CHAPTER 3 Biogeochemical Cycles Water is Earth's "proud setter up and puller down of kings."28 3.1 NUTRIENT CYCLES S TREAMS and rivers are complex ecosystems that take part in the physical and chemical cycles (biogeochemical cycles) that shape our planet and allow life to exist.29 A biogeochemical cycle is composed of bioelements (chemical ele- ments that cycle through living organisms), and it occurs when there is interac- tion between the biological and physical exchanges of bioelements. J Note: Contrary to an incorrect assumption, energy does not cycle through an ecosystem-chemicals do. The inorganic nutrients cycle through more than the organisms; however, they also enter the oceans, atmosphere, and even rocks. Because these chemicals cycle through both the biological and the geological worlds, we call the overall cycles biogeochemical cycles. Each chemical has a unique cycle, but all cycles have some things in com- mon. Reservoirs are those parts of the cycle where the chemical is held in large quantities for long periods of times (e.g., the oceans for water and rocks for phosphorous). In exchange pools, on the other hand, the chemical is held for only a short time (e.g., the atmosphere; a cloud). The length of time a chemical is held in an exchange pool or a reservoir is termed its residence time. The biotic community includes all living organisms. This community may serve as an ex- change pool (although for some chemicals like carbon that can be bound in cer- tain tree species for a thousand years, it may seem more like a reservoir), and it **~hakes~eare's description of Richard-the Kingmaker during the Wars of the Roses. 29~ave, C., Stream Ecology. http:l/home.netcom.com/-cristi, p. 1, 1998. Copyright © 2001 by Technomic Publishing Company, Inc. 26 BIOGEOCHEMICAL CYCLES also may serve to move chemicals (bioelements) from one stage of the cycle to another. For instance, the trees of the tropical rain forest bring water up from the forest floor to be transpired into the atmosphere. Likewise, coral organisms take carbon from the water and turn it into limestone rock. The energy for most of the transportation of chemicals is provided either by the sun or by the heat re- leased from the mantle and core of the earth. In the case of chemical elements that cycle through living things, the follow- ing can be stated:30 All bioelements reside in compartments or defined spaces in nature. A compartment contains a certain quantity, or pool, of bioelements. Compartments exchange bioelements. The rate of movement of bioelements between two compartments is called the flux rate. The average length of time a bioelement remains in a compartment is called the mean residence time (MRT). The flux rate and pools of bioelements together define the nutrient cycle in an ecosystem. Ecosystems are not isolated from one another, and bioelements come into an ecosystem through meteorological, geological, or biological transport mechanisms: -meteorological (e.g., deposition in rain and snow, atmospheric gases) -geological (e.g., surface and subsurface drainage) -biological (e.g., movement of organisms between ecosystems) As a result, biogeochemical cycles can be local or global. Smith categorizes biogeochemical cycles into two types, the gaseous and the sedimentary. Gaseous cycles include carbon and nitrogen cycles. The main pool (or sink) of nutrients in the gaseous cycle is the atmosphere and the ocean. The sedimentary cycles include sulfur and phosphorous cycles. The main sink for sedimentary cycles is soil and rocks of the earth's crust.31 Between 20 to 40 of the earth's 92 naturally occurring elements are ingredi- ents that make up living organisms. The chemical elements carbon, hydrogen, oxygen, nitrogen, and phosphorus are critical in maintaining life. Odum points out that of the elements needed by living organisms to survive, oxygen, hydro- gen, carbon, and nitrogen are needed in larger quantities than some of the other elements.32 These elements exhibit definite biogeochemical cycles, which will be discussed in detail later. The elements needed to sustain life are products of the global environment that consists of three main subdivisions (see Figure 3.1): 30~rom Biogeochernical Cycles 11: The Nitrogen and Phosphorus Cycles. Yahoo.com internet access, pp. 1-2, 2000. 31~mith, R. L., Ecology and Field Biology New York: Harper & Row, p. 49, 1974. 320dum, E. P., Fundamentals of Ecology. Philadelphia: Saunders College Publishing, p. 30, 1971. Copyright © 2001 by Technomic Publishing Company, Inc. Nutrient Cycles Figure 3.1 The global environment. (1) Hydrosphere-includes all components formed of water bodies on the earth's surface. (2) Lithosphere-comprises the solid components on the earth's surface such as rocks. (3) Atmosphere-is the gaseous mantle that envelops the hydrosphere and the lithosphere. To survive, organisms require inorganic metabolites from all three parts of the biosphere. For example, the hydrosphere supplies water as the exclusive source of needed hydrogen. Essential elements such as calcium, sulfur, and phosphorus are provided by the lithosphere. Finally, oxygen, nitrogen, and car- bon dioxide are provided by the atmosphere. Within the biogeochemical cycles, all the essential elements circulate from the environment to organisms and back to the environment. Because of the crit- ical importance of elements in sustaining life, it may be easily understood why biogeochemical cycles are readily and realistically labeled nutrient cycles. Through these biogeochemical or nutrient cycles, nature processes and reprocesses the critical life-sustaining elements in definite inorganic-organic cycles. In some cycles, such as carbon, there is no loss of material for long peri- ods of time. One point to keep in mind is that energy (to be explained later) flows "through an ecosystem, but nutrients are cycled and recycled. Humans need most of these recycled elements to survive. Because of this, Copyright © 2001 by Technomic Publishing Company, Inc. 28 BIOGEOCHEMICAL CYCLES we have speeded up the movement of many materials so that the cycles tend to become imperfect, or what Odum calls acyclic. Odum goes on to explain that our environmental impact on phosphorus demonstrates one example of a some- what imperfect cycle. We mine and process phosphate rock with such careless abandon that severe lo- cal pollution results near mines and phosphate mills. Then, with equally acute myopia we increase the input of phosphate fertilizers in agricultural systems without controlling in any way the inevitable increase in run-off output that se- verely stresses our waterways and reduces water quality through e~tro~hication.~~ As related above, in agricultural ecosystems, we often supply necessary nu- trients in the form of fertilizer to increase plant growth and yield. In natural eco- systems, however, these nutrients are recycled naturally through each trophic level. For example, the elemental forms are taken up by plants. The consumers ingest these elements in the form of organic plant material. Eventually, the nu- trients are degraded to the inorganic form again. The following pages present and discuss the nutrient cycles for carbon, nitrogen, phosphorus, and sulfur. 3.2 CARBON CYCLE Carbon, which is an essential ingredient of all living things, is the basic building block of the large organic molecules necessary for life. Carbon is cy- cled into food chains from the atmosphere, as shown in Figure 3.2. The carbon cycle (see Figure 3.2) is based on carbon dioxide, which makes up only a small percentage of the atmosphere. From Figure 3.2, it can be seen that green plants obtain carbon dioxide (CO2) from the air and, through photo- synthesis, described by Asimov as the "most important chemical process on Earth," it produces the food and oxygen on which all organisms live.34 Part of the carbon produced remains in living matter, and the other part is released as CO2 in cellular respiration. Miller points out that the carbon dioxide released by cellular respiration in all living organisms is returned to the atrn~sphere.~~ J Note: About a tenth of the estimated 700 billion tons of carbon dioxide in the atmosphere is fixed annually by photosynthetic plants. A further trillion tons are dissolved in the ocean, more than half in the photosynthetic layer. Some carbon is contained in buried dead animal and plant materials. Much of these buried plant and animal materials were transformed into fossil fuels. Fossil fuels (coal, oil, and natural gas) contain large amounts of carbon. When 330dum, E. P., Fundamentals of Ecology. Philadelphia: Saunders College Publishing, p. 87, 1971. 34~simov, I., How Did We Find Out About Photosynthesis? New York: Walker & Company, p. 20, 1989. 35~ller, G. T., Environmental Science: An Introduction. Belmont, CA: Wadsworth, p. 43, 1988. Copyright © 2001 by Technomic Publishing Company, Inc. Nitrogen Cycle 29 Figure 3.2 Carbon cycle. fossil fuels are burned, stored carbon combines with oxygen in the air to form carbon dioxide, which enters the atrn~sphere.~~ In the atmosphere, carbon dioxide acts as a beneficial heat screen as it does not allow the heat generated by earth's radiant energy to be emitted into space. This balance is important. The problem is that as more carbon dioxide from burning is released into the atmosphere, balance can and is being altered. Odum warns that recent increase in consumption of fossil fuels "coupled with the de- crease in the 'removal capacity' of the green belt is beginning to exceed the del- icate balance."37 Increased releases of carbon dioxide into the atmosphere tend to increase the possibility of global warming. The consequences of global warming "would be catastrophic. . . and the resulting climatic change would be irre~ersible."~~ NITROGEN CYCLE Nitrogen is important to all life. Nitrogen in the atmosphere or in the soil can go through many complex chemical and biological changes, be combined into living and nonliving material, and return to the soil or air in a continuing cycle. This is called the nitrogen cycle.39 The atmosphere contains 78% by volume of nitrogen. Moreover, as stated 36~oran, J. M,, Morgan, M. D., and Wiersma, J. H., Introduction to EnvironmentalScience. New York: W.H. Free- man and Company, p. 67,1986. 370dum, E. P., Basic Ecology. Philadelphia: Saunders College Publishing, p. 202, 1983. 38~brahamson, D. E. (ed.). The Challenge of Global Warming. Washington, DC: Island Press, p. 4, 1988. 39~illpack, S. C. and Buchholz, D., Nitrogen in the Environment: Nitrogen. Missouri: University of Missouri-CO- lumbia, p. 1, 1993. Copyright © 2001 by Technomic Publishing Company, Inc. 30 BIOGEOCHEMICAL CYCLES previously, nitrogen is an essential element for all living matter and constitutes 1-3% dry weight of cells, yet nitrogen is not a common element on earth. Al- though it is an essential ingredient for plant growth, it is chemically very inac- tive, and before it can be incorporated by the vast majority of the biomass, it must be fixed.40 Price describes the nitrogen cycle as an example "of a largely complete chemical cycle in ecosystems with little leaching out of the system."41 From the waterlwastewater specialist's point of view, nitrogen and phosphorous are both commonly considered limiting factors for productivity. Of the two, nitrogen is harder to control but is found in smaller quantities in wastewater. As stated earlier, nitrogen gas makes up about 78% of the volume of the earth's atmosphere. As such, it is useless to most plants and animals. Fortu- nately, nitrogen gas is converted into compounds containing nitrate ions, which are taken up by plant roots as part of the nitrogen cycle, shown in simplified form in Figure 3.3. Aerial nitrogen is converted into nitrates mainly by microorganisms, bacte- ria, and blue-green algae. Lightning also converts some aerial nitrogen gas into forms that return to the earth as nitrate ions in rainfall and other types of precipi- tation. From Figure 3.3, it can be seen that ammonia plays a major role in the ni- trogen cycle. Excretion by animals and anaerobic decomposition of dead or- ganic matter by bacteria produce ammonia. Ammonia, in turn, is converted by nitrification bacteria into nitrites and then into nitrates. This process is known as nitrification. Nitrification bacteria are aerobic. Bacteria that convert ammo- nia into nitrites are known as nitrite bacteria (Nitrosococcus and Nitrosomonas); they convert nitrites into nitrates and nitrate bacteria (Nitrobacter). In wastewater treatment, ammonia is produced in the sludge digester and nitrates are produced in the aerobic sewage treatment process. In Wastewater Engineering, several pages are devoted to describing the ni- trogen cycle and its impact on the wastewater treatment process. They point out that nitrogen is found in wastewater in the form of urea. During wastewater treatment, the urea is transformed into ammonia nitrogen. Because ammonia exerts a BOD and chlorine demand, high quantities of ammonia in wastewater effluents are undesirable. The process of nitrification is utilized to convert am- monia to nitrates. Nitrification is a biological process that involves the addition of oxygen to the wastewater. If further treatment is necessary, another biologi- cal process called denitrification is used.42 In this process, nitrate is converted into nitrogen gas, which is lost to the atmosphere, as can be seen in Figure 3.3. When attempting to address the important and complex factors that make up 40~orteous, A., Dictionary of Environmental Science and Technology New York: John Wiley & Sons, Inc., p. 83, 1992. 41~rice, P. W., Insect Ecology. New York: John Wiley & Sons, Inc., p. 11, 1984. 42~etcalf & Eddy, Inc., Wastewater Engineering: Treatment, Disposal, Reuse. 3rd ed. New York: McGraw-Hill, pp. 85-87,1991. Copyright © 2001 by Technomic Publishing Company, Inc. Nitrogen Cycle Dissolution Loss to Deep Sediments Nitrites Loss to Deep Sediments Animal 'N' Excretion I Ammonia ++ Organic Nitrogen as Amino Acids Figure 3.3 Nitrogen cycle. the topic of stream ecology and self-purification, it is important to understand the impact that the nitrogen cycle can have on effluent that is dumped (outfalled) into receiving streams. At the same time, one should remember that the nitrogen cycle that occurs in the wastewater stream is not the source of the nitrogen contamination of surface water bodies. As a case in point, Price uses the example of large inputs of nitrogen fertilizer from agricultural systems, which "may result in considerable leaching and unidirection flow of nitrogen into aquatic systems which become polluted with excessive nitrogen . . . ."43 J Note: Nitrogen becomes a concern to stream ecology (quality) when nitro- gen in the soil is converted to nitrate (NO?) form. Nitrate is very mobile and moves with water in the soil. The concern of nitrates and water quality is gen- erally directed at groundwater. However, nitrates can also enter surface wa- ters such as ponds, streams, and rivers. Nitrates in drinking water can lead to nitrate poisoning in infant humans and animals, causing serious health prob- lems and even death. This occurs because of a bacteria commonly found in the intestinal tract of infants that can convert nitrate to high toxic nitrites (NO,). Nitrite can replace oxygen in the bloodstream and result in oxygen starvation that causes a bluish discoloration of the infant ("blue baby" syn- drome). 43~rice, P. W., Insect Ecology New York: John Wiley & Sons, Inc., p. 11, 1984. 44~pellman, F. R., The Science of Water: Concepts & Applications. Lancaster, PA: Technornic Publishing Com- pany, Inc., pp. 175-176, 1998. Copyright © 2001 by Technomic Publishing Company, Inc. 32 BIOGEOCHEMICAL CYCLES 3.4 PHOSPHORUS CYCLE Phosphorus is another element that is common in the structure of living or- ganisms. However, of all the elements recycled in the biosphere, phosphorus is the scarcest and, therefore, the one most limiting in any given ecological sys- tem. It is indispensable to life, being intimately involved in energy transfer and in the passage of genetic information in the DNA of all cells. The ultimate source of phosphorus is rock, from which it is released by weathering, leaching, and mining. Phosphorus occurs as phosphate or other minerals formed in past geological ages. These massive deposits are gradually eroding to provide phosphorus to ecosystems. A large amount of eroded phos- phorus ends up in deep sediments in the oceans and lesser amounts in shallow sediments. Part of the phosphorus comes to land when marine animals are brought out. Birds also play a role in the recovery of phosphorus. The great guano deposit, bird excreta, of the Peruvian coast is an example. Man has has- tened the rate of loss of phosphorus through mining activities and the subse- quent production of fertilizers. Even with the increase in human activities, however, there is no immediate cause for concern, because the known reserves of phosphate are quite large. Figure 3.4 shows the phosphorus cycle. I Protoplasm synthesis \/ Erosion / / Marine birds and fish * Loss to deep sedirnents Figure 3.4 The phosphorus cycle. Copyright © 2001 by Technomic Publishing Company, Inc. Sulfur Cycle 33 Phosphorous has become very important in water quality studies, because it is often found to be a limiting factor (i.e., limiting plant nutrient). Control of phosphorus compounds that enter surface waters and contribute to growth of al- gal blooms is of much interest to stream ecologists. Phosphates, upon entering a stream, act as fertilizer, which promotes the growth of undesirable algae popu- lation~ or algal blooms. As the organic matter decays, dissolved oxygen levels decrease, and fish and other aquatic species die. While it is true that phosphorus discharged into streams is a contributing fac- tor to stream pollution (and causes eutrophication), it is also true that phospho- rus is not the lone factor. Odum warns against what he calls the one-factor con- trol hypothesis, i.e., the one-problemlone-solution syndrome. He goes on to point out that environmentalists in the past have focused on one or two items, like phosphorous contamination, and "have failed to understand that the strat- egy for pollution control must involve reducing the input of all enriching and toxic material^."^^ J Note: Because of its high reactivity, phosphorus exists in combined form with other elements. Microorganisms produce acids that form soluble phos- phate from insoluble phosphorus compounds. The phosphates are utilized by algae and terrestrial green plants, which in turn pass into the bodies of animal consumers. Upon death and decay of organisms, phosphates are released for recycling.46 3.5 SULFUR CYCLE Sulfur, like nitrogen and carbon, is characteristic of organic compounds. However, an important distinction between cycling of sulfur and cycling of ni- trogen and carbon is that sulfur is "already fixed." That is, plenty of sulfate an- ions are available for living organisms to utilize. By contrast, the major biologi- cal reservoirs of nitrogen atoms (Nz) and carbon atoms (CO2) are gases that must be pulled out of the atmosphere. The sulfur cycle (see Figure 3.5) is both sedimentary and gaseous. Tchobanoglous and Schroeder note that "the principal forms of sulfur that are of special significance in water quality management are organic sulfur, hydro- gen sulfide, elemental sulfur and s~lfate."~~ Bacteria play a major role in the conversion of sulfur from one form to an- other. In an anaerobic environment, bacteria break down organic matter pro- 450dum, E. P,, Ecology The Link Between the Natural and the Social Sciences. New York: Holt, Rinehart and Winston, Inc., p. 110, 1975. 46~hosphorus Cycle. Britannica.com Inc., p. 1,2000. 47~chobanoglous, G. and Schroeder, E. D., Water Qualiq. Reading, MA: Addison-Wesley, p. 184, 1985. Copyright © 2001 by Technomic Publishing Company, Inc. BIOGEOCHEMICAL CYCLES Bacterial photosynthesis Oxidation by bacterium \ Digestion \ Bacterial Reduction / Sulfur Dioxide, SO, Plant proteins Animal proteins Figure 3.5 The sulfur cycle. ducing hydrogen sulfide with its characteristic rotten egg odor. A bacteria called Beggiatoa converts hydrogen sulfide into elemental sulfur. An aerobic sulfur bacterium, Thiobacillus thiooxidans, converts sulfur into sulfates. Other sulfates are contributed by the dissolving of rocks and some sulfur dioxide. Sul- fur is incorporated by plants into proteins. Some of these plants are then con- sumed by organisms. Sulfur from proteins is liberated by many heterotrophic anaerobic bacteria, as hydrogen sulfide. 3.6 SUMMARY OF KEY TERMS Hydrosphere-is the water covering the earth's surface of which 80% is salt, 19% is groundwater and, obviously, the other 1% is unsalted fresh surface water (rivers, lakes, streams, etc.). Lithosphere-is comprised of the solid components of the earth's sur- face such as rocks and weathered soil. Atmosphere-is the gaseous mantle enveloping the hydrosphere and lithosphere, which is 78% nitrogen by volume. Organisms-require 20-40 elements for survival. Carbon-is an essential part of all organic compounds; photosynthesis is a source of carbon. Photosynthesis-is the chemical process by which solar energy is stored as chemical energy. Copyright © 2001 by Technomic Publishing Company, Inc. [...]... converted 3 7 CHAPTER REVIEW QUESTIONS 3. 1 Define biogeochemical cycle 3. 2 are those parts of the cycle where the chemical is held in large quantities for long periods of time 3. 3 The length of time a chemical is held in an exchange pool or a reservoir is termed its 3. 4 The average length of time a bioelement remains in a compartment is called the 3. 5 Name the three transport mechanisms 3. 6 Biochemical... Biochemical cycles can be andor 3. 7 Name the two types of biogeochemical cycles 3. 8 The three main subdivisions of the global environment are: 3. 9 The carbon cycle is based on 3. 10 "The most important chemical process on earth: 3. 1 1 In the atmosphere, acts as a 3. 12 The atmosphere contains 78% by volume of Copyright © 2001 by Technomic Publishing Company, Inc heat screen 36 BIOGEOCHEMICALCYCLES 3. 13 Aerial... bacteria, and blue-green algae mainly by microorganisms, 3. 14 Excretion by animals and anaerobic decomposition of dead organic matter by bacteria produce 3. 15 The process whereby ammonia converted by nitrification bacteria into nitrites and then into nitrates is known as 3. 16 The process whereby nitrate is converted into nitrogen gas is known as 3. 17 The ultimate source of phosphorus is 3. 18 Phosphate,.. .35 Chapter Review Questions Nitrogen-is required for the construction of proteins and nucleic acids; the major source is the atmosphere Phosphorus cycle-is a very inefficient cycle; the greatest source is the lithosphere Humans have greatly speeded this cycle through mining Sulfur cycle-is a cycle in which elementary sulfur of the lithosphere which is not available to plants and animals unless... converted into nitrogen gas is known as 3. 17 The ultimate source of phosphorus is 3. 18 Phosphate, upon entering a stream, acts as the growth of , which promotes 3. 19 Explain the "one-factor control hypothesis." 3. 20 The cycle is both sedimentary and gaseous Copyright © 2001 by Technomic Publishing Company, Inc . and other types of precipi- tation. From Figure 3. 3, it can be seen that ammonia plays a major role in the ni- trogen cycle. Excretion by animals and anaerobic decomposition of dead or- ganic. ++ Organic Nitrogen as Amino Acids Figure 3. 3 Nitrogen cycle. the topic of stream ecology and self- purification, it is important to understand the impact that the nitrogen cycle can have. Nitrite can replace oxygen in the bloodstream and result in oxygen starvation that causes a bluish discoloration of the infant ("blue baby" syn- drome). 43~ rice, P. W., Insect Ecology