Nutrient cycles Flashcards
What is the greenhouse effect?
A natural process that occurs all the time and keeps average global temperatures at around 17 degrees - without it the average temperature of the Earth would be around -18 degrees
This effect is the result of the heat and light of the Sun (solar radiation) that reaches our planet
Some solar radiation is reflected back into space, some absorbed by the atmosphere, and some reaches the Earth’s surface
Some of this radiation reaching the Earth’s surface is reflected back as heat and is lost into space, however, some is radiated back to Earth by clouds and the ‘greenhouse gases’ that form part of the atmosphere
The gases trap this heat beneath them close to the Earth’s surface keeping it warm
Carbon dioxide as a greenhouse gas
It is the most important gas as there is so much of it and it remains in the atmosphere for the longest time - around 100 years
It’s estimated that 50-70% of global warming is due to carbon dioxide in the atmosphere
It is mainly the result of human activities that the concentration of carbon dioxide is increasing - enhancing the greenhouse effect and causing environmental concerns
Methane as a greenhouse gas
Produced when microorganisms break down the organic matter of which organisms are made
This occurs mostly in 2 situations:
- when decomposers break down the dead remains of organisms
- when microorganisms in the intestines of primary consumers eg, cattle, digest the food that has been eaten
What is global warming?
Since 1900 the Earth’s temperature has increased by 0.6 degrees - a change known as global warming
As the Earth has always shown periodic fluctuations in temperature, we cannot be certain that this change is due to human activity eg, burning fossil fuels, deforestation and generating additional carbon dioxide
Consequences of global warming
- Melting of polar ice caps could cause extinction of some wild plants and animals eg, polar bears, and cause the sea level to rise
- A rise in sea level due to thermal expansion of oceans could flood low-lying land and flood many major cities - salt water would extend further up rivers and make cultivation of crop plants difficult
- Higher temperatures and less rainfall could lead to the failure of present crops in some areas - more drought-resistant species would have to be grown and land may not sustain crops at all. The distribution of wild plants in these areas would naturally change with only xerophytes being able to survive (species that feed on xerophytes and can sustain hot dry conditions would move in)
- Greater rainfall and intense storms would occur due to the disturbance of climatic patterns - distribution of plants and animals would change in favour of those adapted to withstand such conditions
- Life-cycles and populations of insect pests would alter as they adapt to the changed conditions. As insects carry many human and crop pathogens, tropical diseases could spread towards the poles
Benefits of global warming
- Increased rainfall would fill reservoirs
- Warmer temperatures would allow crops to be grown where it is presently too cold
- The rate of photosynthesis and productivity would increase so may be possible to harvest twice a year instead of once
Why does intensive food production make large demands on the soil?
Mineral ions are continually taken up by the crops being grown on it and are either used directly as food fodder (eg, straw/hay) for animals that are directly eaten
Why is it necessary to replenish mineral ions in agricultural ecosystems?
In agricultural ecosystems the crop is harvested then transported from its point of origin for consumption so the urine, faeces and dead remains of the consumer are rarely returned to the same area of land so the levels of mineral ions will fall and become a limiting factor to plant growth therefore reduce productivity
What are the two types of fertilisers?
- Natural (organic) fertilisers - consist of the dead and decaying remains of plants and animals as well as animal wastes such as manure and bone
- Artificial (inorganic) fertilisers - mined from rocks and deposits and then converted into different forms and blended together to give the appropriate balance of minerals for a particular crop. Compounds containing the three elements (nitrogen, phosphorus and potassium) are almost always present
How do fertilisers increase productivity?
Plants require minerals for growth eg, nitrogen which is needed for proteins and DNA
Where nitrates are readily available, plants are likely to develop earlier, grow taller and have a greater leaf area thus increasing the rate of photosynthesis and improving crop productivity
Nitrogen fertilisers have been a benefit in providing us with cheaper food
Harmful effects due to nitrogen-containing fertilisers
- Reduced species diversity - nitrogen-rich soils favour the growth of grasses, nettles and other rapidly growing species which out-compete many other species which die as a result
- Leaching - may lead to pollution of watercourses
- Eutrophication - caused by leaching of fertiliser into watercourses
Explain leaching
The process by which nutrients are removed from the soil
Rain water will dissolve any soluble nutrients (such as nitrates) and carry them deep into the soil, eventually beyond plant roots
The leached nitrates find their way into watercourses eg, streams/rivers that may drain into freshwater lakes
They can have a harmful effect on humans if the lake is a source of drinking water
High nitrate levels in drinking water can prevent efficient oxygen transport in babies and a link to stomach cancer in humans has been suggested
Leached nitrates are also harmful to the environment and can cause eutrophication
Explain eutrophication
The process by which nutrients build up in bodies of water - a natural process that occurs mostly in freshwater lakes and the lower reaches of rivers
It consists of the following events:
- In most lakes and rivers there is naturally very little nitrate and so it is a limiting factor for plant/algal growth
- As the nitrate concentration increases due to leaching, it ceases to be a limiting factor so plants and algae grow exponentially
- Algae mostly grow at the surface so the upper layers of water become densely populated with algae - an algal bloom
- This dense surface layer of algae absorbs light and prevents it from penetrating to lower depths
- Light then becomes the limiting factor for growth of plants and algae at lower depths so they eventually die
- The lack of dead plants and algae is no longer a limiting factor for the growth of saprobiotic bacteria and so these grow exponentially, using the dead organisms as food
- The saprobiotic bacteria require oxygen for their respiration, creating an increased demand
- The concentration of oxygen in the water is reduced and nitrates are released from the decaying organisms
- Oxygen then becomes the limiting factor for aerobic organisms such as fish so they die as the oxygen is used up altogether
- Without the aerobic organisms, there is less competition for anaerobic organisms, whose population now rises exponentially
- Anaerobic organisms further decompose dead material, releasing more nitrates and some toxic wastes such as hydrogen sulphide which makes the water putrid
Other than leaching of artificial fertilisers, what can lead to eutrophication?
Organic manures Animal slurry Human sewage Ploughing old grassland Natural leaching
