13. Decay and Recycling - IGCSE Coordinated Sciences
1 of 46 Boardworks Ltd 2007 What happens to nutrients? Throughout an ecosystem, energy is constantly lost by wasted heat. In contrast, nutrients are constantly recycled through the carbon cycle and the nitrogen cycle. nutrients in the soil and atmosphere energy and biomass
energy and biomass microbial decomposition of dead organisms and waste 2 of 46 Boardworks Ltd 2007 What is decay? In all ecosystems, dead organisms and waste material are broken down by bacteria and fungi called decomposers. This process is decay or decomposition, and it releases nutrients back into the environment ready to be reused by other organisms.
Some food chains have decaying matter as the first stage. Under what conditions will decay occur the fastest? A warm, moist, oxygen-rich environment is the most favourable for decay to occur. 3 of 46 Boardworks Ltd 2007 How do humans use micro-organisms? The average person in the UK creates just over half a tonne of waste a year. How are micro-organisms used to break some of this down? Decomposers break down plant waste to
make compost. Micro-organisms are used in sewage plants to break down human waste. Biodegradable plastics are broken down by micro-organisms. 4 of 46 Boardworks Ltd 2007 Why is nitrogen so important? Nitrogen is essential for growth because it is used by plants and animals to make proteins.
Nitrogen makes up about 78% of the atmosphere. However, nitrogen deficiency is the most common cause of poor plant growth. Why are plants unable to use the nitrogen straight from the air? Nitrogen gas (N2) is unreactive and is not easily converted into other compounds. Most plants can only take up nitrogen in the form of ammonia or nitrate. How is atmospheric nitrogen changed into a useable form? 5 of 46 Boardworks Ltd 2007 What is the nitrogen cycle?
6 of 46 Boardworks Ltd 2007 Labelling the nitrogen cycle 7 of 46 Boardworks Ltd 2007 Components of the nitrogen cycle 8 of 46
Boardworks Ltd 2007 How are nitrates produced? Nitrates are important because they are a form of nitrogen that plants can absorb. Nitrogen is used to make protein, and is passed from plants to animals along food chains. What processes add nitrates to the soil? Decomposers release ammonium compounds from waste (such as urine) and dead matter. Nitrifying bacteria then convert the ammonium compounds into nitrates. Some nitrogen compounds form during lightning strikes and are washed into the soil by rain water. Lightning provides the high level of energy required for
nitrogen to react and form compounds. 9 of 46 Boardworks Ltd 2007 Can plants add nitrogen to the soil? This rugged place is in Iceland. Deforestation by the original settlers and high levels of volcanic activity have left much of the country as bare lava or sand; an environment in which few plants grow. In the 1960s, the country
began to manage its soil, and dropped millions of lupin seeds from the air. Why might lupins survive where nothing else grows? 10 of 46 Boardworks Ltd 2007 What are legumes? Most plants need nitrates from the soil because the nitrogen in air is too unreactive. These plants rely on the presence of nitrifying bacteria in the soil or artificial fertilizers. By contrast, lupins and other legumes, such
as clover and peas, are self-sufficient. Nitrogen-fixing bacteria living in the root nodules of legumes convert nitrogen gas into nitrates, improving the fertility of poor-quality soils. 11 of 46 Boardworks Ltd 2007 How can nitrates be added to soil? Nitrates are vital for plant growth, but levels in the soil are gradually
depleted as crops grow. What methods can farmers use to increase soil nitrate levels? Modern, intensive farming uses artificial fertilizers. These are made by the Haber process. However, run-off into nearby rivers and lakes can cause eutrophication. Organic farming uses manure a natural fertilizer. Crop rotation, a system that varies the crops planted each season, is also used to maintain soil fertility. 12 of 46 Boardworks Ltd 2007 Keeping soil fertile
13 of 46 Boardworks Ltd 2007 Why is carbon important? Proteins, fats and sugar all contain carbon. Life without carbon would be very different and might be impossible. Carbon is present in the atmosphere as carbon dioxide. Plants use carbon dioxide during photosynthesis to produce sugars. The carbon is then transferred to animals along food chains. What happens to the carbon in organisms when they die? As dead matter decomposes, carbon is released back
into the atmosphere in the form of carbon dioxide. The carbon from dead organisms can also form fossil fuels and sedimentary rocks such as limestone. These are long-term carbon stores. 14 of 46 Boardworks Ltd 2007 What is the carbon cycle? 15 of 46 Boardworks Ltd 2007 Labelling the carbon cycle
16 of 46 Boardworks Ltd 2007 How is carbon recycled? Carbon is constantly recycled by photosynthesis and respiration. carbon dioxide + water In a sealed ecosphere, photosynthesis
carbon dioxide concentrations biomass + algae shrimp fluctuate but the respiration respiration oxygen mean level does not change. How are carbon dioxide levels changing in the atmosphere of the Earth?
17 of 46 feeding Boardworks Ltd 2007 Are carbon dioxide levels rising? CO2 conc. (ppm) Although the total amount of carbon in the environment is fixed, carbon dioxide levels are constantly fluctuating. 400 300 200
100 0 1960 1980 2000 Currently, the general trend shows an increasing level of carbon dioxide. Why might this be happening?
year Many scientists believe that human activity, such as burning fossil fuels and making cement from limestone, is responsible for increasing carbon dioxide levels. What environmental problems might this cause? 18 of 46 Boardworks Ltd 2007 Carbon: true or false? 19 of 46
Boardworks Ltd 2007 Glossary (1/2) carbon cycle The process by which carbon is continuously recycled in the environment. denitrifying bacteria Soil bacteria that convert nitrates into nitrogen gas. fertilizer A chemical added to soil to provide essential mineral salts supporting plant growth. 20 of 46 Boardworks Ltd 2007
Glossary (2/2) legume A group of plant species that contain nitrogenfixing bacteria in their roots and can therefore make their own nitrates. nitrifying bacteria Soil bacteria that convert ammonium ions from protein decomposition into nitrates. nitrogen cycle The process by which nitrogen is continuously recycled in the environment. nitrogen-fixing bacteria Bacteria that live in the roots of legumes and convert nitrogen gas into nitrates. 21 of 46
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