Nutrient cycles

Succession

Succession is the change in an ecosystem over time. It includes abiotic (non-living) and biotic (living) factors.

A typical sequence of succession, which might occur over a period of 100-200 years, is shown here:

bare rock ⟶ lichens ⟶ mosses and liverworts (bryophytes) ⟶ grasses and small shrubs ⟶ fast growing trees ⟶ slower growing trees

Secondary succession

Secondary succession occurs where there has been a land clearance, perhaps by fire or landslip. An ecosystem has been established but is replaced as conditions have changed. Soil is already present so secondary succession is usually much quicker than primary succession and a variety of plants such as annual grasses and low-growing perennials can colonise rapidly.

Over time, a sequence of colonisation takes place. Some plants thrive, and are then replaced by others as the abiotic environment changes. The exact sequence depends on local conditions but eventually a stable community, known as the climax community develops.

Case study: Surtsey

Surtsey is an island that formed as a result of volcanic activity off the coast of Iceland in 1963. For the first 20 years after it was formed, the sands and lava of Surtsey were quite barren and soil development was poor. Mosses and lichens were found early on the island but few other species are adapted to such conditions. Shore plants that grow on sandy beaches were the first pioneers to colonise Surtsey and were characteristic of the vegetation during the first decades.

The first higher plant species found on Surtsey was sea rocket (Cakile arctica) in 1965 and in the following year sea lyme grass (Leymus arenarius) was found. In 1967, these species were joined by the oyster plant (Mertensia maritima).

Between 1977 and 1979, sea lyme grass and oyster plant started seeding and spreading throughout the sands and pumices of the island. Sea lyme grass is hardier species and is now one of the most common species present on Surtsey.

The first birds to nest on Surtsey were the fulmar (Fulmarus glacialis) and the black guillemot (Cepphus grille) in 1970. In 1974, they were joined by the great black-backed gull (Larus marinus), in 1975 by the kittiwake (Rissa tridactyla).

Dwarf willow (Salix herbacea) was discovered in 1995 and was the first willow species to colonise the island. The improved soil conditions following the gull colonisation are probably the main reason for this invasion of willows on Surtsey.

Northern green orchid (Platanthera hyperborean) and lady’s bedstraw (Galium verum) were found for the first time in the summer of 2003 and both these plants were found in lush vegetation in the gull colony where bird droppings have acted as a natural fertiliser. They are both examples of species that colonise land where vegetation has been developing for sometime.

Since 1963, the primary production and biomass of all species have increased significantly as stages of succession have taken place.

How living organisms influence the abiotic environment

Over time, an area that has been colonised as a result of a primary succession changes. The amount of soil increases, and so too does the diversity of plants, animals and other species. Some of the changes to the abiotic environment that result from the increasing presence of living organisms during primary succession are summarised below.

• Organic matter is released by plants and other organisms when they die and their tissues are broken down by decomposers. Animals also release organic matter in the form of faeces. This material forms humus in the soil, which provides a medium for plant growth.
• Soil depth increases as more organic matter accumulates and binds mineral fragments together.
• Soil structure improves as moisture and minerals are retained in it by the humus.
• Erosion is prevented as plant roots stabilise the ground.
• Minerals are recycled so that more nutrients are retained in the developing ecosystem, rather than being leached away by rainwater.

The carbon cycle

Carbon is one of the most important elements that are recycled in an ecosystem. Most of the chemicals that make up living tissue contain carbon. When organism die the carbon is recycled so that it can be used by future generations. Four main processes are involved: photosynthesis, respiration, decomposition, combustion. Inorganic carbon dioxide in the atmosphere is trapped or ‘fixed’ as organic carbon compounds during photosynthesis. Some of this carbon is soon returned to the atmosphere as the plants respire. The other steps in the cycle follow the same path as food chains. As herbivores eat plants, and carnivores eat herbivores, the carbon compounds move from plants to animals. Respiration by any organism in this sequence returns carbon to the atmosphere as carbon dioxide and when a plant or animal dies, carbon compounds that move to detritivores and saprotrophs may also be respired.

In some conditions, plants and animals do not decay when they die. They become compressed and fossilised in a process that takes millions of years and forms fossil fuels. Vast coal, oil and natural gas deposits have been formed and the carbon trapped in these fuels cannot return to the atmosphere unless the fuels are burned. Over a very long period of time, fossil fuel formation has gradually lowered the carbon dioxide level of the Earth’s atmosphere, but in more recent times this balance has been upset.

Removal of carbon dioxide from the atmosphere

Photosynthesis

Green plants remove carbon dioxide from the atmosphere as a result of their photosynthesis. The carbon from the carbon dioxide is built first into a carbohydrate such as sugar. Some of this is changed into starch or the cellulose of cell walls, and the proteins, pigments and other compounds of a plant. When the plants are eaten by animals, the organic plant material is digested, absorbed and built into the compounds making up the animals tissues. Thus the carbon atoms form the plant become part of the animal.

Fossilisation

Any environment that prevents rapid decay may produce fossils. The carbon in the dead organisms becomes trapped and compressed and can remain there for millions of years. The carbon may form fossil fuels such as coal, oil and natural gas. Some animals make shells or exoskeletons containing carbon and these can become fossils.

Addition of carbon dioxide to the atmosphere

Respiration

Plants and animals obtain energy by oxidising carbohydrates in their cells to carbon dioxide and water. The carbon dioxide and water are excreted so the carbon dioxide returns once again to the atmosphere.

Decomposition

A crucial factor in carbon recycling is the process of decomposition, or decay. If it were not for decay, essential materials would not be release from dead organisms. When an organism dies, the enzymes in its cells, freed from normal controls, start to digest its own tissues (auto-digestion). Soon, scavengers appear on the scene and eat much of the remains; blowfly larvae devour carcases, earthworms consume dead leaves.

Finally the decomposers, fungi and bacteria (collectively called micro-organisms), arrive and invade the remaining tissues. These saprophytes secrete extracellular enzymes into the tissues and reabsorb the liquid products of digestion. When the micro-organisms themselves die, auto-digestion takes place, releasing the products such as nitrates, sulfates, phosphates, etc. into the soil or the surrounding water to be taken up again by the producers in the ecosystem.

The speed of decay depends on the abundance of micro-organisms, temperature, the presence of water and, in many cases, oxygen. High temperatures speed up decay because they speed up respiration of the micro-organisms. Water is necessary for all living processes and oxygen is needed for aerobic respiration of the bacteria and fungi. Decay can take place in anaerobic conditions but it is slow and incomplete, as in the waterlogged conditions of peat bogs.

Combustion (burning)

When carbon-containing fuels such as wood, coal, petroleum and natural gas are burned, the carbon is oxidised to carbon dioxide (C + O₂ ⟶ CO₂). The hydrocarbon fuels, such as coal and petroleum, come from ancient plants, which have only partly decomposed over the millions of years since they were buried.

So, an atom of carbon which today is in a molecule of carbon dioxide in the air may tomorrow be in a molecule of cellulose in the cell wall of a blade of grass. When the grass is eaten by a cow, the carbon atom may become part of a glucose molecule in the cow’s bloodstream. When the glucose molecule is used for respiration, the carbon atom will be breathed out into the air once again as carbon dioxide.

The same kind of cycling applies to nearly all the elements of the Earth. No new matter is created, but it is repeatedly rearranged. A great proportion of the atoms of which you are composed will, at one time, have been part of other organisms.

The effects of combustion of fossil fuels

If you look back at the carbon cycle, you will see that the natural processes of photosynthesis, respiration and decomposition would be expected to keep the CO₂ concentration at a steady level. However, since the Industrial Revolution, we have been burning the fossil fuels such as coal and petroleum and releasing extra CO₂ into the atmosphere. As a result, the concentration of CO₂ has increased from 0.029% to 0.035% since 1860. It is likely to go on increasing as we burn more and more fossil fuel.

Photosysnthesis takes CO₂ out of the atmosphere and replaces it with O₂ . Respiration and combustion both do the opposite: they use up O₂ and replace with CO₂.

The equations are essentially the same but reversed:

Although it is not possible to prove beyond all reasonable doubt that production of CO₂ and other ‘greenhouse gases’ is causing a rise in the Earth’s temperature, i.e. global warming, the majority of scientists and climatologists agree that it is happening now and will get worse unless we take drastic action to reduce the output of these gases.

Another factor contributing to the increase in atmospheric CO₂ is deforestation. Trees are responsible for removing gaseous CO₂ and trapping the carbon in organic molecules (carbohydrates, proteins and fats). When they are cut down the amount of photosynthesis globally is reduced. Often deforestation is achieved by a process called ‘slash and burn’, where the felled trees are burned to provide land for agriculture and this releases even more atmospheric CO₂.

The water cycle

The water cycle is somewhat different from other cycles because only a tiny proportion of the water that is recycled passes through living organisms.

Animals lose water by evaporation, defecation, urination and exhalation. They gain water from their food and drink. Plants take up water from the soil and lose it by transpiration. Millions of tonnes of water are transpired, but only a tiny fraction of this has taken part in the reactions of respiration or photosynthesis.

The great proportion of water is recycled without the intervention of animals or plants. The Sun shining and the wind blowing over the oceans evaporate water from their vast, exposed surfaces. The water vapour produced in this way enters the atmosphere and eventually condenses to form clouds. The clouds release their water in the form of rain or snow (precipitation). The rain collects in streams, rivers and lakes and ultimately finds its way back to the oceans. The human population diverts some of this water for drinking, washing, cooking, irrigation, hydroelectric schemes and other industrial purposes, before allowing it to return to the sea.

Environmental change and the water cycle

Environmental changes affect the distribution of Organism

Environmental changes can cause the distribution of organisms to change. A change in distribution means change in where an organism lives. Environmental changes that can affect organsims in this way include:

A change in the availability of water. For example:

The distribution of some animal and plant species in the tropics changes between the wet and the dry season – i.e. the times of year where there is more or less rainfall, and so more or less water available. E.g. each year in Africa, large numbers of giant wildebeest migrate, moving north and then back south as the rainfall patterns change.

A change in the temperature. For example:

The distribution of bird species in Germany is changing because of a rise in average temperature. E.g. the European bee-eater bird is a Mediterranean species but it’s now present in parts of Germany.

A change in the composition of atmospheric gases. For example:

The distribution of some species changes in areas where there is more air pollution. E.g. some species of lichen can’t grow in areas where sulfur dioxide is given out by certain industrial processes.

These environmental changes can be caused by seasonal factors, geographic factors or human interaction. For example, the rise in average temperature is due to global warming, which has been caused by human activity.

The nitrogen cycle

When a plant or animal dies, its tissues decompose, partly as a result of the action of saprotrophic bacteria. One of the important products of the decay of animal and plant protein is ammonia (NH₃, a compound of nitrogen), which is washed into the soil. It dissolves readily in water to form ammonium ion NH_4^–).

The excretory products of animals contain nitrogenous waste products such as ammonia, urea and uric acid. Urea is formed in the liver of humans as a result of deamination. The organic matter in animal droppings is also decomposed by soil bacteria.

Processes that add nitrates to soil

Nitrifying bacteria

These are bacteria living in the soil, which use the ammonia from excretory products and decaying organisms as a source of energy (as we use glucose in respiration). In the process of getting energy from ammonia, called nitrification, the bacteria produce nitrates. Nitrification is thus the oxidation of ammonia to nitrate. This is carried out in the soil by Nitrosomonas, which uses oxygen to convert ammonia to nitrite, and also by Nitrobacter, which also uses oxygen and converts nitrite to nitrate. This is important part of the cycle because both ammonia and nitrite are toxic to plants. P

• The ‘nitrite’ bacteria oxidise ammonium compounds to nitrites (NH_4^– ⟶ NO_2^–)
• ‘Nitrate’ bacteria oxidise nitrites to nitrates (NO_2^– ⟶ NO_3^–).

Although plant roots can take up ammonia in the form of its compounds, they take up nitrates more readily, so the nitrifying bacteria increase the fertility of the soil by making nitrates available to the plants. Nitrification is favoured by neutral pH, warmth and well-aerated soil, as it is an oxidative process.

Nitrogen-fixing bacteria

Lightning

The high temperature of lightning discharge causes some of the nitrogen and oxygen in the air to combine and form oxides of nitrogen. These dissolve in the rain and are washed into the soil as weak acids where they form nitrates. Although several million tonnes of nitrate may reach the Earth’s surface in this way each year, this forms only a small fraction of the total nitrogen being recycled.

Processes that remove nitrates from the soil

Uptake by plants

Plant roots absorb nitrates from the soil and combine them with carbohydrates to make amino acids, which are built up into proteins. These proteins are then available to animals, which feed on the plants and digest the proteins in them.

Leaching

Nitrates are very soluble (i.e. dissolve easily in water), and as rainwater passes through the soil it dissolves the nitrates and carries them away in the run-off or to deeper layers of the soil. This is called leaching.

Denitrifying bacteria

These are bacteria that obtain their energy by breaking down nitrates to nitrogen gas, which then escapes from the soil into the atmosphere. Denitrification is the conversion of useful nitrate in the soil to nitrogen. Pseudomonas denitrificans completes the cycle by converting nitrite and nitrate into gaseous nitrogen. This reduces the fertility of the soil. Denitrification is favoured by anaerobic conditions found in compacted or waterlogged soils, with a high nitrogen input.

The pollution of waterways

Raw sewage

Raw sewage produced by human activity is high in nitrates and phosphates and also contains many pathogens. If it is released into rivers and streams that are used for drinking water or bathing, diseases such as cholera and typhoid can easily be spread.

Nitrate and phosphates in sewage also cause ecological problems if raw sewage leaks into rivers and streams. In time, the river may recover as long as no other pollution occurs.

1. Saprotrophic bacteria and fungi feed on the organic material in the raw sewage as a source of nutrients, and multiply. These aerobic organisms use up a large amount of oxygen and reduce its concentration in the water. They are said to have a high biochemical oxygen demand (BOD).
2. When the oxygen level drops, river organisms, including fish and many invertebrates that are highly dependent on high oxygen levels, die or move to another unpolluted areas if they can.
3. Death and decay of the sensitive organisms leads to a build up of ammonia, phosphate and minerals.
4. Ammonia is converted to nitrate and with this increased concentration of nutrients, algae reproduces rapidly. This is known as eutrophication.

5. In time, the increased photosynthesis by the large amounts of algae that use the nitrate to grow restores the levels of oxygen, so the river returns to normal.
6. If the algae produce an algal bloom and then die and decay, this may cause a cycle of events that reduce oxygen concentration again and lead to death of other organisms, so that the river takes longer to recover. If the algae do not die, then the river can recover from sewage pollution, although this may be several kilometres downstream.

Nitrate fertilisers in rivers

Farmers are sometimes blamed for causing eutrophication through inappropriate use of fertilisers, which are high in nitrates and phosphates. Excess fertiliser can run off the land and flow into rivers.

1. Nitrates and phosphates are very soluble and cause algae in water to proliferate.
2. If algae grow very rapidly, they may form an algal bloom, which deprives other plants of light so that they die. Saprotrophs decompose the dead plants and decrease levels of oxygen in the water, as there is high BOD.
3. Oxygen-dependent organisms become threatened, and die or move away.

In many countries, fertiliser use is controlled, and in modern farming the requirements of crop plants are closely monitored. It is difficult to blame farmers if, for example, it rains heavily after fertiliser is applied, so much of it passes through the soil to ground water, before crop plants have absorbed it.

The treatment of sewage

Saprotrophic bacteria play an important role in the treatment of sewage using trickle filter beds and reed bed systems.

Sewage treatment is very important for human health and different methods are used in different countries. Inorganic material is removed first and then bacteria are used to break down and remove the organic material.

Trickle filter beds

A trickle filter bed consists of a large tank containing a layer of gravel or clinker, which provides a large surface area on which a film of saprotrophic bacteria can grow. Raw sewgae is sprayed onto the rocks while keeping the environment well aerated. The sprayer is usually turned in a circle over the rocks so there is even coverage.

The bacteria feed on the organic material in sewage and break it down to simple inorganic compounds such as nitrates and phosphates.

The effluent is sent to a second tank where the bacteria sink to the bottom as sludge and are removed. One problem with this system is that the purified water may contain relatively high concentrations of nitrate and phosphate.

Reed beds

A reed bed system can also be used to treat sewage. Reeds are grown on sand or gravel medium, which maintains an oxygenated area for the roots of the plants and the saprotrophic bacteria in the medium. Sewage flows into the reed bed and the bacteria break down organic material to nitrates and phosphate, which can be taken in by the plant roots. The nutrients provided for the reeds enable them to grow; they are later harvested and composted for fertiliser. While this system solves the problem of high levels of nitrates and phosphate in the purified water, reed beds must be large to be effective as they can only cope with a limited flow of sewage.

Biomass for fuel

Biomass in the form of wood and agricultural waste, such as straw and animal manure, already provides a useful source of fuel. Now many countries are looking at the use of biofuels to reduce their dependence on foosil fuels. Biofuels are produced by converting biomass into ethanol or methane. This is done in bioreactors, either on a large industrial scale or on a domestic scale at a farm or in a village. A simple, small scale bioreactor is shown in Figure.

Methane produced from animal manure and agricultural waste is known as biogas. Manure and straw are fed into the bioreactor, where they decompose anaerobically as different groups of bacteria present in the manure break down the organic material. The slurry that remains is a useful fertiliser.

Three groups of bacteria produce the enzymes that digest organic material, and each stage breaks down the complex carbohydrates, fats and proteins into simpler compounds.

• Organic material is first converted to organic acids and ethanol by anaerobic, acidogenic bacteria, which occur naturally in manure.
• Acetogenic bacteria then use the organic acids and alcohol to produce acetate, carbon dioxide and hydrogen.
• Finally, methanogenic bacteria produce methane either from carbon dioxide and hydrogen or by breaking down acetate:

carbon dioxide + hydrogen ⟶ methane + water

CO₂ + 4H₂ ⟶ CH₄ + 2H₂O

acetate ⟶ methane + carbon dioxide

CH₃COOH ⟶ CH₄ + CO₂

The sequence of these processes is shown in Figure below

Biodiesel made from oils and fats is most often used as a diesel additive to reduce levels of emissions, and is the most common biofuel used in Europe. In 2008, biofuels accounted for almost 2% of world’s transport fuel.

Greenhouse gases, human activity and global warming

Carbon dioxide currently forms only 0.04% of the atmospheric gases but it plays a significant part in the greenhouse effect. Other greenhouse gases include water vapour, methane, oxides of nitrogen and fluorocarbons (FCs). Chlorofluorocarbons (CFCs) were used in aerosols and as refrigerants but were found to damage the ozone layer when released into the atmosphere. They are being replaced by hydrofluorcarbons (HFCs), but this is leading to an additional problem because HFCs are greenhouse gases.

The human population has increased dramatically in recent history, with a consequent increase in demand for energy in industry, transport and homes. Most of this energy demand has been met by burning fossil fuels, mainly oil, coal and gas. Burning fossil fuels releases both carbon dioxide and oxides of nitrogen. This activity has raised the concentration of carbon dioxide in the Earth’s atmosphere by more than 20% since 1959 see Figure and Table below.

Year	Carbon dioxide concentration/parts per million (ppm)	% increase from the previous value	% increase from 1959
1959	316
1965	320	1.3	1.3
1970	326	1.9	3.2
1975	331	1.5	4.7
1980	339	2.4	7.3
1985	346	2.1	9.5
1990	354	2.3	12.0
1995	361	2.0	14.2
2000	369	2.2	16.8
2002	373	1.1	18.0
2004	378	1.3	19.6
2006	382	1.1	20.9
2008	386	1.0	22.2

In the tropical regions of the world vast rainforests trap carbon dioxide through photosynthesis and have been important in maintaining the low level of atmospheric carbon dioxide. Humans have upset this balance by deforesting vast areas of forest for agriculture and timber production. Forest destruction has multiple effects, but the important for the atmosphere are the loss of carbon dioxide uptake by photosynthesis and the increase in carbon dioxide released from the rotting or burnt vegetation.

Methane is another important greenhouse gas. It is produced by human activity when organic waste decomposes in waste tips. It also comes from rice paddies and from cattle farming. More rice is being planted as the human population increases and more cattle are being farmed for meat. Cattle release methane from their digestive systems as they process their food.

Climatologists are concerned that, as a result of all this activity, humans are adversely affecting our atmosphere. Rising levels of greenhouse gases are believed to be causing an enhancement of the natural greenhouse effect. Scientists have shown that the Earth is experiencing a rise in average global temperature, known as global warming, which is thought to be happening because of the enhanced greenhouse effect.

Some possible results of global warming might be:

• melting of ice caps and glaciers
• a rise in sea levels, causing flooding to low-lying areas
• changes in the pattern of the climate and winds – climate change – leading to changes in ecosystems and the distribution of plants and animals
• increase in photosynthesis as plants receive more carbon dioxide.

The precautionary principle

The precautionary principle suggests that if the effect of a change caused by humans is likely to be very harmful to the environment, action should be taken to prevent it, even though there may not be sufficient data to prove that the activity will cause harm. The precautionary principle is most often applied to the impact of human actions on the environment and human health, as both are complex and the consequences of action may be unpredictable.

One of the cornerstones of the precautionary principle, and a globally accepted definition, comes from the Earth Summit held in Rio in 1992.

‘In order to protect the environment, the precautionary approach shall be widely applied by States according to their capabilities. Where there are threats of serious or irreversible damage, lack of full scientific certainty shall not be used as a reason for postponing cost-effective measures to prevent environmental degradation.’

There are many warning signs to indicate that climate change will have a serious effect on ecosystems across the world. It is clear that global temperatures are increasing and there is a significant probability that this is caused by human activities. It is also likely that weather patterns will alter and cause changes in sea levels and the availability of land for farming. The precautionary principle challenges governments, industries and consumers to take action without waiting for definitive scientific proof to be forthcoming.

Evaluating the precautionary principle

Should the precautionary principle be used to justify action to reduce the impact of the release of greenhouse gases into the atmosphere before irreparable harm is done? Here are some arguments to discuss:

• Global warming has consequences for the entire human race and an international solution is needed to tackle the problems. It is not always the case that those who produce the most greenhouse gases suffer the greatest harm so it is essential that measures to reduce emissions are taken with full international cooperation.
• If industries and farmers in one area invest money to reduce their greenhouse gases while those in other areas do not, and economic imbalance may be created in favour of the more polluting enterprises, who can offer services more cheaply.
• Consumers can be encouraged to use more environmentally friendly goods and services.
• Scientists can argue that it is better to invest in a sustainable future and prevent further harm.
• It is unethical for one generation to cause harm to future generations by not taking action to address the problem of greenhouse gases.

Global temperature rise and Arctic ecosystems

The ecosystem of the Arctic include the tundra, permafrost and the sea ice, a huge floating ice mass surrounding the North Pole. (Figure below)

Scientists studying these regions have recorded considerable changes in recent years. Average annual temperatures in the Arctic have increased by approximately double the increase in global average temperatures and the direct impacts of this include melting of sea ice and glaciers.

Melting sea ice affects many species. Algae, which are important producers in Arctic food chains, are found just beneath the sea ice. As the ice disappears, so do the algae and this affects numerous other organisms that use them as food.

Populations of marine mammals, caribou and polar bears are also affected and have already been forced to adapt to changes in their habitats. According to scientists, the retreat of sea ice has reduced the platform that seals and walruses traditionally use to rest between searches for fish and mussels. Caribou are falling through once-soild sea ice and polar bears, which live on sea ice while hunting their prey, now have shorter feeding periods and decreased access to the seals that they hunt. Polar bear numbers are decreasing at an alarming rate.

Forest and tundra ecosystems are important features of Arctic environment. In Alaska, substantial changes in forest life, including increases in insect pests, have been observed. Rising temperatures have allowed spruce bark beetles to reproduce rapidly and one outbreak of the beetles caused the loss of over 2.3 million acres of trees. Outbreaks of other leaf-eating insects in the boreal forest, such as spruce budworm and larch sawfly, have also increased sharply. In addition, some species from temperate climates are extending their ranges to the north. One insect causing concern is the mosquito, which is responsible for the transmission of malaria.

Detritus trapped in frozen tundra is released as the ground thaws. The detritus decomposes and the carbon dioxide and methane produced are released into the atmosphere, contributing to rising greenhouse gas levels.