The Biosphere

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THE BIOSPHERE

CONCEPT

The biosphere is simply "life on Earth"—the sum total, that is, of all living things on Earth. Yet the whole is more than the sum of the parts: not only is the biosphere an integrated system whose many components fit together in complex ways, but it also works, in turn, in concert with the other major earth systems. The latter include the geosphere, hydrosphere, and atmosphere, through which circulate the chemical elements and compounds essential to life. Among these elements is carbon, a part of all living things, which also cycles through the nonliving realms of soil, water, and air—just one of many vital biogeo-chemical cycles. As for the compounds on which life depends, none is more important than water, which, though it is the focal point of the hydrosphere, passes through the various earth systems as well. Organisms participate in the hydrologic cycle by providing moisture to the air through the process of transpiration, and they likewise benefit from the downward movement of moisture in the form of precipitation. These and many other interactions make it easy to see why scientists speak of Earth as a system—and why some go even further and call it a living thing.

HOW IT WORKS

Earth Systems

Chances are that the mention of the word system calls to mind something mechanical or electrical, produced by humans: for example, a heating and cooling system. This application of the word is close to the scientific meaning, but in the sciences system identifies a wider range of examples. In scientific terms a system is any set of interactions that can be set apart mentally from the rest of the universe for the purposes of study, observation, and measurement. Thus, virtually by definition, a system is something in which the various parts fit together harmoniously, as though they were designed—or adapted over millions of years—to do so.

Anything outside the system is known as the environment, and numerous qualifying terms identify the level of interaction between the system and its environment. An isolated system is one so completely sealed off from its environment that neither matter nor energy passes through its boundaries. This is a merely theoretical construct, however, because, in practice, some matter always flows between system and environment. For example, regardless of how tightly a vault or other interior chamber is sealed, there is always room for matter at the microscopic or atomic level to pass through the barrier; moreover, energy, which in many forms does not require any material medium for its transmission, will pass through as well.

Earth itself is an approximation of a closed system, or a system in which, despite the sound of its name, exchanges of energy (but not matter) with the environment are possible. Earth absorbs electromagnetic energy from the Sun and returns that energy to space in a different form, but very little matter enters or departs Earth's system. Earth is not a perfectly closed system, however, since meteorites can enter the atmosphere and hydrogen can escape. Without the intrusion of meteorites, in fact, it is unlikely that life could exist on the planet, because these projectiles from space first brought water (and possibly even the carbon-based rudiments of life) to the planet Earth more than four billion years ago.

Nevertheless, Earth more closely resembles a closed system than it does an open system, or one that allows the full and free exchange of both matter and energy with its environment. The human circulatory system is an example of an open system, as are the various "spheres" of Earth (geosphere, hydrosphere, biosphere, and atmosphere) that we discuss later in this essay. The distribution of matter and energy within these earth systems may vary over time, but the total amount of energy and matter within the larger earth system is constant.

THE FOUR "SPHERES."

On the other hand, the four subsystems, or "spheres," within the larger Earth system are very much open systems. Of these four subsystems, the name of only one, the atmosphere, is familiar from everyday life, whereas those of the other three sectors (geosphere, hydrosphere, and biosphere) may sound at first like scientific jargon. Yet each has a distinct identity and meaning, and each represents a part of Earth that is at once clearly defined and virtually inseparable from the rest of the planet.

The geosphere is the upper part of the planet's continental crust, the portion of the solid earth on which human beings live and which provides them with most of their food and natural resources. It is also the oldest, followed by the hydrosphere, which had its beginnings with several hundred millions years' worth of rains that took place about four billion years ago. Today the hydrosphere includes all water on Earth, except for water vapor in the atmosphere. The latter, incidentally, was probably the last of the four subsystems to take shape: though Earth in its early stages had a blanket of gases around it, there was no oxygen. (See Paleontology for more about the early atmosphere, oxygen, and early life.)

THE ATMOSPHERE.

Today the atmosphere is 78% nitrogen, 21% oxygen, and 0.93% argon. The remaining 0.07% is made up of water vapor, carbon dioxide, ozone (a form of oxygen in which three oxygen atoms bond chemically), and noble gases. The noble gases, including argon and neon, are noted for their lack of reactivity, meaning that they are extremely resistant to chemical bonding with other elements.

Nitrogen also tends to be unreactive, and the reason for its abundance in the atmosphere lies in the fact that it never attempted to bond with other elements. Therefore, nitrogen, along with the noble gases, is simply "hanging in the air" (literally), left over from the time when volcanoes hurled it into the atmosphere several billion years ago. By contrast, oxygen (both in O₂ and O_3, or ozone, molecules) and the other elements in air are vital to life. Furthermore, oxygen is one of two elements, along with hydrogen, that goes into the formation of water.

OVERLAP BETWEEN SUBSYSTEMS.

The present atmosphere would not exist without the biosphere. In order to put oxygen into the air, there had to be plants, which take in carbon dioxide and release oxygen in the process of photosynthesis. This resulted from an exceedingly complex series of evolutionary developments from anaerobic, or non-oxygen-breathing, single-cell life-forms to the appearance of algae. As plant life evolved, eventually it put more and more oxygen into the atmosphere, until the air became breathable for animal life. Thus, the atmosphere and biosphere have sustained one another.

Such overlap is typical and indeed inevitable where the open earth subsystems are concerned, and examples of this overlap are everywhere. For instance, plants (biosphere) grow in the ground (geosphere), but to survive they absorb water (hydrosphere) and carbon dioxide (atmosphere). Nor are plants merely absorbing: they also give back oxygen to the atmosphere, and by providing nutrition to animals, they contribute to the biosphere. At the same time, the many components of the picture just described are involved in complex biogeochemical cycles, which we look at later.

THE BIOSPHERE IN CONTEXT.

The biosphere is, of course, integral to the functioning of earth systems. First of all, the present atmosphere, as we have noted, is the product of respiration on the part of plants, which receive carbon dioxide and produce oxygen. In addition, transpiration, a form of evaporation from living organisms (primarily plants), is a mechanism of fundamental importance for moving moisture from the hydrosphere through the biosphere to the atmosphere.

We examine transpiration later, within the larger context of evapotranspiration, along with another area in which the biosphere interacts closely with one or more of the other earth systems: soil. Though soil is part of the geosphere, its production and maintenance is an achievement of all spheres. The role of the biosphere in this instance is particularly important: the amount of decayed organic material (i.e., dead plants and animals) is critical to the quality of the soil for sustaining further life in the form of plants and other organisms that live underground.

As important as the biosphere is, it may be surprising to learn just what a small portion of the overall earth system it occupies. As living beings, we tend to have a bias in favor of the living world, but the overwhelming majority of the planet's mass and space is devoted to nonliving matter. The geosphere alone accounts for almost 82% of the combined mass of the four subsystems. (This is not the combined mass of Earth, which would be much larger; remember that the geosphere is only the extreme upper layer of Earth, and does not include the vast depths of the lower mantle and core.)

Of the remaining mass that makes up the four earth systems, the hydrosphere is a little more than 18%, the atmosphere less than 1%, and the biosphere a tiny 0.00008%. Note just how much greater the amount of mass is in the air, which we tend to think of as being weightless (though, of course, it is not), than in the biosphere. Even within the biosphere's almost infinitesimal fraction of total mass, the animal kingdom accounts for less than 2%, the remainder being devoted to other kingdoms: plants, fungi, monera (including bacteria), and protista, such as algae. (See Taxonomy and Species for more about the kingdoms of living things.) It need hardly be added that humans, in turn, are a very, very small portion of the animal world.

Water and the Hydrologic Cycle

As any backyard horticulturist knows, plants need good soil and water. In the course of circulating throughout Earth, water makes its way through organisms in the biosphere as well as reservoirs housed within the geosphere. It also circulates continuously between the hydrosphere and the atmosphere. This movement, known as the hydrologic cycle, is driven by the twin processes of evaporation and transpiration.

The first of these processes, of course, is the means whereby liquid water is converted into a gaseous state and transported to the atmosphere, while the second one—a less familiar term—is the process by which plants lose water through their stomata, small openings on the undersides of leaves. Scientists usually speak of the two as a single phenomenon, evapotranspiration. The atmosphere is just one of several "compartments" in which water is stored within the larger environment. In fact, the atmosphere is the only major reservoir of water on Earth that is not considered part of the hydrosphere.

ACCOUNTING FOR EARTH'S WATER SUPPLY.

The water that most of us see or experience is only a very small portion of the total. Actually, that statement should be qualified: the oceans, parts of which most people have seen, make up about 5.2% of Earth's total water supply. This may not sound like a large portion, but, in fact, the oceans are the second-largest water compartment on Earth. If the oceans are such a small portion yet rank second in abundance, two things are true: there must be a lot of water on Earth, and most of it must be in one place.

In fact, the vast majority of water on Earth is stored in aquifers, or underground rock formations, that hold 94.7% of the planet's water. Thus, deep groundwater and oceans account for 99.9% of the total. Glaciers and other forms of permanent and semipermanent ice take third place, with 0.065%. Another 0.03% appears in the form of shallow groundwater, the source of most local water supplies. Next are the inland surface waters, including such vast deposits as the Great Lakes and the Caspian Sea as well as the Mississippi-Missouri, Amazon, and Nile river systems and many more, which collectively make up just 0.003% of Earth's water.

ATMOSPHERIC MOISTURE AND WEATHER.

That leaves only 0.002%, which is the proportion taken up by moisture in the atmosphere: clouds, mist, and fog, as well as rain, sleet, snow, and hail. While it may seem astounding that atmospheric moisture is such a small portion of the total, this fact says more about the vast amounts of water on Earth than it does about the small amount in the atmosphere. That "small" amount, after all, weighs 1.433 × 10¹³ tons (1.3 × 10¹³ tonnes), or 28,659,540,000,000,000 pounds (12,999,967,344,000,002 kg).

This moisture in the atmosphere is the source of all weather, which clearly has an effect on Earth's life-forms. (Weather is the condition of the atmosphere at a given time and in a given place, whereas climate is the pattern of weather in a particular area over an extended period of time.) On the one hand, rain is necessary to provide water to plants, and desert conditions can sustain only very specific life-forms; on the other hand, storms, icy precipitation, and flooding can be deadly.

Biogeochemical Cycles

Water is not the only substance that circulates through the various earth systems. So, too, do six other substances or, rather, chemical elements. These elements are composed of a single type of atom, meaning that they cannot be broken down chemically to make a simpler substance, as is the case with such compounds as water. The six elements that cycle throughout Earth's systems are hydrogen, oxygen, carbon, nitrogen, phosphorus, and sulfur. The two following lists provide rankings for their abundance. The first shows their ranking and share in the entire known mass of the planet, including the crust, living matter, the oceans, and atmosphere. The second list 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%)

Note 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 relative abundance 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. All six of these elements take part in biogeochemical cycles, a term used to refer to 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.

THE CARBON AND NITROGEN CYCLES.

Carbon, for instance, is present in all living things and is integral to the scientific definition of the word organic. The latter term does not, as is popularly believed, refer only to living and formerly living things, their parts, and their products, such as sweat or urine. Organic refers to the presence of compounds containing carbon and hydrogen. The realm of organic substances encompasses not only the world of the living, the formerly living, and their parts and products, but also such substances as plastics that have never been living.

The carbon cycle itself involves movement between the worlds of the living and nonliving, the organic and inorganic. This highly complex biogeochemical cycle circulates carbon from soil and carbonate rocks (which are inorganic because they do not contain carbon-hydrogen compounds) to plants and hence to animals, which put carbon dioxide into the air. Likewise, the nitrogen cycle moves that element among all these reservoirs.

Because nitrogen is highly unreactive, the participation of microorganisms in the nitrogen cycle is critical to moving that element between the various earth systems. These organisms "fix" nitrogen, meaning that by processing the element through their bodies, they bring about a chemical reaction that makes nitrogen usable to plant life. Additionally, detritivores and decomposers in the soil are responsible for transforming nitrites and nitrates (compounds of nitrogen and oxygen) from the bodies of dead animals into elemental nitrogen that can be returned to the atmosphere.

REAL-LIFE APPLICATIONS

Soil and the Life in It

The soil is a sort of anchor to the biosphere. It teems with life like few other areas within the earth system, and, indeed, there are more creatures—plant, animal, monera, protista, and fungi—living in the soil than above it. Minerals from weathered rock in the soil provide plants with the nutrients they need to grow, setting in motion the first of several steps whereby organisms take root in and contribute to the soil.

Plants provide food to animals, which, when they die, likewise become one with the soil. So, too, do plants themselves. Organisms from both the plant and animal kingdoms leave behind material to feed such decomposers as bacteria and fungi, which, along with detritivores, are critical to the functioning of food webs (see entry). Detritivores, of which earthworms are a great example, are much more complex organisms than the typically single-cell decomposers.

We cannot see bacteria, but almost anyone who has ever dug in the dirt has discovered another type of organism: the Annelida phylum of the animal kingdom, which includes all segmented worms, among them, the earthworm. (Incidentally, the leech family also falls within phylum Annelida.) These slimy creatures at first might seem disgusting, but without the appropriately named earthworm, our world could not exist as it does.

Detritivores consume the remains of plant and animal life, which usually contain enzymes and proteins far too complex to benefit the soil in their original state. By feeding on organic remains, detritivores cycle these complex chemicals through their internal systems, thus causing the substances to undergo chemical reactions that result in the breakdown of their components. As a result, simple and usable nutrients are made available to the soil.

Earthworms, which are visible and relatively large, are not the only worms at work in the soil. There are also the colorless creatures of the phylum Nematoda, or roundworms. Included in this phylum are hookworms and pinworms, which can be extremely detrimental to the body when they live inside it as parasites. (See Parasites and Parasitology.) This is one good reason (among many) not to eat dirt. But nematodes in the soil, most of them only slightly larger than microorganisms, perform the vital function of processing organic material by feeding on dead plants. Even in a soil situation, however, some nematodes are parasites that live off the roots of such crops as corn or cotton.

In addition to earthworms, ants and other creatures are also significant inhabitants of the soil. Like earthworms, ants aerate soil and help bring oxygen and organic material from the surface while circulating soils from below. Among the larger creatures that call the soil home are moles, which live off earthworms, grubs (insect larvae), and the roots of plants. By burrowing under the ground, they help to loosen the soil, making it more porous and thus receptive to both moisture and air. Other large burrowing creatures include mice, ground squirrels, and, in some areas, even prairie dogs.

Soils and the Environments They Help Create

Five different factors determine the quality of soil: parent material (the decayed organisms and weathered rock that make it up), climate, the presence of living organisms, topography (the shape of the land, including prominent natural features), and the passage of time. These factors influence the ability of the soil to sustain life.

For example, in a desert, a place that obviously has a smaller abundance and complexity of life-forms (see Biological Communities), the soil itself is lacking in this life-sustaining quality. Desert soil, in fact, is usually referred to as immature soil. Healthy soil normally has a deep A horizon, the area in which decayed organic material appears and atop which humus sits. In general, the deeper the A horizon, the better the soil. Immature soil, on the other hand, has a very thin A horizon and no B horizon, which is the subsoil that typically separates the A horizon from a layer of weathered material that rests even lower, at the C horizon just above bedrock.

In deserts, by definition, the water supply is very limited, and only those species that require very little water—for example, the varieties of cactus that grow in the American Southwest—are able to survive. But lack of water is not the only problem. Desert subsoils often contain heavy deposits of salts, and when rain or irrigation adds water to the topsoil, these salts rise. Thus, watering desert topsoil actually can make it a worse environment for growth.

TROPICAL RAIN FORESTS.

The soil in rain forests has just the opposite problem of desert soil: instead of being immature, it has gone beyond maturity and reached old age, a point at which plant growth and water percolation (the downward movement of water through the soil) have removed most of its nutrients. In environments located near the equator, whether these regions be desert or rain forest, soils tend to be "old." This helps to explain the fact that equatorial regions are usually low in agricultural productivity, despite the fact that they enjoy an otherwise favorable climate.

Given the poor quality of equatorial soil, one might wonder how it is that some of the most lush rain forests are located in the equatorial regions of Africa and South America. The answer is that a rain forest has such a wide assortment of life-forms that there is never a shortage of decayed organic material to "feed" the soil. Due to the sheer breadth and scope of the rain forest's ecological diversity, there are bound to always be plants and animals dying, replenishing what would otherwise be poor soil. The rapid rate of decay common in warm, moist regions (which are extremely hospitable to bacteria and other microbes) further supports the process of renewing minerals in the ground.

The fact that decayed organic material is critical to the life of soil helps to explain why many environmentalists have long been concerned about the destruction of tropical rain forests. For example, in Brazil, vast portions of the Amazon rain forest have been clear-cut, subjected to an extremely destructive form of slash-and-burn agriculture that is motivated by modern economic concerns—the desire of the country's leaders to create jobs and exports—but which resembles practices applied (albeit on a much smaller scale) by premodern peoples in Central and South America.

By removing the heavy jungle canopy of tall trees, clear-cutting exposes the ground to the heat of the Sun and the pounding of monsoon rains. Sun and rain, thanks to the removal of this protection, fall directly on the ground, parching it in the first instance and eroding it in the second. Furthermore, when trees and other vegetation are removed, the animal life that these plants supported disappears as well, and this has a direct impact on the soil by removing organisms whose waste products and bodies eventually would have decayed and enriched it.

WHAT MAKES GOOD SOIL?

The soil of the Brazilian rain forest may be old and weak, but thankfully there are places in the world where the soil tells an entirely different story. Instead of being nutrient-poor, this soil is nutrient-rich, and instead of being red, such soil is a deep, rich black. Sometimes regions of good and bad soil exist in close proximity, as in ancient Egypt, where the Nile made possible a narrow strip of extraordinarily productive land, running the length of the country, flanked by deserts. The latter the Egyptians called "the red land" because of its nutrient-poor soil, whereas their own fertile region was "the black land."

Just as Egypt, after its annexation by the Romans in 30 b.c., became known as the bread-basket of the Roman Empire, there are other "black lands" that serve as the breadbaskets of today's world. Unlike Egypt, however, most are located in temperate rather than equatorial regions. (See Biomes for more about temperate regions.) Examples include the midwestern United States, western Canada, and southern Russia, regions characterized by vast plains of fertile black soil. Below this rich topsoil is a thick sub-soil that helps hold in moisture and nutrients.

Rivers such as the Nile, the Mississippi-Missouri, or the Volga in Russia make possible the richest variety of soil on Earth, alluvial soil, a youngish sediment of sand, silt, and clay. A river pulls soil along with it as it flows, and with this comes nutrients from the regions through which the river has passed. The river then deposits these nutrients in the alluvial soil at the delta, the area where it enters a larger body, usually of salt water. (A delta is so named because, as it widens in the region near the sea, the shape of the river is like that of the Greek letter delta, or Δ .)

Because they are depositories for accumulated alluvial soil, delta regions such as Mississippi and Louisiana in the United States (where the mighty Mississippi-Missouri, the largest river system in North America, empties into the Gulf of Mexico), are exceedingly fertile. The same is true in the Volga and Nile deltas, the deltas of the Danube and other major European rivers, and those of lesser-known rivers in Canada or Australia.

FERTILIZER.

It is also possible to artificially improve soil that is not in a river delta, or that has not otherwise been blessed by nature. The most significant way to achieve this is by using fertilizer, which augments the nutrients in the soil itself. As noted earlier, nitrogen is highly nonreactive, meaning that it tends not to bond chemically with other substances. However, because it is a necessary component of biogeo-chemical cycles, it is critical that nitrogen be introduced to the soil in such a way that it becomes useful, and typically this is done by combining it with a highly reactive element: oxygen.

Fertilizers may contain nitrogen in the form of a nitrate, which is a compound of nitrogen and oxygen, or they may include a nitrogen-hydrogen compound. The latter may be ammonia (NH₃) or ammonium (NH₄⁺). Note that ammonia is much more than the product with which it is most readily associated: a household cleaner. In fact, ammonia is an extremely abundant substance, occurring naturally, for instance, in the atmospheres of Venus and other planets in our solar system. The importance of ammonia is reflected by the fact that it and water are the only two substances that chemists regularly refer to by their common names, as opposed to a scientific name such as carbon dioxide.

As for ammonium, its extra hydrogen atom makes it a substance that dissolves in water and is attracted to negatively charged surfaces of clays and organic matter in soil. Therefore, it tends to become stuck in one place rather than to move around, as nitrate does. Plants in acidic soils typically receive their nitrogen from ammonium, but in nonacidic soils, nitrate is typically the more useful form of fertilizer. The two fertilizers are also combined to form ammonium nitrate, which is powerful both as a fertilizer and as an explosive. (Ammonium nitrate was used both in the first World Trade Center bombing, in 1993, and in the even more devastating Oklahoma City bombing two years later.)

Erosion and Soil Conservation

The mismanagement of agricultural lands, and/or the influence of natural forces, can produce devastating results, as illustrated by events during the years 1934 and 1935 in a region including Texas, Oklahoma, Kansas, and eastern Colorado. In just a few months, once-productive farmland turned into worthless fields of stubble and dust, good for virtually nothing. By the time it was over, the region had acquired a bitter nickname: the "dust bowl."

Ironically, in the years leading up to the early 1930s the future dust bowl farmlands had seemed remarkably productive. Farmers happily reaped abundant yields, year after year, not knowing that they were actually preparing the way for soil erosion on a grand scale. Farmers in the 1930s had long known about the principle of crop rotation as a means of giving the soil a rest and restoring its nutrients. But to be successful, crop rotation must include fallow years (i.e., no crops are planted), and must make use of crops that replenish the soil of nutrients.

Cotton and wheat are examples of crops that deplete nutrient content in the soil, and in fact wheat was the crop of choice in the future dust bowl. In some places, farmers alternated between wheat cultivation and livestock grazing on the same plot of land. The hooves of the cattle further damaged the soil, already weakened by raising wheat. The land was ready to become the site of a full-fledged natural disaster, and in the depths of the Great Depression, that disaster came in the form of high winds. These winds scattered vast quantities of soil from the Great Plains of the Midwest to the Atlantic seaboard, and acreage that once had rippled with wheat turned into desert-like wastelands.

The farmlands of the plains states have long since recovered from the dust bowl, and farming practices have changed considerably. Instead of alternating one year of wheat with a year of grazing livestock, farmers in the dust bowl region apply a three-year cycle of wheat, sorghum, and fallow land. They also have planted trees to serve as barriers against wind.

Years after the dust bowl, the American West could have again become the site of another disaster, had not farmers and agricultural officials learned from the mistakes of an earlier generation. During the 1970s, American farms enjoyed such a great surplus that farmers increasingly began to sell their crops to the Soviet Union, and farmers were encouraged to cultivate even marginal croplands to increase profits. This alarmed environmental activists, who called attention to the flow of nutrients from croplands into water resources. As a result of public concerns over these and related issues, Congress in 1977 passed the Soil and Water Resource Conservation Act, mandating measures to conserve or protect soil, water, and other resources on private farmlands and other properties.

Leaching and Its Effect on Soil

Like erosion, leaching moves substances through soil, only in this case it is a downward movement. Leached water can carry all sorts of dissolved substances, ranging from nutrients to contaminants. The introduction of manufactured contaminants to the soil, and hence the water table, is of course a serious threat to the environment. On the other hand, where human waste and other, more natural forms of toxin are concerned, nature itself is able to achieve a certain amount of cleanup on its own.

In a septic tank system, used by people who are not connected to a municipal sewage system, anaerobic bacteria process wastes, removing a great deal of their toxic content in the tank itself. (These bacteria usually are not introduced artificially to the tank; they simply congregate in what is a natural environment for them.) The waste-water leaves the tank and passes through a drain field, in which the water leaches through layers of gravel and other filters that help remove more of its harmful content. In the drain field, the waste is subjected to aerobic decay by other forms of bacteria before it either filters through the drain-pipes into the ground or is evaporated.

In addition to purifying water, leaching also passes nutrients to the depths of the A horizon and into the B horizon—something that is not always beneficial. In some ecosystems, leaching removes large amounts of dissolved nitrogen from the soil, and it becomes necessary to fertilize the soil with nitrate. However, soil often has difficulty binding to nitrate, which tends to leach easily, and this leads to an overabundance of nitrogen in the lower levels of the soil and groundwater. This is a condition known as nitrogen saturation, which can influence the eutrophication of waters (a topic discussed later) and cause the decline and death of trees with roots in an affected area of ground.

How Deserts Are Formed

Let us now consider what one might call an extreme ecosystem: a desert. Of the world's deserts, by far the most impressive is the Sahara, which today spreads across some 3.5 million sq. mi. (9.06 million sq km), an area that is larger than the continental United States. Only about 780 acres (316 hectares) of it, a little more than 1 sq. mi. (2.6 sq km), is fertile. The rest is mostly stone and dry earth with scattered shrubs—and here and there the rolling sand dunes typically used to depict the Sahara in movies.

Just 8,000 years ago—the blink of an eye in terms of Earth's timescale—it was a region of flowing rivers and lush valleys. For thousands of years it served as a home to many cultures, some of them quite advanced, to judge from their artwork. Though they left behind an extraordinary record in the form of their rock-art paintings and carvings, which show an understanding of realistic representation that would not be matched until the time of the Greeks, the identity of the early Saharan peoples themselves remains largely a mystery.

EVOLUTION OF THE ANCIENT SAHARAN ECOSYSTEM.

The phases in ancient Saharan cultures are identified by the names of the domesticated animals that dominated at given times, and collectively these names tell the story of the Saharan ecosystem's transformation from forest to grassland to desert. First was the Hunter period, from about 6000 to about 4000 b.c., when a Paleolithic, or Old Stone Age, people survived by hunting the many wild animals then available in the region. Next came the Herder period, from about 4000 to 1500 b.c. As their name suggests, these people maintained herds of animals and also practiced basic agriculture.

A high point of ancient Saharan civilization came with the Herder period, which, not surprisingly, also marked the high point of the local ecosystem in terms of its ability to sustain varied life-forms. Yet through a complex set of circumstances that scientists today do not understand fully, desertification—the slow transformation of ordinary lands to desert—had begun to set in. As the Sahara became drier and drier, the herds disappeared.

Eventually, the Egyptians began bringing in domesticated horses to cross the desert: hence the name of the Horse period (ca. 1500-ca. 600 b.c.), when the Sahara probably resembled the dry grasslands of western Texas or the sub-Saharan savanna in Africa today. By about 600 b.c., however, the climate had become so severe, the water supply so limited, and the ecosystem so depleted of supporting life-forms that not even horses could survive in the forbidding climate. There was only one mammal that could: the hardy, seemingly inexhaustible creature that gave its name to the Camel era, which continues to the present day.

CONTROLLING DESERTIFICATION.

What happened to the Sahara? The answer is a complex one, as is the subject of desertification. Desertification does not always result in what people normally think of as a desert; rather, it is a process that contributes toward making a region more dry and arid, and because it is usually gradual, it can be reversed in some cases. Nor is it necessary for a society to undertake large-scale mechanized agricultural projects, such as those of the American dust bowl of the 1930s, to do long-term damage that can result in desertification. The Pueblan culture of what is now the southwestern United States depleted an already dry and vulnerable region after about a.d. 800 by removing its meager stands of mesquite trees.

And though human causes, either in the form of mismanagement or deliberate damage, certainly have contributed to desertification, sometimes nature itself is the driving force. Long-term changes in rainfall or general climate, as well as water erosion and wind erosion such as caused the dust bowl, can turn a region into a permanent desert. An ecosystem may survive short-term drought, but if soil is forced to go too long without proper moisture, it sets in motion a chain reaction in which plant life dwindles and, with it, animal life. Thus, the soil is denied the fresh organic material necessary to its continued sustenance, and a slow, steady process of decline begins.

The Phosphorus Cycle

Having examined what can go wrong (and right) in soil, a key component of the biosphere, we will devote the remainder of this essay to two other aspects of the biosphere mentioned earlier: biogeochemical cycles and the hydrologic cycle. In the context of biogeochemical cycles, we will look at the phosphorus cycle, along with eutrophication—another instance of "what can go wrong."

Because of 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. In the early 1800s chemists recognized that the critical component in the bones was the phosphorus, which plants use in photosynthesis—the biological conversion of energy from the Sun into chemical energy. 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 phosphate, a type of phosphorus-based mineral.

Microorganisms in the biosphere absorb insoluble phosphorus compounds and, through the action of acids within the microorganisms, turn them into soluble phosphates. These soluble phosphates then are absorbed by algae and other green plants, which are eaten by animals. When they die, the animals, in turn, 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 vast 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.

Because fish absorb particles of phosphorus, some of it returns to dry land through the catching and consumption of seafood. Also, guano or dung from birds that live in an ocean environment (e.g., seagulls) returns portions of phosphorus to the terrestrial environment. Nevertheless, scientists believe that phosphorus is steadily being transferred 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 would otherwise be steadily lost. However, phosphorus still ends up making its way through the waters, and this creates a serious problem in the form of eutrophication.

EUTROPHICATION.

Eutrophication (from a Greek term meaning "well nourished") is a state of heightened biological productivity in a body of water. One of the leading causes of eutrophication is a high rate of nutrient input, in the form of phosphate or nitrate, a nitrogen-oxygen compound. 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 of doing so, they 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.

Lake Erie—one of the Great Lakes on the border of the United States and Canada—became an extreme example of eutrophication in the 1960s. 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 the phosphate content in fertilizers and detergents drastically. Within a few decades, thanks to the new measures, the lake once again teemed with life. Thus, Lake Erie became an environmental success story.

Evapotranspiration

Many of the phenomena and processes we have described tie together the biosphere with other "spheres" of Earth. Such is the case with evapo-transpiration, the sum total of evaporation and transpiration. The second of these terms is less well-known than the first, but the words in fact refer to the same process. The only difference is that evaporation involves the upward movement of water from nonliving sources, while transpiration is the evaporation of moisture from living sources.

Transpiration is at least as important as evaporation when it comes to putting moisture into the atmosphere. It actually puts more water into the air than evaporation does: any large area of vegetation tends to transpire much larger quantities of moisture than an equivalent nonfoliated region, such as the surface of a lake or moist soil. Though animals can play a part in transpiration, plant transpiration has much greater environmental significance.

Water in plants is lost through moist membranes of a tissue known as spongy mesophyll, found in the tiny cavities that lie beneath the microscopic leaf pores called stomata. Stomata remain open most of the time, but when they need to be closed, guard cells around their borders push them shut. Because plants depend on stomata to "breathe" by pulling in carbon dioxide, they keep them open—just as a human's pores must remain open. Otherwise, the person would not take in enough oxygen, and would perish.

The fact that the stomata are exposed in order to receive carbon dioxide for the plant's photosyn-thesis also means that the stomata are open to allow the loss of moisture to the atmosphere. It can be said, then, that transpiration in plants—vital as it is to the functioning of our atmosphere—is actually an unavoidable consequence of photosynthesis, an unrelated process. (See Carbohydrates for more about photosynthesis.)

ANIMAL TRANSPIRATION.

Transpiration in animals (including humans) takes place for much the same reason as it does with plants: as a by-product of breathing. Animals have to keep their moist respiratory surfaces, such as the lungs, open to the atmosphere. We may not think of our own breathing as transferring moisture to the air, but the presence of moisture in our lungs can be proved simply by breathing on a piece of glass and observing the misty cloud that lingers there.

Transpiration can cause animals to become dehydrated, but it also can be important in cooling down their bodies. When human bodies become overheated, they produce perspiration, which cools the surface of the skin somewhat. If the air around us is too humid, however, then it already is largely saturated with water, and the perspiration has no place to evaporate. Therefore, instead of continuing to cool our bodies, the perspiration simply forms a sticky film on the skin. But assuming the air is capable of absorbing more moisture, the sweat will evaporate, cooling our bodies considerably.

EFFECTS OF HEAT AND COLD.

When summer is at its height, air temperatures are warm and the trees are fully foliated (i.e., covered in leaves), and a high rate of transpiration occurs. So much water is pumped into the atmosphere through foliage that the rate of evapotranspiration typically exceeds the input of water to the local environment through rainfall. The result is that soil becomes dry, some streams cease to flow, and by late summer in extremely warm temperate areas, such as the southern United States, there is a great threat of drought and related problems, such as forest fires.

As trees drop their leaves in the autumn, transpiration rates decrease greatly. This makes it possible for the parched soil to become recharged by rainfall and for streams to flow again. Such is the case in a temperate region, which, by definition, is one that has the four seasons to which most people in the United States (outside Hawaii, Alaska, and extreme southern Florida and Texas) are accustomed. In a tropical region, by contrast, there is a "dry season," in which transpiration takes place, and a "rainy season," in which moisture from the atmosphere inundates the solid earth. This rainy season may be so intense that it produces floods.

WHERE TO LEARN MORE

Bial, Raymond. A Handful of Dirt. New York: Walker, 2000.

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

Bocknek, Jonathan. The Science of Soil. Milwaukee, WI: Gareth Stevens, 1999.

Global Hydrology and Climate Center (Web site). <http://wwwghcc.msfc.nasa.gov/>.

Hancock, Paul L., and Brian J. Skinner. The Oxford Com panion 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>.

Richardson, Joy. The Water Cycle. Illus. Linda Costello. New York: Franklin Watts, 1992.

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.

Smith, David. The Water Cycle. Illus. John Yates. New York: Thomson Learning, 1993.

KEY TERMS

A HORIZON:

Topsoil, the uppermost of the three major soil horizons. This layer and the humus that lies above it house all the organic content in soil.

AEROBIC:

Oxygen-breathing.

ANAEROBIC:

Non-oxygen-breathing.

ATMOSPHERE:

Earth's atmosphere is a blanket of gases that includes nitrogen (78%), oxygen (21%), argon (0.93%), and a combination of water vapor, carbon dioxide, ozone, and noble gases such as neon (0.07%). Most of these gases are con tained in the troposphere, the lowest layer, which extends to about 10 mi. (16 km) above the planet's surface.

B HORIZON:

Subsoil, beneath topsoil and above the C horizon. Though the B horizon contains no organic material, its presence is critical if the soil is to be suit able for sustaining a varied ecosystem.

BIOGEOCHEMICAL CYCLES:

The changes that particular elements undergo as they pass back and forth through the various earth systems (e.g., the biosphere) and particularly between living and non-living 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, animals, birds, marine life, insects, viruses, single-cell organisms, and so on—as well as all formerly living things that have not yet decomposed.

C HORIZON:

The bottommost of the soil horizons, between subsoil and bedrock. The C horizon is made of regolith, or weathered rock.

CANOPY:

The upper portion or layer of the trees in a forest. A forest with a closed canopy is one so dense with vegeta tion that the sky is not visible from the ground.

CLIMATE:

The pattern of weather conditions in a particular region over an extended period. Compare with weather.

CLOSED SYSTEM:

A system that per mits the exchange of energy with its external environment but does not allow matter to pass between the environment and the system. Compare with isolated system on the one hand and open system on the other.

COMPOUND:

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

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.

DECOMPOSITION REACTION:

A chemical reaction in which a compound is broken down into simpler compounds, or into its constituent elements. In the bios phere, 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.

ECOLOGY:

The study of the relation ships between organisms and their environments.

ECOSYSTEM:

A community of inter dependent organisms along with the in organic components of their environment.

ELEMENT:

A substance made up of only one kind of atom. Unlike compounds, elements cannot be broken down 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.

EVAPORATION:

The process whereby liquid water is converted into a gaseous state and transported to the atmosphere. When discussing the atmosphere and precipitation, usually evaporation is distinguished from transpiration. In this context, evaporation refers solely to the transfer of water from nonliving sources, such as the soil or the surface of a lake.

EVAPOTRANSPIRATION:

The loss of water to the atmosphere via the combined (and related) processes of evaporation and transpiration.

FOOD WEB:

A term describing the interaction of plants, herbivores, carnivores, omnivores, decomposers, and detritivores in an ecosystem. Each of these organisms consumes nutrients and passes it along to other organisms. Earth scientists typically prefer this name to food chain, an everyday term for a similar phenomenon. A food chain is a series of singular organisms in which each plant or animal depends on the organism that precedes it. Food chains rarely exist in nature.

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.

HUMUS:

Unincorporated, often par tially decomposed plant residue that lies at the top of soil and eventually will decay fully to become part of it.

HYDROCARBON:

Any organic chemical compound whose molecules are made up of nothing but carbon and hydrogen atoms.

HYDROLOGIC CYCLE:

The continuous circulation of water throughout Earth and between various earth systems.

HYDROSPHERE:

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

ISOLATED SYSTEM:

A system that is separated so fully from the rest of the uni verse that it exchanges neither matter nor energy with its environment. This is an imaginary construct, since full isolation is impossible.

LEACHING:

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

MINERAL:

A naturally occurring, typically inorganic substance with a specific chemical composition and a crystalline structure. A crystalline structure is one in which the constituent parts have a simple and definite geometric arrangement that is repeated in all directions.

OPEN SYSTEM:

A system that allows complete, or near complete, exchange of matter and energy with its environment.

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 miner als) and oxides, such as carbon dioxide.