of groundwater has resulted in the little water that remains containing increased concentrations of pollutants. For example up to 1 million wells in Bangla Desh were found to contain arsenic resulting in slow poisoning, skin leasions and cancer for the 15 million people dependent on them.

• Industries have used water to wash away harmful chemicals used as part of the manufacturing process. As a result pesticides, nitrates, petrochemicals, fluorides, heavy metals and mining wastes pollute major aquifers in the ‘developed’ world.

• Disruption to ecosystems and wildlife: Wetland and river ecosystems have been disrupted through the damming of rivers and the pumping of too much groundwater often for irrigation.

• Conflicts between countries: Water resources have rarely been the sole source of violent conflict but they increasingly underlie international and regional tensions. According to the UN, 158 international river basins upon which millions of people depend for drinking water, irrigation, fisheries and hydropower

could prove to be future sources of conflict. Jordan, Israel and Turkey have long been in dispute over the use of the river Jordan. Bangla Desh disagrees with India’s plans to divert its rivers while in southern Africa, Zimbabwe, Zambia and Mozambique all have plans to divert and use the Zambezi that could have serious downstream effects.

• Disruption in livelihoods: Increasingly governments are thinking of bigger water supply schemes. Zimbabwe wishes to divert the Zambezi to bring water to Bulawayo; the Indian government plans to divert and link 14 major rivers including the Brahmaputra and the Ganges, evicting 3 million people as 300 reservoirs are built; China’s three gorges dam will create an inland sea 650km long relocating over 1m people.

Cycle 3: Nitrogen Nitrogen is essential to all life on earth. Nitrogen gas makes up 78% of the air we breathe. In its various forms it passes through a series of natural processes from the air, to the soil, to plants and animals and then back to the soil and the air . All life requires nitrogen compounds to make proteins and nucleic acids. But most organisms cannot use the nitrogen that is in the air. Plants must secure it in a ‘fixed’ or inorganic form such as nitrates (NO3), ammonia (NH3) or urea (NH2)2CO2. Animals secure their nitrogen (and all other) compounds from plants or animals that have fed on plants. Four processes participate in the cycling of nitrogen through the biosphere: Nitrogen fixation: Nitrogen (N2) is quite inert and must be broken apart before it combines with other atoms to form useful compounds. This can happen in three ways:

• Atmospheric fixation: where the energy released by lightning breaks apart the nitrogen molecules and enables their atoms to combine with oxygen forming nitrogen oxides. These dissolve in rain forming



nitrates that are carried to the ground. Atmospheric nitrogen contributes about 5-8% of all nitrogen fixed.

• Industrial fixation: through the Haber process (that imitates an electrical storm) where nitrogen is combined with hydrogen under great pressure and heat to make ammonia (NH3) Ammonia can be used as a fertiliser, but most of it is processed further to make urea and ammonium nitrate.

• Biological fixation: certain bacteria can fix nitrogen. Some live in a symbiotic relationship with plants of the legume family (soybeans, alfalfa) while others live in a similar relationship with other plants. eg acacia trees. Others exist freely in the soil while cyanobacteria are essential to maintaining the fertility of semi aquatic environments such as rice paddies.In the process ammonia is formed which is quickly incorporated into protein and other organic nitrogen compounds.

Decay: The proteins made by plants are eaten by herbivores. Carnivores then prey on herbivores for their protein. Most of this protein is used to make muscle and other tissue but some is used for energy and the nitrogen is not needed. Mammals excrete waste nitrogen as urea in their urine. Animal dung doesn’t contain much nitrogen based matter but the urine that is mixed with it is an important nitrogen source, though other material in the dung helps to conserve water and nutrients and adds to soil fertility. These materials are then used by two types of micro-organisms of decay: Nitrification: Nitrifying bacteria turn them first into ammonia, then to nitrites and finally to nitrates a form that plants can take up and use De-nitrification: In the absence of air, denitrifying bacteria break down the nitrogen compounds and release them back into the air as nitrogen molecules.



Human activity and the nitrogen cycle 500 years ago famine was a regular occurrence all over the world. In some years the crops would grow well and in other years the yield would not be as good and the food supply would not last. Over the years people discovered that by adding animal waste (particularly guano) to the soil, its fertility improved. By the 19th century however the guano from Peru was exhausted. So it was perhaps fortunate that soon after Fritz Haber discovered how nitrates could be made. Although the main reason for the research was to make salt peter for explosives, ammonium nitrate was also an important fertiliser. Today as a result of the ‘green revolution’ the use of artificial fertilisers is widespread. Agriculture is now responsible for around a half of all the nitrogen fixation taking place on earth through:

• The use of fertilisers

• The growing of legumes While yields have increased, the misuse or overuse of fertilisers has had serious environmental consequences. Eutrophication is the name given to the process that occurs in lakes, dams or bays when excess nitrates or phosphates get into the water. Although it is a natural process in the aging of lakes, human activities may greatly accelerate it. Agricultural run-off, urban run-off, the discharge from food factories, leaking septic and sewage systems and the discharge of sewage all increase the flow of these ‘nutrients’. When this is washed into a lake it has an excessive ‘fertilising’ effect causing algae to grow quickly or ‘bloom’. Algal blooms hurt water ecosystems in two ways: firstly they cloud the water and block sunlight underwater grasses to die. Because these grasses provide food and shelter for water organisms, the spawning and nursery habitat is destroyed and waterfowl have less to eat when grasses die off. Secondly when the algae die and decompose, oxygen is used up. Dissolved oxygen in the water is essential to most organisms living in water such as fish and crabs. In the absence of oxygen other bacteria multiply giving the water a bad

smell and rendering it undrinkable. In Namibia algal blooms have begun to occur in the larger dams particularly in early summer after inflows from ephemeral rivers. This increases the cost of water purification. Limiting fertiliser use treating sewage properly composting and recycling and controlling run-off all help to prevent eutrophication. Acid rain. Burning fossil fuels, forests and the increased use of road and air transport have all resulted in large quantities of nitrogen oxide (as well as other gases) being released into the air. As a result local concentrations of nitrogen oxide have risen so much that when dissolved in rain water it has been precipitated as nitric acid or ‘acid rain’. (For more about this process and its consequences see the Information Sheet on Acid Rain) Health Issues: We are exposed to nitrogen in the food we eat, the water we drink and the air we breathe. Drinking water sources are susceptible to watershed run-off. Ground water contamination from over-fertilisation of plant life, human and animal waste discharges can occur. Excessive amounts of nitrogen as nitrate in drinking water can cause health problems especially in babies and young infants. Removing the nitrates is expensive and adds to the cost of water.



Important ideas to stress in your teaching and learning The Carbon Cycle

• Carbon dioxide is normally found in very small amounts in the atmosphere

• Burning fossil fuels produces carbon dioxide.

• The carbon cycle consists of a natural balance between photosynthesis and respiration in living things

• Fossil fuels form when decay is prevented. Their formation happened slowly over millions of years, locking up huge amounts of carbon.

• Human beings have disturbed the natural balance by unlocking and burning fossil fuels as well as cutting and burning forests.

• Since today more carbon dioxide is being released than the carbon sinks can absorb, CO2 levels are rapidly increasing to their highest ever. Global warming with all its consequences is resulting.

The Water Cycle

• The amount of accessible fresh water in the earth is limited. Global consumption is doubling every 20 years. New sources are becoming expensive to develop and treat.

• Water supplies are not evenly distributed. Many countries already experience a water shortage and water costs are rising.

• Agriculture uses most water followed by industry and cities.

• Water supplies today are increasingly becoming polluted.

• The consequences of today’s growing water use include a disruption of groundwater supplies, ecosystems, wildlife and people’s livelihoods, increased pollution and an increased likelihood of conflict between cities, states and nations over access to water.

The Nitrogen Cycle

• All life requires nitrogen in order to live and grow (make proteins and nucleic acids)

• Nitrogen in the air is ‘fixed’ in a cycle that involves four processes: fixation, decay, nitrification and denitrification. These processes change nitrogen gas to nitrates , decaying materials to ammonia and then finally back to nitrates or nitrogen.

• Human activities have disturbed this cycle and added more nitrogen to both land and air through the manufacture of fertilisers, the burning of fossil fuels and car emissions.

• An excess of nitrates in the ground has resulted in lake eutrophication and excess nitrates contaminating water and food supplies. An excess of nitrogen oxide in the air has led to acid rain.

Glossary Acid rain rain that contains diluted acid which is derived from burning fossil fuels and is

harmful to the environment. Anaerobic living or taking place in the absence of oxygen, and not needing oxygen for

metabolism. Aquifer a layer of permeable rock, sand, or gravel through which groundwater flows. Biological relating to living organisms.



Cyanobacteria Bacteria belonging to a large group that carry out photosynthesis. Cycle A sequence of events that is repeated over and over. Ecosystem A group of interdependent organisms together with the environment they

inhabit and depend on. Eutrophication The process by which a body of water becomes rich in dissolved nutrients,

thereby encouraging the growth and decomposition of oxygen-depleting plant life and resulting in harm to other organisms.

Evapotranspiration The return of moisture to the air through both evaporation from the soil and transpiration by plants.

Fluorides Any chemical compound consisting of fluorine and another element or group. Fossil fuels Any carbon-containing fuel, for example coal, peat, petroleum, and natural gas,

derived form the decomposed remains of prehistoric plants and animals. Geological Relating to the structure of the Earth, its rocks, soil, minerals and origin. Global warming An increase in the world’s temperatures, believed to be caused by the

greenhouse effect and the depletion of the ozone layer. Hydropower Electric power generated using water power. Microbes A microscopic organism, especially one that transmits a disease. Nitrates A compound such as a salt with NO3 Organic/inorganic Relating to or derived from living things/ not derived from living substance. Ozone The layer of the upper atmosphere 15-50km above the Earth’s surface, where

atmospheric ozone collects. It serves to absorb harmful ultraviolet radiation from the Sun.

Perennial river A river that consistently runs throughout the year. Petrochemicals A substance derived form petroleum or natural gas, such as paraffin. Sedimentary Used to describe rocks formed form material, including debris of organic

material, deposited as a sediment by water, wind, or ice and then consolidated by pressure.

Sub-tropical Just below the tropics where it is very hot and humid. Symbiotic A cooperative, mutually beneficial relationship between two entities.

Sources/Further Reading Wikipedia encyclopedia One small step: The science of understanding environmental issues, Summers et al. Water Pollution, National Water Awareness Campaign. The world’s water, The Guardian Saturday August 23 2003 Earth, health check for a planet., The Guardian August 2002 Wikipedia encyclopedia

natural resources and their management environmental cycles

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