THE CYCLING OF NUTRIENTS

Transcription

1 Unit 4 THE CYCLING OF NUTRIENTS LEARNING OBJECTIVES 1. Recognize the need for the recycling of the earth s chemicals and the consequences if this is not done. 2. Learn the difference between a global cycle and a local cycle and the significance of both. 3. Be able to outline the hydrologic cycle. 4. Be able to outline the carbon cycle and understand the effects of plants and animals on this cycle. 5. Understand the relationship between the carbon cycle and the earth s temperature. 6. Obtain a general understanding of the nitrogen cycle. 7. Obtain a general understanding of the phosphorus cycle. INTRODUCTION The concept of earth as a spaceship helps one to realize that there is a limited amount of chemicals and unless they are recycled they will soon be exhausted. Because certain chemicals get tied up in living organisms, death is a necessary part of recycling. And although organisms in nature have always recycled, it is only in recent years that humans have realized the importance and necessity for recycling. Scarcity of certain minerals and the filling of landsites have been major factors in bringing the danger of straight lining the earth s chemicals to our attention. Nature has provided us with convenient ways of recycling certain chemicals such as carbon, nitrogen, water, phosphorus, etc, if we don t interfere with natural processes. Humans have the ability to make major changes in an ecosystem and to take many chemicals out of circulation. Hopefully, we will combine our abilities with good ecological judgment so that we assist, or at least don t damage, the recycling of materials through an ecosystem. In this unit, we will examine recycling of several major chemicals. TWO GENERAL TYPES OF BIOGEOCHEMICAL CYCLES Recall that the earth receives an inexhaustible supply of energy from the sun; but it has a limited supply of chemical elements. This requires the recycling of essential elements. There are major elements that are found in organic molecules, which are important constituents in all living organisms. Carbon, oxygen and nitrogen are found in carbohydrates, lipids, proteins, and nucleic acids. In addition proteins contain nitrogen 27

2 and sometimes sulfur and phosphorus, and nucleic acids always contain nitrogen and phosphorus. There are many other elements that are required as major or minor (trace) minerals. Calcium is an example of one required in relatively large amounts for bones, teeth, and certain physiological activities. Iron is needed for the production of hemoglobin, and iodine for the production of thyroxin. Figure 4-1 shows the recycling of chemicals in both the global and local cycles. In the global cycle, the atmosphere is the reservoir for gaseous elements Carbon, Nitrogen, Oxygen and Hydrogen. Because the atmosphere is the reservoir, changes in the concentration of the various elements or the additions of man made pollutants is more apt to have an effect on the entire earth than that which occurs in local cycles. An example is the fallout of radioactive material from the Chernobyl accident. Another example is the effect of Nitrous oxide and Sulphur dioxide-causing acid rain in areas far distant from the source of the gases. In the local cycle the soil is the reservoir for less mobile elements Phosphorus (P), Sulphur (S), Potassium (K), Calcium (Ca), and trace elements. Materials cycled in local cycles tend to have a direct affect on a rather limited area; although there are ways in which the local chemicals can become more widely spread. An example is when the chemical gets into the ground water or runs off into a stream or river and is carried a great distance. Also chemicals absorbed by plants or ingested by animals can result in foods containing the chemical being shipped great distances and thus having more than a local effect. Figure 4-1. Two general types of biogeochemical cycles, global (A) and local (B). WATER (HYDROLOGIC) CYCLE. All living material contains a relatively large percentage of water, and thus, an adequate water supply is essential to life. Many organisms live in water and most others have their cells surround by an aqueous medium and their cytoplasm is largely water. Materials are moved within both plants and animals by a transport system whose major constituent is water. Excretion, getting rid of waste products of metabolism, requires water in a large percentage of animals. Plants require water to produce a simple carbohydrate by the process of photosynthesis. These examples should indicate the reason that water is so essential to life. Figure 4-2 illustrates the hydrologic cycle. Water returns to earth by the process of precipitation. The precipitation can be in the form of rain, snow, sleet, or hail. This 28

3 water can be taken up by plants and animals, fall in bodies of water, or run off the land into bodies of water. Water returns from the earth to the atmosphere by evaporation. Animals return water to the atmosphere by sweating, respiration, and excretion. Plants return it to the atmosphere by transpiration (water passes from the roots of the plant through the vascular tissue and exits the plant by way of small openings, stomates), and by respiration. Also water evaporates from bodies of water. It takes heat energy to change water from a liquid to a gaseous stage (water vapor). Thus much heat is lost from the earth by the process of evaporation. The condensing of water vapor to liquid water in the atmosphere gives up energy. Figure 4-2. Water cycle. CARBON CYCLE The fact that carbon is an essential element of all organic molecules, is indicative of its necessity for living organisms. Figure 4-3 is an example of how the cycle of carbon can be represented. Carbon in the form of carbon dioxide is taken out of the air by two major ways: 1) photosynthesis in which plants take in CO 2 and use it to make a carbohydrate, and 2) formation of bicarbonate ions, which occurs as a result of carbon dioxide mixing with water to form H 2 CO 3, carbonic acid, which in turn breaks down to H + and HCO 3 -, bicarbonate ion. The bicarbonate ions can be taken up by aquatic plants and can be trapped in sediments that fall to the bottom of the body of water. Although carbon dioxide is only 0.03% of the gases in the atmosphere, it is estimated that this is the equivalent of roughly 700 billion metric tons. In addition another 1 trillion metric tons 29

4 are dissolved in the oceans. The fossil remains of plants and animals contain another 5 trillion tons of carbon in the form of fossil fuels gas, oil, and coal. Roughly 750 million metric tons are tied up in living organisms and will be released when they die and are decomposed. Carbon as carbon dioxide is released to the atmosphere by respiration, decay, and burning of organic materials (burning of fossil fuels is the major contributor). The increase in carbon dioxide in the atmosphere is a concern to many scientists as a contributor to global warming. Table 4-1 from page 43, Vital Signs shows the world carbon dioxide emissions from fossil fuel burning from 1950 to Table 4-1 from page 43, Vital Signs also shows the average global temperature from 1950 to Note the correlation between the increases in both the atmospheric carbon dioxide content and the average global temperature. Figure 4-3. Carbon cycle. Table 4-1. Global Average Temperature and Carbon Emissions from Fossil Fuel Burning, , and Atmospheric Concentrations of Carbon Dioxide, Year Emissions (mill. tons of carbon) Carbon Dioxide (parts per million) Temperature degrees C n.a n.a

5 (prelim) n.a Source: Worldwatch estimates based on ORNL, BP Amoco, DOE, LBL, IEA, IGU, and Scripps NITROGEN CYCLE Figure 4-4 shows a nitrogen cycle. The atmosphere contains roughly 79% nitrogen, but with the exception of a relatively few organisms, this nitrogen is not available to organisms. The gaseous nitrogen becomes avail to living organisms by two processes: 1) nitrogen fixation certain bacteria can fix nitrogen to hydrogen forming ammonium, NH - 4. Some of these bacteria are free living and others live in nodules in the roots of certain plants called legumes; and 2) nitrification which can occur by 1) atmospheric nitrogen reacting with oxygen in the presence of cosmic radiation to produce nitrate, NO - 3 ; and 2) certain soil bacteria change NH to NO 2, Nitrite, and then other - - bacteria change NO 2 to NO 3, Nitrate. These bacteria are called nitrifying bacteria. Green plants can then assimilate the nitrate, NO - 3 into amino acids and ultimately into proteins. Dead plants and animals and their nitrogen containing waste products are converted to ammonium by bacteria and fungi by the process of decomposition. The 31

6 process is called ammonification. These processes keep nitrogen in the cycle of living organisms. But another process called denitrification carried on by certain bacteria coverts NO 3 - to gaseous nitrogen and it returns to the atmosphere. These bacteria live under anaerobic conditions and gain their oxygen from the nitrates. Figure 4-4. Nitrogen cycle. PHOSPHORUS CYCLE Figure 4-5 shows a model of the phosphorus cycle. Phosphorus does not enter the atmosphere. Sedimentary rock contains phosphates and when the rocks are exposed by upheaval and undergo weathering some of the phosphate runs off into water supplies and is made available to aquatic plants. Animals can eat the plants and both of these sources (dead plants and animals) ultimately end up as sediment at the bottom of the body of water. Long periods of time convert the sediment into sedimentary rocks and the cycle continues. Phosphate is taken up in bones, teeth, shells, etc. and these sources of phosphate become available after decomposition. Phosphorus is an important element in DNA, RNA, and ATP, genetic material and energy currency respectively. Phosphate is often a limiting nutrient in ecosystems because it is tied up in living organisms or in remains of organisms and sedimentary rock. As we will discuss in a future unit, it can also become a pollutant due to the large amount that runs off of fertilized soil into bodies of water causing algal bloom and eutrophication. Eutrophication refers to the process of a body of water containing an abundant supply of 32

7 minerals and organic matter, which results in flourishing of algae and microorganisms and can result in oxygen depletion and the eventual dying of life in the water. Figure 4-5. Phosphorus cycle. 33

8 Unit 4 OBJECTIVE QUESTIONS OVER CYCLING OF NUTRIENT 1. All organic molecules contain all but one of the following elements. Which one is not found in all molecules? (A) Carbon (B) Hydrogen (C) Nitrogen (D) Oxygen. 2. Which of the following does not cause water to enter the atmosphere? (A) Evaporation (B) Precipitation (C) Transpiration (D) Respiration. 3. In general less mobile elements are found in the (A) atmospheric reservoir (B)soil reservoir 4. Carbon is removed from the atmosphere by (A) decay (B) photosynthesis (C) respiration. 5. Farmers do not appreciate the process of (A) nitrification (B) assimilation (C) nitrogen fixation (D) denitrification. 6. Lightening fixes gaseous nitrogen as (A) NH 4 (ammonium) (B) NO 3 (nitrate) (C) amino acid (D) protein. 7. Phosphorus is an important element in (A) DNA (B) Bones (C) Teeth (D) two of the preceding (E) all the preceding. 8. Match the following elements with the type of cycle: Local (soil) a. Calcium (Ca) b. Carbon c. Hydrogen (H) d. Potassium (K) e. Nitrogen (N) f. Oxygen (O) Global g. Phosphorus (P) h. Sulphur (S) 9. Which of the following does not return CO 2 to the atmosphere? (A) burning fossil fuels (B) decomposition (C) photosynthesis (D) respiration. 10. Which of the following returns nitrogen to the atmosphere? (A) nitrogen fixation (B) denitrification (C) assimilation (D) nitrification. 11. Which of the following do green plants accomplish? (A) nitrogen fixation (B) denitrification (C) assimilation (D) nitrification. 34

9 12. Denitrification occurs under (A) aerobic (B) anaerobic (C) both A and B (D) neither A or B condition(s). DISCUSSION QUESTIONS OVER CYCLING OF NUTRIENTS. 1. It is said that ecosystems have an inexhaustible supply of energy. On the basis of this statement, what is the reason that we talk about energy shortages? 2. If all the chemical elements are always present on or in the earth in the same amounts, how can there be shortages of certain elements? 3. Your body can convert carbohydrates to lipids, lipids to carbohydrates, and proteins to carbohydrates and lipids; but you can not convert either lipids or carbohydrates to proteins. Explain the reason for this inability. 4. As far as humans are concerned what is the significance of the two types of biogeochemical cycles? 5. How does replacing grassland with a factory affect the carbon cycle? 35

10 6. What is the reason(s) that the Industrial Revolution had such an effect on the carbon cycle? 7. Water is recycled via precipitation and evaporation. Explain the reason(s) that many of the parts of the U.S. suffer water shortages. 8. What affect does draining swamp lands have on the nitrogen cycle? 9. *What is the reason that phosphorus has been removed from laundry detergents? 10. Which element (part of the water cycle) is missing from Figure 4.2? 36