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Part of the Local Ecosystems Module Spotlight Biology Preliminary Text Chapter 4 Authors: D. Heffernan, J. Bastina, B. Grieve, K. Humphreys, A. Sartor.

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Presentation on theme: "Part of the Local Ecosystems Module Spotlight Biology Preliminary Text Chapter 4 Authors: D. Heffernan, J. Bastina, B. Grieve, K. Humphreys, A. Sartor."— Presentation transcript:

1 Part of the Local Ecosystems Module Spotlight Biology Preliminary Text Chapter 4 Authors: D. Heffernan, J. Bastina, B. Grieve, K. Humphreys, A. Sartor Science Press 2002 Ecosystems Topic 4.5 Continuned: Biogeochemical Cycles

2 The Nitrogen Cycle The nitrogen cycle is another important biogeochemical cycle. Primary producers absorb nitrogen in a simple form, usually as either ammonia (NH ₃ ), ammonium ions (NH ₄), or nitrate ions (NO ₃).

3 The Nitrogen Cycle Plants use energy from photosynthesis to concentrate nitrogen in their tissues and then use more solar energy to enable the combining of nitrogen, hydrogen, carbon, oxygen and other elements into proteins and amino acids (these are the complex molecules used as the building blocks in living systems)

4 The Nitrogen Cycle When animals eat plants or other animals, they digest proteins, breaking them down into their component nitrogen-containing amino acids. Some are re-formed into the proteins of the animal that has eaten them while others are broken down to release energy they contain releasing nitrogen in the form of ammonia.

5 The Nitrogen Cycle Because ammonia is toxic to many cells, it must be eliminated. Most aquatic animals excrete it into the water around them while terrestrial animals often convert nitrogen wastes into urea and concentrate it in urine before eliminating.

6 The Nitrogen Cycle Once the nitrogen is back into the environment, it does not last long. A group of bacteria called Nitrosomonas combine ammonia with oxygen and convert it into nitrite ions (NO ₂ ). This is then changed into nitrate (NO ₃) ions by bacteria called Nitrobacter. These two processes are called nitrification.

7 The Nitrogen Cycle Although they rarely appear in food-web diagrams, the bacteria and fungi of decay are vital to the nitrogen cycle. Nitrogen in dead animals and plants is useless to primary producers. Therefore these decomposers break up the organic molecules into simpler forms which plants are then able to use.

8 The Nitrogen Cycle When nitrate ions, nitrite ions and ammonia are released into the soil of a healthy forest or water filled with algae, they may quickly be reabsorbed by the primary producers. Many terrestrial plants absorb nitrate ions best which is why most fertilisers contain a lot of nitrogen as a nitrate compound. And the cycle starts over

9 The Nitrogen Cycle In the open sea, dead marine organisms and their nutrient rich solid wastes sink rapidly out of the surface layers. Large quantities of nutrients end up on the bottom of the ocean where primary producers cannot reach them.

10 The Nitrogen Cycle Some areas however have ocean currents which force this nutrient laden water back to the surface into a zone where photosynthesis can occur. This is called upwelling. In areas where this is steady, phytoplankton grow at rapid rates. Up to 50% of fish caught world wide come from upwelling. Why do you think this may be?

11 Earth’s Nitrogen Reserve While aquatic and terrestrial primary producers compete for available nitrogen, an enormous reservoir of gaseous nitrogen exists out of reach in the atmosphere. The paired atoms (N ₂ ) in nitrogen gas are held together by a covalent bond which cannot be broken down by most organisms.

12 Earth’s Nitrogen Reserve Only a few bacteria can break this bond and incorporate atmospheric nitrogen into living tissue. This process is known as nitrogen fixation (taking nitrogen gas from the air and turning it into living tissue). The most familiar are the nitrogen-fixing bacteria that live on the roots of plants (peas and soybeans). The most important nitrogen fixers in the sea are photosynthetic cyanobacteria (blue green algae).

13 Earth’s Nitrogen Reserve Denitrifying bacteria do the opposite. They convert organic nitrogen into nitrogen gas. The constant addition of nitrogen fertilisers to farm soil can encourage their growth. Why would this be a bad thing? (pumping nitrogen out of the soil into the atmosphere)

14 Review/Overview Plants and animals need nitrogen to make proteins. But they cannot get nitrogen directly from the air because, as a gas, nitrogen is fairly unreactive. Plants are able to take up nitrogen compounds such as nitrates and ammonium salts from the soil. Making nitrogen compounds from nitrogen in the air is called nitrogen fixation.

15 Earth’s Nitrogen Reserve Nitrogen fixation happens in three ways: The energy in lightning splits nitrogen molecules into individual nitrogen atoms. These react with oxygen to form nitrogen oxides. Nitrogen oxides are washed to the ground by rain, where they form nitrates in the soil. Nitrogen-fixing bacteria found in the soil and in the root nodules of leguminous plants, such as peas, beans and clover, fix nitrogen gas into nitrogen compounds. The Haber process is used by industry to produce ammonia from nitrogen and hydrogen. Ammonia is used to make nitrogen compounds that are used as fertiliser by farmers.

16 Homework 1. Outline what happens during nitrogen fixation 2. How does nitrification differ from denitrification 3. Where is most of the worlds nitrogen stored? 4. How are you part of the nitrogen cycle today? 5. Is the flow of nitrogen in balance?

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