BIOGEOCHEMICAL CYCLES

CONCEPT

Of the 92 elements produced in nature, only six are critical to the life of organisms: hydrogen, carbon, nitrogen, oxygen, phosphorus, and sulfur. Though these elements account for 95% of the mass of all living things, their importance extends far beyond the biosphere. Hydrogen and oxygen, chemically bonded in the form of water, are the focal point of the hydrosphere, while oxygen and nitrogen form the bulk of the atmosphere. All six are part of complex biogeochemical cycles in which they pass through the biosphere, atmosphere, hydrosphere, and geosphere. These cycles circulate nutrients through the soil into plants, microbes, and animals, which return the elements to the earth system through chemical processes that range from respiration to decomposition.

HOW IT WORKS

The Elements

An element is a substance composed of a single type of atom, meaning that it cannot be broken down chemically to make a simpler substance. They are listed on the periodic table of elements, a chart that renders them in order of their atomic numbers, or the number of protons in the nucleus of the atom. The elements we want to discuss in the context of biogeochemical cycles are all low in atomic number, starting with hydrogen, which has just one proton in its nucleus. In the following list, the elements are cited by atomic number along with their chemical symbols, or the abbreviation by which they are known to chemists.

Elements Involved in Biogeochemical Cycles

• 1. Hydrogen (B)
• 6. Carbon (C)
• 7. Nitrogen (N)
• 8. Oxygen (O)
• 15. Phosphorus (P)
• 16. Sulfur (S)

Given the fact, as noted earlier, that 92 elements appear in nature, it should come as no surprise that the highest atomic number for any naturally occurring element is 92, for uranium. Beyond uranium there are about two dozen artificially created elements, but they are of little interest outside the realm of certain specialties in chemistry and physics. The naturally occurring elements are the ones that matter to the earth sciences, and of these elements, only a handful play a significant role.

In the essays Minerals and Economic Geology, other elements—most notably silicon—are discussed with regard to their importance in forming minerals, rocks, and ores. Though they are critical to Earth's systems, elements other than the six discussed here play no role in biogeochemical cycles. Indeed, it is a fact of the physical sciences that not all elements are created equal: certainly, the universe is not divided evenly 92 ways, with equal amounts of all elements. In fact, hydrogen and helium account for 99% of the mass of the entire universe.

ABUNDANCE.

On Earth the ratios are quite different, however. Oxygen and silicon constitute the preponderance of the known mass of Earth's crust, while nitrogen and oxygen form the overwhelming majority of the atmosphere. Hydrogen is proportionally much, much less abundant on Earth than in the universe as a whole, but owing to its role in forming water, a substance essential to the sustenance of life, it is unquestionably of great significance.

The two following lists provide rankings for the abundance of the six elements discussed in this essay. The first table shows their ranking and share in the entire known mass of the planet, including the crust, living matter, the oceans, and atmosphere. The second shows their relative abundance and ranking in the human body.

Abundance of Selected Elements on Earth (Ranking and Percentage)

• 1. Oxygen (49.2%)
• 9. Hydrogen (0.87%)
• 12. Phosphorus (0.11%)
• 14. Carbon (0.08%)
• 15. Sulfur (0.06%)
• 16. Nitrogen (0.03%)

Abundance of Selected Elements in the Human Body (Ranking and Percentage)

• 1. Oxygen (65%)
• 2. Carbon (18%)
• 3. Hydrogen (10%)
• 4. Nitrogen (3%)
• 6. Phosphorus (1%)
• 9. Sulfur (0.26%)

Several things are interesting about these figures. First and most obviously, there is the fact that the ranking of all these elements (with the exception of oxygen) is relatively low in the total known elemental mass of Earth, whereas their ranking is much, much higher within the human body. This is significant, given the fact that these elements are all essential to the lives of organisms.

Furthermore, note that it does not take a great percentage to constitute an "abundant" element: even nitrogen, with its 0.03% share of Earth's total known mass, still is considered abundant. The presence of the vast majority of elements on Earth is measured in parts per million (ppm) or even parts per billion (ppb).

Chemistry and Geochemistry

In a general sense, chemistry can be defined as an area of the physical sciences concerned with the composition, structure, properties, and changes of substances, including elements, compounds, and mixtures. This definition unites the phases in the history of the development of the discipline, from early modern times—when it arose from alchemy, a set of mystical beliefs based on the idea that ordinary matter can be perfected—to modern times. Our modern understanding of chemistry, however, is quite different from the model of chemistry that prevailed until about 1800, a difference that relates to a key discovery: the atom.

This change, in fact, should be described not in terms of a discovery so much as the development of a model. Long before chemists and physicists comprehended the structure of the atom, they developed an understanding of all chemical substances as composed of atomic units, each representing one and only one element. Until the development of this model—thanks to a number of chemists, most notably, John Dalton (1766-1844) of England, Antoine Lavoisier (1743-1794) of France, and Amedeo Avogadro (1776-1856) of Italy—chemistry was concerned primarily with mixing potions and observing their effects. Thanks to the atomic model, chemists never again would confuse mixtures with chemical compounds.

CHEMICAL REACTIONS.

The difference between a mixture and a compound goes to the heart of the distinction between physics and chemistry. A mixture, such as coffee, is the result of a physical process—in this case, the heating of water and coffee beans—and the result does not have a uniform chemical structure. On the other hand, a compound results from chemical reactions between atoms, which form enormously powerful bonds in the process of joining to create a molecule. A molecule is the basic particle of a compound, just as an atom is to an element. It should be noted that some elements, such as nitrogen, typically appear in diatomic form, that is, two atoms bond to form a molecule of nitrogen.

A substance may undergo physical changes without experiencing any alteration in its underlying structure; on the other hand, a chemical reaction makes a fundamental change to the substance. In a chemical reaction, a substance may experience a change of state (i.e., from solid to liquid or gas) without undergoing any physical process of being heated or cooled by an outside source. Chemical reactions involve the breaking of bonds between atoms in a molecule and the formation of new bonds. As a result, an entirely new substance is created—something that could never be achieved through mere physical processes.

REAL-LIFE APPLICATIONS

Geochemistry

Just as geochemistry is a branch of the geologic sciences that weds physics and geology, so there is a geologic subdiscipline, geochemistry, in which chemistry and the geologic sciences come together. Geochemistry is concerned with the chemical properties and processes of Earth—in particular, the abundance and interaction of chemical elements and their isotopes. (Isotopes are atoms that have an equal number of protons, and hence are of the same element, but differ in their number of neutrons. Isotopes may be either stable or unstable, in which case they are subject to the emission of high-energy particles. Some elements have numerous stable isotopes, others have only one or two, and some have none.)

Before the mid-twentieth century, geochemistry had a relatively limited scope, confined primarily to the identification of elements in rocks and minerals and the determination of the relative abundance of those elements. Since that time, however, this subdiscipline has come to encompass many other concerns, particularly those discussed in the present context. Geochemistry today focuses on such issues as the recycling of elements between the various sectors of the earth system, especially between living and non-living things.

Biogeochemical Cycles and Earth Systems

The changes that a particular element undergoes as it passes back and forth through the various earth systems, and particularly between living and nonliving matter, are known as biogeochemical cycles. The four earth systems involved in these cycles are the atmosphere, the biosphere (the sum of all living things as well as formerly living things that have not yet decomposed), the hydrosphere (the entirety of Earth's water except for vapor in the atmosphere), and the geosphere. The last of these spheres is defined as the upper part of Earth's continental crust, or that portion of the solid earth on which human beings live and which provides them with most of their food and natural resources.

Carbon, for instance, is present in all living things on Earth. Hence, the phrase carbon-based life-form, a cliché found in many an old sciencefiction movie, is actually a redundancy: all life-forms contain carbon. In the context of the physical sciences, organic refers to all substances that contain carbon, with the exception of oxides, such as carbon dioxide and carbon monoxide, and carbonates, which are found in minerals. Still, carbon circulates between the organic world and the inorganic world, as when an animal exhales carbon dioxide.

The carbon cycle is of such importance to the functioning of Earth that it is discussed separately (see Carbon Cycle). So, too, is the nitrogen cycle (see Nitrogen Cycle), whereby nitrogen passes between the soil, air, and biosphere as well as the hydrosphere. The hydrosphere, as noted earlier, is based on a single substance, water, created by the chemical bonding of hydrogen and oxygen, and it is likewise discussed in detail elsewhere (see Hydrologic Cycle).

Despite the emphasis here on carbon in the biosphere, nitrogen in the geosphere, and hydrogen and oxygen in the hydrosphere, it should be noted that biogeochemical cycles involving these four elements take them through all four "spheres." The same is true of sulfur, whose biogeochemical cycle is discussed later in this essay. On the other hand, phosphorus, also discussed later, is present in only three of Earth's systems; it plays little role in the atmosphere.

Decomposers and Detritivores

Most biogeochemical cycles involve a special type of chemical reaction known as decomposition, and for this to take place, agents of decomposition—known as decomposers and detritivores—are essential. Decomposition occurs when a compound is broken down into simpler compounds or into its constituent elements. This is achieved primarily by decomposers, organisms that obtain their energy from the chemical breakdown of dead organisms as well as from animal and plant waste products.

The principal forms of decomposer are bacteria and fungi. These creatures carry enzymes, which they secrete into the materials they consume, breaking them down chemically before taking in the products of this chemical breakdown. They thus take organic matter and render it in inorganic form, such that later it can be taken in again by plants and returned to the biosphere.

Detritivores are much more complex organisms, but their role is similar to that of decomposers. They, too, feed on waste matter, breaking this organic material down into inorganic substances that then can become available to the biosphere in the form of nutrients for plants. Examples of detritivores are earthworms and maggots. As discussed in Energy and Earth, detritivores are key players in the food web, the set of nutritional interactions—sometimes called a food chain—between living organisms.

Phosphorus and the Phosphorus Cycle

There are a few elements that were known in ancient or even prehistoric times, examples being gold, iron, lead, and tin. The vast majority, on the other hand, have been discovered since the beginning of the modern era, and the first of them was phosphorus, which is also the first element whose discoverer is known.

In 1674 the German alchemist Hennig Brand (ca. 1630-ca. 1692) was searching for the philosopher's stone, a mythical substance that allegedly would turn common or base metals into gold. Convinced that he would find this substance in the human body, Brand evaporated water from a urine sample and burned the precipitate (the solid that remained) along with sand. The result was a waxy, whitish substance that glowed in the dark and reacted violently with oxygen. Brand named it phosphorus, a name derived from a Greek term meaning "light-bearer."

Owing to its high reactivity with oxygen, phosphorus is used in the production of safety matches, smoke bombs, and other incendiary devices. It is also important in various industrial applications and in fertilizers. In fact, ancient humans used phosphorus without knowing it when they fertilized their crops with animal bones.

PHOSPHATES.

In the early 1800s, chemists recognized that the critical component in bones was phosphorus, which plants use in photosynthesis—the biological conversion of energy from the Sun into chemical energy (see Energy and Earth). With this discovery came the realization that phosphorus would make an even more effective fertilizer when treated with sulfuric acid, which makes it soluble, or capable of being dissolved, in water. This compound, known as superphosphate, can be produced from phosphates, a type of mineral.

Phosphates represent one of the eight major classes of mineral (see Minerals). All phosphates contain a characteristic formation, PO₄, which is bonded to other elements or compounds—for example, with aluminum in aluminum phosphate, or AlPO₄. Phosphorus fertilizer is typically calcium phosphate, known as bone ash, the most important industrial mineral (see Economic Geology) produced from phosphorus. Another significant phosphate is sodium phosphate, used in dishwashing detergents. In fact, phosphates once played a much larger role in the detergent industry—with disastrous consequences, as we shall see.

THE PHOSPHORUS CYCLE.

The majority of phosphorus in the earth system is located in rocks and deposits of sediment, from which it can be removed by one of three processes: weathering, the breakdown of rocks and minerals at or near the surface of Earth as the result of physical, chemical, or biological processes (see Erosion, Sediment and Sedimentation); leaching, the removal of soil materials that are in solution, or dissolved in water; and mining.

Phosphorus is highly reactive, meaning that it is likely to bond with other elements, and for this reason it often is found in compounds. Microorganisms absorb insoluble phosphorus compounds (ones that are incapable of being dissolved) and, through the action of acids within the microorganisms, turn them into soluble phosphates. Algae and other green plants absorb these phosphates and, in turn, are eaten by animals. When they die, the animals release the phosphates back into the soil.

As with all elements, the total amount of phosphorus on Earth stays constant, but the distribution of it does not. Some of the phosphorus passes from the geosphere into the biosphere, but the majority of it winds up in the ocean. It may find its way into sediments in shallow waters, in which case it continues to circulate, or it may be taken to the deep parts of the seas, in which case it is likely to be deposited for the long term.

Fish absorb particles of phosphorus, and thus some of the element returns to dry land through the catching and consumption of seafood. In addition, guano, or dung, from birds that live in an ocean environment (e.g., seagulls) also returns portions of phosphorus to the terrestrial environment. Nonetheless, geochemists believe that phosphorus is being transferred steadily to the ocean, from whence it is not likely to return. It is for this reason that phosphorus-based fertilizers are important, because they feed the soil with nutrients that otherwise would be continually lost.

EUTROPHICATION.

To be sure, phosphorus, in the proper quantities, is good for the environment. But as with medicine or any other beneficial substance, if a little is good, that does not mean that a lot is necessarily better. In the case of phosphorus, an overabundance of the element in the environment can lead to a phenomenon called eutrophication, a state of heightened biological productivity in a body of water. One of the leading causes of eutrophication (from a Greek term meaning "well nourished") is a high rate of nutrient input, in the form of phosphates or nitrates, a nitrogen-oxygen compound (see Nitrogen Cycle).

As a result of soil erosion, fertilizers make their way into bodies of water, as does detergent runoff in wastewater. Excessive phosphates and nitrates stimulate growth in algae and other green plants, and when these plants die, they drift to the bottom of the lake or other body of water. There, decomposers consume the remains of the plants and, in the process, also use oxygen that otherwise would be available to fish, mollusks, and other forms of life. As a result, those species die off, to be replaced by others that are more tolerant of lowered oxygen levels—for example, worms. Needless to say, the outcome of eutrophication is devastating to the lake's ecosystem.

During the 1960s, Lake Erie—one of the Great Lakes on the U.S.-Canadian border—became an example of eutrophication gone mad. As a result of high phosphate concentrations, Erie's waters were choked with plant and algae growth. Fish were unable to live in the water, the beaches reeked with the smell of decaying algae, and Erie became widely known as a "dead" body of water. This situation led to the passage of new environmental standards and pollution controls by both the United States and Canada, whose governments acted to reduce drastically the phosphate content in fertilizers and detergents. Lake Erie proved to be an environmental success story: within a few decades the lake once again teemed with life.

Sulfur and the Sulfur Cycle

If there is any element that can be said to have a bad image—and a falsely bad one at that—it is sulfur. As everyone "knows," sulfur has a foul smell, and this smell, combined with its combustibility, led to the biblical association of brimstone —the ancient name for the element—with the fires of hell. It may come as a surprise, then, to learn that sulfur has no smell of its own. Only in combination with other elements does it acquire the offensive odor that has led to its unpleasant reputation.

An example of such a compound is hydrogen sulfide, a poisonous substance present in intestinal gas. The May 2001 National Geographic included two stories relating to the presence of natural hydrogen sulfide deposits on opposite sides of the earth, and in both cases the presence of these toxic fumes created unusual ecosystems.

A system of caves known as Villa Luz in southern Mexico contains some 20 underground springs that carry large quantities of hydrogen sulfide. Among the strange creatures that have made Villa Luz their home are species of fish colored bright red; the pigmentation is a result of the fact that they have to produce high quantities of hemoglobin (a component in red blood cells) to survive on the scant oxygen. The waters of the cave also are populated by microorganisms that oxidize the hydrogen sulfide and turn it into sulfuric acid, which dissolves the rock walls and continually enlarges the cave.

Thousands of miles away, in the Black Sea, explorers examining evidence of a great ancient flood like the one depicted in the Bible (see Earth, Science, and Nonscience) found an unexpected ally in the form of hydrogen sulfide. Because the Black Sea lacks the temperature differences that cause water to circulate from the bottom upward, hydrogen sulfide had gathered at the bottom and stayed there, covered by dense layers of saltwater. Oxygen could not reach the bottom of the Black Sea, and thus wood-boring worms could not live in the toxic environment. As a result, a 1,500-year-old shipwreck had been virtually undisturbed.

THE SULFUR CYCLE.

Sulfur is removed from rock by weathering, at which point it reacts with oxygen in the air to form sulfate, or SO₄. This sulfate is taken in by plants and microorganisms, which convert it to organic materials and pass it on to animals in the food web. Later, when these organisms die, decomposers absorb the sulfur from their bodies and return it to the environment. As with phosphorus, however, sulfur is being lost continually to the oceans as it drains through lakes and streams (and through the atmosphere) on its way to the sea.

In the ocean ecosystem, sulfur can take one of three routes. Some of it circulates through food webs, and some drifts to the bottom to bond with iron in the form of ferrous sulfide, or FeS. Ferrous sulfide contributes to the dark color of sediments at the bottom of the ocean. On the other hand, sulfur may be returned to the atmosphere, released by spray from saltwater. In addition, sulfur can pass into the atmosphere as the result of volcanic activity or through the action of bacteria, which release it in the form of hydrogen sulfide, the foul-smelling gas discussed earlier.

As with all biogeochemical cycles, humans play a part in the sulfur cycle, and the role of modern industrial society is generally less than favorable, as is true of most such cycles. A particularly potent example is the production of acid rain. Among the impurities in coal is sulfur, and when coal is burned (as it still is, for instance, in electric power plants), it results in the production of sulfur dioxide and sulfur trioxide—SO₂and SO₃, respectively. Sulfur trioxide reacts with water in the air to produce sulfuric acid, or H₂SO₄. This mixes with moisture in the atmosphere to create acid rain, which is hazardous to both plant and animal life.

WHERE TO LEARN MORE

Beatty, Richard. Sulfur. New York: Benchmark Books, 2000.

——. Phosphorus. New York: Benchmark Books, 2001.

Biogeochemical Cycles (Web site). <http://www.bsi.vt.edu/chagedor/biol_4684/Cycles/cycles.html>.

General Chemistry Online (Web site). <http://antoine.fsu.umd.edu/chem/senese/101/index.shtml>.

Geochemistry on the World Wide Web (Web site). <http://www.geo.cornell.edu/geology/classes/Geochemweblinks.HTML>.

Hancock, Paul L., Brian J. Skinner. The Oxford Companion to the Earth. New York: Oxford University Press, 2000.

Kump, Lee R., James F. Kasting, and Robert G. Crane. The Earth System. Upper Saddle River, NJ: Prentice Hall, 2000.

Life and Biogeochemical Cycles (Web site). <http://essp.csumb.edu/esse/climate/climatebiogeo.html>.

Skinner, Brian J., Stephen C. Porter, and Daniel B. Botkin. The Blue Planet: An Introduction to Earth System Science. 2d ed. New York: John Wiley and Sons, 1999.

WebElements Periodic Table of the Elements (Web site). <http://www.webelements.com/webelements/scholar/index.html>.

KEY TERMS

ALCHEMY:

A set of mystical beliefs based on the idea that ordinary matter can be perfected. Though it was a pseudo-science, alchemy, which flourished in the late Middle Ages, was a forerunner of scientific chemistry.

ATMOSPHERE:

In general, an atmosphere is a blanket of gases surrounding a planet. Unless otherwise identified, however, the term refers to the atmosphere of Earth, which consists of nitrogen (78%), oxygen (21%), argon (0.93%), and other substances that include water vapor, carbon dioxide, ozone, and noble gases such as neon (0.07%).

ATOMIC NUMBER:

The number of protons in the nucleus of an atom. Since this number is different for each element, elements are listed on the periodic table in order of atomic number.

BIOGEOCHEMICAL CYCLES:

The changes that particular elements undergo as they pass back and forth through the various earth systems and particularly between living and nonliving matter. The elements involved in biogeochemical cycles are hydrogen, oxygen, carbon, nitrogen, phosphorus, and sulfur.

BIOSPHERE:

A combination of all living things on Earth—plants, mammals, birds, amphibians, reptiles, aquatic life, insects, viruses, single-cell organisms, and so on—as well as all formerly living things that have not yet decomposed.

CARNIVORE:

A meat-eating organism.

CHEMICAL BONDING:

The joining through electromagnetic force of atoms that sometimes, but not always, represent more than one chemical element. The result is the formation of a molecule.

CHEMICAL SYMBOL:

A one-letter or two-letter abbreviation for the name of an element.

COMPOUND:

A substance made up of atoms of more than one element chemically bonded to one another.

DECOMPOSERS:

Organisms that obtain their energy from the chemical breakdown of dead organisms as well as from animal and plant waste products. The principal forms of decomposers are bacteria and fungi.

DECOMPOSITION REACTION:

A chemical reaction in which a compound is broken down into simpler compounds or into its constituent elements. In the earthsystem, this often is achieved through the help of detritivores and decomposers.

DETRITIVORES:

Organisms that feed on waste matter, breaking organic material down into inorganic substances that then can become available to the biosphere in the form of nutrients for plants. Their function is similar to that of decomposers;however, unlike decomposers—which tend to be bacteria or fungi—detritivores are relatively complex organisms, such as earthworms or maggots.

ECOSYSTEM:

A term referring to a community of interdependent organisms along with the inorganic components of their environment.

ELEMENT:

A substance made up of only one kind of atom. Unlike compounds, elements cannot be broken chemically into other substances.

EUTROPHICATION:

A state of heightened biological productivity in a body of water, which is typically detrimental to the ecosystem in which it takes place. Eutrophication can be caused by an excess of nitrogen or phosphorus in the form of nitrates and phosphates, respectively.

FOOD WEB:

A term describing the interaction of plants, herbivores, carnivores, omnivores, decomposers, and detritivores, each of which consumes nutrientsand passes it along to other organisms.

GEOCHEMISTRY:

A branch of the earth sciences, combining aspects of geology and chemistry, that is concerned with the chemical properties and processes of Earth—inparticular, the abundance and interaction of chemical elements and their isotopes.

GEOSPHERE:

The upper part of Earth's continental crust, or that portion of the solid earth on which human beings live and which provides them with most of their food and natural resources.

HERBIVORE:

A plant-eating organism.

HYDROSPHERE:

The entirety of Earth's water, excluding water vapor in the atmosphere but including all oceans, lakes, streams, groundwater, snow, and ice.

LEACHING:

The removal of soil materials that are in solution, or dissolved in water.

MINERAL:

A naturally occurring, typically inorganic substance with a specific chemical composition and a crystalline structure.

MIXTURE:

A substance with a variable composition, meaning that it is composed of molecules or atoms of differing types. Compare with compound.

MOLECULE:

A group of atoms, usually but not always representing more than one element, joined in a structure. Compounds typically are made up of molecules.

OMNIVORE:

An organism that eats both plants and other animals.

ORGANIC:

At one time chemists used the term organic only in reference to living things. Now the word is applied to most compounds containing carbon, with the exception of carbonates (which are minerals) and oxides, such as carbondioxide.

PERIODIC TABLE OF ELEMENTS:

A chart that shows the elements arranged in order of atomic number along with their chemical symbols and the average atomic mass for each particular element.

PHOTOSYNTHESIS:

The biological conversion of light energy (that is, electromagnetic energy) from the Sun to chemical energy in plants.

REACTIVITY:

A term referring to the ability of one element to bond with others. The higher the reactivity, the greater the tendency to bond.

SEDIMENT:

Material deposited at or near Earth's surface from a number of sources, most notably preexisting rock. There are three types of sediment: rocks, or clastic sediment; mineral deposits, or chemical sediment; and organic sediment, composed primarily of organic material.

SOLUBLE:

Capable of being dissolved.

SOLUTION:

A homogeneous mixture(i.e., one that is the same throughout) in which one or more substances is dissolved in another substance—for example, sugar dissolved in water.

WEATHERING:

The breakdown of rocks and minerals at or near the surface of Earth due to physical, chemical, or biological processes.

Biogeochemical Cycles

Biogeochemical cycles refer to the movement of chemical elements between living (biotic) and nonliving (abiotic) forms in the environment. Although many elements undergo this type of cycling to some extent, four elements—carbon, nitrogen, phosphorus, and sulfur—are most commonly discussed because of their importance (along with hydrogen and oxygen) for living organisms. The extent and rate of the cycling of these elements has important consequences, such as influencing the amount of phosphate available to forests and the ability of the oceans to slow down global warming by absorbing carbon dioxide.

Common Compounds

All the elements that undergo cycling are incorporated into compounds. Carbon may be found as inorganic CO₂ gas, carbonate ions (CO₃²⁻) in rocks or the oceans, or in organic compounds, such as sugars and proteins , within living organisms. Nitrogen exists in the atmosphere as N₂ or ammonia (NH₃), in the soil as an ion such as nitrate (NO₃⁻), and in living organisms in a variety of organic compounds, including proteins and nucleic acids. Wherever it occurs, phosphorus is largely bound to oxygen to make a phosphate ion (PO₄³⁻). Sulfur exists as sulfur dioxide gas (SO₂), sulfate ions (SO₄²⁻) in rocks, and in living organisms incorporated into proteins.

The atmosphere, oceans, fresh water, rocks, soil, and living organisms can each be thought of as a "pool" for storing these compounds. The time spent in any one pool is quantified as the mean residence time (MRT). For instance, the MRT for phosphate in rock may be thousands of years, whereas the MRT for the phosphate in a stand of corn is less than one year.

Transport Mechanisms

Elements move from one pool to another through meteorological, geologic, biological, or anthropogenic mechanisms. Meteorological mechanisms revolve around precipitation, such as rain carrying SO₂ into the soil. Geologic mechanisms include erosion, which can bring rock ions into solution, as well as sedimentation and volcanoes.

Biological mechanisms are those carried on by living organisms, such as photosynthetic conversion of CO₂ to sugar, or conversion of soil NH₃ to gaseous N₂ by soil bacteria. Marine birds can have a significant local impact on transport of phosphate and nitrogen from ocean to land. Many islands off the western coast of South America, for instance, are covered with a layer of white guano, dropped by generations of birds feasting on anchovies. Harvest of this rich fertilizer forms part of the economies of Peru, Chile, and Ecuador.

Anthropogenic mechanisms are those carried on by humans and are therefore a subset of biological mechanisms. Humans have a profound effect on biogeochemical cycles through agriculture (for example, adding nitrogen to the global nitrogen cycle through fertilizer applications), forestry, and especially the use of carbon-based fossil fuels. The release of vast amounts of carbon from stored pools is likely to raise the world's temperature by at least several degrees over the coming decades, with the potential for significant consequences on many forms of life. An important, yet unanswered, question is whether the forests, soil, and especially the ocean can absorb this extra CO₂ and thereby reduce the extent of global warming.

see also Carbon Cycle; Ecosystem; Global Climate Change; Nitrogen Cycle; Photosynthesis; Plankton

Richard Robinson

Bibliography

Berner Elizabeth Kay, and Robert A. Berner. Global Environment: Water, Air, and Geochemical Cycles. Upper Saddle River, NJ: Prentice Hall, 1996.

Biogeochemical Cycles

Nutrients are elements that plants require for growth. In most terrestrial ecosystems , a lack of essential nutrients may limit plant primary productivity. Net primary productivity (NPP), other ecosystem processes, and ecosystem structure and function may be best understood by examining and studying the cycles of these nutrients. To trace the movement of a nutrient—such as nitrogen (N), phosphorus (P), sulfur (S), or carbon (C)—as it travels between the living (biotic) and nonliving (abiotic) components of an ecosystem is to trace its biogeochemical cycle.

The Phosphorus Cycle

A generalized biogeochemical cycle of phosphorus, for example, starts with its release from apatite (phosphorus-containing rock). Inorganic, plant-available forms of phosphorus in most soils derive from apatite. Mechanical and chemical weathering reactions release phosphorus from apatite into the soil solution. Plants take up available forms of phosphorus, such as orthophosphate (H₂ PO₄^3-), from the soil solution into their roots. After uptake from the soil, phosphorus travels as the phosphate anion HPO₄^2- through the plant before accumulating in leaves and other living tissues. Phosphate is present in plant cells and circulating fluids at a concentration of about 10^-3m. Plants also incorporate inorganic phosphate into organic forms that may be used for various metabolic processes. Because there are no gaseous forms of phosphorus and soil reserves are small, phosphorus is difficult for plants to acquire. As a result, plants hoard phosphorus. Rather than releasing it back into the environment, plants send the phosphorus to the roots for storage before dropping their leaves. This process, called translocation , ensures that the plant will have a sufficient supply of phosphorus for the next growing season. Any phosphorus remaining in the dead leaves falls to the ground in leaf litter. This phosphorus gradually returns to the soil organic matter after microorganisms, such as fungi and bacteria, break down the litter through decomposition, and the cycle commences again.

The time that it takes for the phosphorus to move from apatite into the soil solution is called the flux rate. This represents the amount of time it takes a given amount of a certain element to move between the pools, or reservoirs. Flux rates can be very slow or very rapid. It may take hundreds of years for the phosphorus in the apatite to move into the soil solution. In contrast, once plant-available phosphorus is in the soil solution it is rapidly taken up by the plant roots. The mean residence time (MRT) is the length of time that elements remain in a pool. The MRT for phosphorus in apatite may be thousands of years, but within a plant, the MRT may be only one year. Nutrients move between pools through meteorological, geological, hydrological, biological, or anthropogenic mechanisms.

Transport Mechanisms

Meteorological mechanisms of nutrient transport are generally related to precipitation in the form of rain, fog, snow, or ice. For example, nitrogen has various gaseous phases including ammonia (NH₃) and nitrous oxides (NO_x). The nitrogen-containing gas may dissolve in precipitation, whereupon the nitrogen is subsequently deposited on plant and soil surfaces. In contrast, phosphorus, which has no gaseous phase, is not incorporated in rain but may be transported as dust by wind currents through the atmosphere. Nutrients also move slowly over the long term (hundreds of thousands of years) and over long distances via geological mechanisms, such as sedimentation , uplift , and volcanism. For example, carbon (C) may be stored in combination with calcium (Ca) as calcium carbonate (CaCO₃) in seashells. The shells fall to the ocean floor and through sedimentation processes become calcite, or calcium carbonate rock. Over thousands or millions of years, this carbon may be released slowly to the atmosphere from near-shore sedimentary rocks. Eventually, the nitrogen deposited on the land by precipitation and the carbon released from the calcite become incorporated into organic matter, the biological component of ecosystems.

Biological mechanisms generally refer to the microbial transformations of elements that are stored in organic matter into inorganic forms of nutrients that may be used by plants. For example, soil bacteria and fungi release acids that break down leaf litter and release the phosphorus and nitrogen that are bound in it. The phosphorus and nitrogen then combine with oxygen or hydrogen to form plant-available compounds .

Biological mechanisms of nutrient distribution can also include movement of nitrogen or phosphorus from one area to another via mammals or birds. Studies of bison movement in Oklahoma's tallgrass prairie ecosystem show that, when nitrogen-containing bison fecal pats decompose due to fire or chemical breakdown, they create a spatially patchy distribution of soil nitrogen; this patchiness of nitrogen may influence plant distributions. Likewise, a trip to the Caribbean island chain Los Roques, off the coast of Venezuela, provides a striking example of phosphorus distribution by seabirds. Guano, the white bird droppings that coat the island's rock out-crops, contains some of the highest phosphorus concentrations in the world.

Humans are probably the most important biological vectors for nutrient transport on Earth, particularly for carbon, nitrogen, sulfur, and phosphorus. Anthropogenic combustion of fossil fuels releases carbon dioxide(CO₂) to the atmosphere in quantities that exceed the combined releases of CO₂ from plant, animal, and microbial respiration, natural forest and grassland fires, and volcanic emissions. This has contributed to the build-up of CO₂ in Earth's atmosphere and may alter the biogeochemical cycles of other elements. Fossil fuel combustion also releases nitrogen and sulfur, which ultimately contributes to the formation and deposition of acid rain. Mining of phosphorus, such as in Los Roques, has altered the long-term storage of phosphorus, increased the flux rate of the global phosphorus cycle, and contributed to the phosphorus pollution of freshwater ecosystems worldwide. These and other human activities are altering the biogeochemical cycles of nitrogen, phosphorus, sulfur, and carbon at the global scale, with largely unknown consequences for Earth's inhabitants and ecosystems.

see also Biogeography; Carbon Cycle; Nitrogen Fixation; Nutrients.

Anne Fernald Cross

Bibliography

Berner, E. K., and R. A. Berner. Global Environment: Water, Air, and Geochemical Cycles. Upper Saddle River, NJ: Prentice-Hall, 1996.

Mackenzie, F. T., and Judith A. Mackenzie. Our Changing Planet: An Introduction to Earth System Science and Global Environmental Change. Upper Saddle River, NJ: Prentice-Hall, 1995.

Schlesinger, W. H. Biogeochemistry: An Analysis of Global Change. San Diego, CA: Academic Press, 1997.

Biogeochemical cycles

The term biogeochemical cycle refers to any set of changes that occur as a particular element passes back and forth between the living and non-living worlds. For example, carbon occurs sometimes in the form of an atmospheric gas (carbon dioxide ), sometimes in rocks and minerals (limestone and marble ), and sometimes as the key element of which all living organisms are made. Over time, chemical changes occur that convert one form of carbon to another form. At various points in the carbon cycle, the element occurs in living organisms and at other points it occurs in the earth's atmosphere, lithosphere , or hydrosphere.

The universe contains about ninety different naturally occurring elements. Six elements—carbon, hydrogen, oxygen , nitrogen, sulfur, and phosphorus—make up over 95% of the mass of all living organisms on Earth. Because the total amount of each element is essentially constant, some cycling process must take place. When an organism dies, for example, the elements of which it is composed continue to move through a cycle, returning to the earth, to the air, to the ocean, or to another organism.

All biogeochemical cycles are complex. A variety of pathways are available by which an element can move among hydrosphere, lithosphere, atmosphere, and biosphere . For instance, nitrogen can move from the lithosphere to the atmosphere by the direct decomposition of dead organisms or by the reduction of nitrates and nitrites in the soil . Most changes in the nitrogen cycle occur as the result of bacterial action on one compound or another. Other cycles do not require the intervention of bacteria. In the sulfur cycle, for example, sulfur dioxide in the atmosphere can react directly with compounds in the earth to make new sulfur compounds that become part of the lithosphere. Those compounds can then be transferred directly to the biosphere by plants growing in the earth.

Most cycles involve the transport of an element through all four parts of the planet—hydrosphere, atmosphere, lithosphere, and biosphere. The phosphorous cycle is an exception since phosphorus is essentially absent from the atmosphere. It does move from biosphere to the lithosphere (when organisms die and decay) to the hydrosphere (when phosphorous-containing compounds dissolve in water ) and back to the biosphere (when plants incorporate phosphorus from water).

Hydrogen and oxygen tend to move together through the planet in the hydrologic cycle . Precipitation carries water from the atmosphere to the hydrosphere and lithosphere. It then becomes part of living organisms (the biosphere) before being returned to the atmosphere through respiration, transpiration, and evaporation .

All biogeochemical cycles are affected by human activities. As fossil fuels are burned, for example, the transfer of carbon from a very old reserve (decayed plants and animals buried in the earth) to a new one (the atmosphere, as carbon dioxide) is accelerated. The long-term impact of this form of human activity on the global environment, as well as that of other forms, is not yet known. Some scientists assert, however, that those affects can be profound, resulting in significant climate changes far into the future.

See also Atmospheric composition and structure; Chemical elements; Dating methods; Evolution, evidence of; Evolutionary mechanisms; Fossil record; Fossils and fossilization; Geochemistry; Geologic time; Global warming; Greenhouse gases and greenhouse effect; Hydrologic cycle; Origin of life; Petroleum; Stellar life cycle

Biogeochemical cycles

The term biogeochemical cycle refers to any set of changes that occur as a particular element passes back and forth between the living and non-living worlds. For example, carbon occurs sometimes in the form of an atmospheric gas (carbon dioxide), sometimes in rocks and minerals (limestone and marble), and sometimes as the key element of which all living organisms are made. Over time, chemical changes occur that convert one form of carbon to another form. At various points in the carbon cycle, the element occurs in living organisms and at other points it occurs in the Earth's atmosphere, lithosphere, or hydrosphere.

The universe contains about ninety different naturally occurring elements. Six elements, carbon, hydrogen, oxygen, nitrogen, sulfur, and phosphorus, make up over 95% of the mass of all living organisms on Earth. Because the total amount of each element is essentially constant, some cycling process must take place. When an organism dies, for example, the elements of which it is composed continue to move through a cycle, returning to the Earth, to the air, to the ocean, or to another organism.

All biogeochemical cycles are complex. A variety of pathways are available by which an element can move among hydrosphere, lithosphere, atmosphere, and biosphere. For instance, nitrogen can move from the lithosphere to the atmosphere by the direct decomposition of dead organisms or by the reduction of nitrates and nitrites in the soil. Most changes in the nitrogen cycle occur as the result of bacterial action on one compound or another. Other cycles do not require the intervention of bacteria . In the sulfur cycle, for example, sulfur dioxide in the atmosphere can react directly with compounds in the earth to make new sulfur compounds that become part of the lithosphere. Those compounds can then be transferred directly to the biosphere by plants growing in the earth.

Most cycles involve the transport of an element through all four parts of the planet—hydrosphere, atmosphere, lithosphere, and biosphere. The phosphorous cycle is an exception since phosphorus is essentially absent from the atmosphere. It does move from biosphere to the lithosphere (when organisms die and decay) to the hydrosphere (when phosphorous-containing compounds dissolve in water) and back to the biosphere (when plants incorporate phosphorus from water).

Hydrogen and oxygen tend to move together through the planet in the hydrologic cycle. Precipitation carries water from the atmosphere to the hydrosphere and lithosphere. It then becomes part of living organisms (the biosphere) before being returned to the atmosphere through respiration , transpiration, and evaporation.

All biogeochemical cycles are affected by human activities. As fossil fuels are burned, for example, the transfer of carbon from a very old reserve (decayed plants and animals buried in the earth) to a new one (the atmosphere, as carbon dioxide) is accelerated. The long-term impact of this form of human activity on the global environment, as well as that of other forms, is not yet known. Some scientists assert, however, that those affects can be profound, resulting in significant climate changes far into the future.

See also Biodegradable substances; Carbon cycle in microorganisms; Composting, microbiological aspects; Economic uses and benefits of microorganisms; Evolution and evolutionary mechanisms; Evolutionary origin of bacteria and viruses; Nitrogen cycle in microorganisms; Oxygen cycle in microorganisms