HUMAN IMPACTS ON SOIL FORMATION

Jonathan Sandor, C. Lee Burras, Michael Thompson,Iowa State University, Ames, Iowa USA

Introduction

Humans-like climate, plants, and many other organisms-are active agents of soil formation. Because soils comprise the dynamic, vibrant skin of the Earth’s terrestrial surface, people have always interacted with, and therefore changed, soils and the course of their formation. While soils are subject to major change and even destruction by natural forces on the scale of geologic time, changes resulting from human activity usually occur on a much shorter time scale. People have impacted soil in a multitude of ways and extents, through farming, building, mining, and even war. In some cases, human activities enhance soils for particular uses. However, in a number of cases the interplay between humans and soils has resulted in soil degradation, which is fundamentally a negative process of formation. Recognizing that soil use is literally and figuratively the base for most civilizations and that soil resources are essentially nonrenewable on the human time scale, understanding soil formation is imperative to developing agricultural and natural resource sustainability, and to protecting environmental quality.

Scales and Scope of Human Impacts on Soil Formation

To understand the effects of humans on soil, it is helpful to first consider soil formation processes and their rates in natural environments to provide a frame of reference. From the perspective of pedology, the study of soil formation, soil is a complex assemblage of mineral and organic materials formed at the Earth’s surface. In spite of the increasing impact of human activities on soil and the likelihood that all of Earth’s ecosystems have been influenced to some degree by humans, many soils still retain their basic morphology imparted by natural processes and environmental factors. A hallmark of soil formation is the differentiation of horizons (layers). Soils are developed and organized into horizons by complex, interrelated physical, chemical, and biological processes that are determined by the factors of soil formation: climate, organisms, geology, topography, and time. The morphology of every soil is the expression of fundamental processes interacting with one another over multiple spatial scales (from micrometers to kilometers) and temporal scales (from days to millennia). Horizons at the surface (A horizons) are often enriched with organic matter, while deeper horizons (B horizons) may have accumulations of clay, calcium carbonate, metal-organic complexes, or other materials. Formation of A horizons is relatively rapid because organic matter accumulates in a few centuries to a millennium. Formation of B horizons commonly takes many thousands of years to become fully expressed. In this sense, soils can range from young to middle-age to old. As soil formation progresses, soils generally tend to

become increasingly anisotropic as they differentiate into greater kinds and numbers of horizons.

Landscape stability is a prerequisite for soil horizon development to proceed. Even so, disturbance and change are integral to the functioning of all natural ecosystems and their soils. Soils, landscapes, and associated biological communities are subject to disturbances ranging from minor perturbations such as low-intensity fires to major events such as volcanism, and to long-term climatic and environmental change. On the geologic time scale, soils are subject to major alteration, destruction, and renewal. In contrast to the relatively slow pace of natural soil development, soil changes resulting from human activity are often more rapid and far-reaching. Human-caused change may be so fast and irreversible that the impacted soil bears little resemblance to its original form. Human impacts usually reverse the anisotropic trend of soil formation, making soils simpler, less organized, and more homogeneous.

A wide array of land use and other human activities have altered paths of formation in many soils (Table 1). Corresponding soil changes also vary greatly in kind, intensity, time and spatial scale, and significance for soil and environmental quality (Tables 2 and 3). Human actions that change soil may act directly or indirectly by changing both soil morphology and the underlying soil-forming processes. They may be intentional changes based on land management strategies to increase soil productivity, or rearrangements of landscapes and soil materials to construct buildings, roads, and other structures. Conversely, they may be unintended changes that can lead to degradation such as soil erosion and burial under sediment. Indirect soil change may result from off-site processes physically remote from the impacted soil, such as subsidence following groundwater pumping or downstream sedimentation. In land uses such as agriculture, surface horizons have usually been more altered than subsurface horizons, while engineering activities often change the entire sequence of soil horizons. Engineering activities lead to some of the most intensive impacts by removing soils entirely and replacing them with different earth materials, as well as by reshaping landforms or constructing new ones. Agriculture and engineering merge in management practices such as in paddy rice production and terracing of slopes, which also involve major geomorphic and soil change.

Some human activities, such as agriculture, have influenced soil processes and morphology for thousands of years, while others, such as global climate change induced by human activity, will become increasingly important in the future. Some anthropogenic impacts on the environment, such as acid rain and other pollution, incur less visible chemical and biological changes, while processes such as desertification result in more sweeping changes through loss of vegetative cover, desiccation, wind and water erosion, and salt accumulation. While the atmospheric effects of global climate change are just now being detected, their potential impacts on soil formation and distribution are large because climate is a major determinant of soil formation over time.

Soils differ in their response to human actions in terms of their resistance to change, and their resilience, that is, their ability to rebound towards their original state. Processes of human-caused soil change may be reversible or irreversible, and the resulting changes may be ephemeral to permanent. Response depends on both external factors, such as the type of impact and environmental conditions, and internal soil properties. For example, soils with uniform textures or that are rich in organic matter tend to be resistant to compaction. After many thousands of years of natural formation, some subsurface soil

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horizons opened by deep plowing may return to their original condition in a few years (e.g., clay-rich argillic horizons), whereas more indurated horizons such as silica-cemented duripans remain fragmented longer.

Evaluating Human Impacts On Soil Formation

Human-caused soil change has been studied by pedologists in several ways. Hans Jenny and others have framed their views in the context of the factors of soil formation, placing humans within the biotic factor, while recognizing the unique role of human culture as distinct from other organisms. Jenny showed that human land use often alters soils through changes in the other soil-forming factors. For example, irrigation alters arid land soils by changing the climate factor, effectively increasing precipitation. Other researchers set humans apart from the natural factors of soil formation to emphasize the extraordinary scale and rate of anthropogenic effects on soils. In working with drastically altered soils, such as mine soils and urban soils, pedologists have defined new soil taxonomic classes because these soils bear little or no resemblance to their original state.

How is soil change detected and measured? While modern, large impacts on soil may be obvious, such as with wholesale change or destruction of soils and landscapes in urban development, longer-term soil changes are often more subtle and complex. Histories of soil change may be difficult to reconstruct, complicated by imprints of multiple land use activities and changing environmental conditions. An approach to evaluating long-term anthropogenic influence on soil has been to identify soils that are relatively undisturbed, or at least that have documented land use histories, and to use these soils as reference points from which to measure soil change. Finding truly comparable sets of reference and altered soils is challenging and imprecise, and may not be possible in some situations. Although results of comparisons must be interpreted carefully, the paired-site approach represents one of the few methods available to actually measure long-term soil change. Soil changes from agriculture have been monitored using controlled experiments at a few locations such as Rothamsted Experimental Station (United Kingdom), Morrow Plots (Illinois, USA), and Sanborn Plots (Missouri, USA) for more than a century. Evaluating ancient agricultural soils up to about 1,000-2,000 years in age has extended the time perspective on anthropogenic soil change. Much can be learned about long-term human effects on soils, and about successes and failures in soil use and management, from past cultures at archaeological sites, as well as from contemporary traditional cultures who have lived on the same land for many generations. Studies of past and present traditional land use contribute information for evaluating soils on longer time scales inherent in the concept of sustainable land use, as well as for modeling and predicting the future condition of land resources.

Changes in Soil Properties and Processes Resulting From Human Activities

To facilitate understanding of impacts and appreciation of the many ways people depend on soil for sustenance and support, soil changes are presented here in the context of the type of land use or other human activity. Examples from agriculture, engineering for urban development, mining, waste disposal, war, and global environmental change are selected to represent the myriad of ways people change soil. It is important to

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recognize that human effects on soil are complex because soils are dynamic systems with interrelated components; they are themselves part of ecosystems within Earth’s biosphere. Soils vary in their sensitivity to disturbance and threshold for change. Single land use practices can affect many soil properties in a cascading process, and different land uses may initiate similar processes of change in soils. Soils may be impacted by more than one land use practice simultaneously or diachronically. Land uses and their variations often differ in the way they impact soils in terms of magnitude, spatial extent, rate, and duration.

Agriculture

Agriculture has profoundly and extensively impacted soils since its inception about 10,000 years ago. In this discussion, agriculture is framed broadly to include all plant and animal production for food, feed, fiber, and fuel, including crop and livestock farming, as well as forestry. All of these land uses rely on many forms of soils world-wide.

Some soils are so impacted by agriculture that their original horizons are wholly transformed or buried. Examples of truly anthropogenic soils are plaggen soils, common in western Europe (Figure 1). These constructed surface horizons that can be a meter thick are the product of centuries of cultivation, with additions of organic materials such as manure and sod and inorganic amendments like sand or marl. Soils that have been similarly transformed through management practices such as terracing and long-term applications of fertilizing materials are found in other regions with long histories of intensive agriculture such as Southeast Asia and the Andes.

In contrast to both the intent and effect of constructed soils, many soils on slopes have been impacted by erosion accelerated by agriculture, some to the point of obliteration or deep burial under eroded sediments through mismanagement. Surface horizons, which generally contain the most plant nutrients and organic matter, are the most immediately vulnerable to erosion. Especially subject to major change are those soils with well-developed, organic matter-rich surface horizons such as Mollisols. The transformation of Mollisols by erosion to soils classified in other orders such as Alfisols, Inceptisols, and Entisols has been documented for past and present agriculture. A key reason for increased soil erosion is the loss of protective vegetation cover as natural ecosystems such as forests or prairies are converted to agricultural use. While accelerated erosion has been difficult to quantify on regional to global scales, estimated average erosion rates on cropland ranges from about 12 (USA) to 30-40 (Africa, Asia, South America) metric tons per hectare annually, much greater than natural erosion rates. Human-induced water and wind erosion is estimated to have resulted in the degradation of approximately one quarter of the world’s cropland.

As surface horizons are eroded, subsurface horizons or bedrock effectively rise closer to the surface and may become exposed. Their incorporation into the topsoil under moderate to severe erosion alters soil properties such as color, structure and texture, often lowering soil quality. If subsurface horizons are problematic for plant growth, serious degradation of productivity can ensue. In some tropical regions such as West Africa and Southeast Asia, exposure of horizons of cemented, iron-rich clay (petroplinthite) through erosion has resulted in abandonment of agricultural land. Erosion of Ultisols with highly weathered, acidic horizons of clay accumulation in warm, temperate to tropical

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environments has significantly reduced soil productive potential. Even in thick sediment parent materials that are considered “forgiving” to erosion, productivity can diminish through exposure or shallow occurrence of dense or cemented horizons and layers that limit rooting, decrease available water capacity, or greatly slow drainage. For example, thick deposits of loess (wind-blown sediment rich in silt) such as in China and the Palouse and Midwest regions of the USA are favorable for crops but particularly susceptible to erosion. Erosion rates in loess can reach 100 metric tons per hectare per year, exceeding natural erosion rates by one to two orders of magnitude. This has resulted in the loss of A horizons, followed by the exhumation of buried soils (paleosols) or dense brittle fragipans at shallower depths that reduce productivity.

Erosion also impacts soil formation off-site. As eroded sediments are transported downslope, soils in the lower terrain of watersheds may become buried by sediment that does not reach streams. Studies in several regions have measured burial of original soils by sediment that is centimeters to meters thick. Also, the freshly deposited sediment constitutes another parent material in which soil formation may begin anew. An opposite situation is where natural sedimentation processes and renewal of soil fertility are impaired by dam construction, as in the case of the Aswan Dam on the Nile River. The extensive erosion and sedimentation that occurs in many agricultural landscapes indicates that massive transformation of many soils is continuing. This situation underscores the need for documenting and responding to changes that undermine soil quality and productivity.

Because water deficiency or excess is of major concern in most agricultural systems, many soils have been altered by water management practices. Some changes are direct such as the effects of flooding soils for wet rice production, creating paddy soils with distinctive anthropogenic characteristics (Figure 2). “Irragric” soils in arid to semiarid central Asia have been highly altered by long-term irrigation, including addition of suspended sediment in the irrigation water. Unintended chemical effects from irrigation in some regions have lead to severe soil degradation through salt accumulation and transformation to saline or sodic soils. In a practice opposite that for paddy soils, conversion of wetlands to row crop production by artificial drainage in regions such as the Midwest USA has changed soils from dominantly anaerobic to dominantly aerobic states, leading to changes like organic matter oxidation. In some areas of the world, many meters of organic soil thickness have vanished through oxidation processes, resulting in significant land subsidence.

More subtle agricultural impacts may not completely alter the original soil, yet over time cause significant change. For example, since the mid 19th century, conventional cultivation of organic matter-rich, prairie-derived soils (Mollisols) throughout the Midwest USA has lead to marked decreases in organic matter in upper soil horizons (Figure 3). Many Mollisols have lost roughly one third to one half of their original organic matter. In addition to erosion, a principal cause is the disruption of soil aggregates by cultivation, making previously protected humus accessible to microorganisms. Compared with native prairie ecosystems that are characterized by abundant organic matter and conservative nutrient cycling, agricultural soils have lower inherent fertility and are more “leaky” with respect to nutrients such as nitrogen. Soil organic matter loss has a cascading effect on soil properties such as structure, reducing aggregate stability and making soils more prone to compaction. Organic matter contents

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can be at least partially restored to soils through management, by using more diverse, conservation-oriented cropping systems, or by a return to more natural vegetation.

Cities and Industry

The growth of cities and industry has profound impacts on soils and their continued formation. Some of the impacts are direct; others are indirect. The creation of constructed urban soils is perhaps the most easily recognized direct impact of city growth on soils (Figure 4). Constructed soils have distinct profiles that are typically the product of the cheapest engineering fill available at the moment of creation. Soil scientist Phillip J. Craul wrote that the history of urban development has been based upon the premise “dirt is dirt and it’s cheap.” These human-created soils may be highly stratified and composed of very different types of fill in terms of their texture, mineralogy, and chemistry. Total fill – and consequently soil - thicknesses can reach multiple meters, with a soil’s maximum age corresponding with the age of the city. That is, a constructed urban soil in Rome, Athens, Mexico City and Beijing can be several thousands years old whereas a similar soil in New York or Buenos Aires must be less than 500 years. Of course, many new hectares of these soils are created each year in every city as new projects are completed. Many ancient cities occur on tells, human-created mounds or hills formed through time as cities were rebuilt on top of previous ones. Other construction materials used in urban soils include composted sewage sludge, municipal solid wastes, heavy metals, glass, plastic, metal, etc. Still others consist of clean sand mined from nearby rivers or topsoil transported from local farms. In other words, urban soils have an exceptionally high degree of disorganized variability.

A second direct impact of city development on soil formation is at the landscape scale as houses, yards, parks, and streets replace the natural terrain. The result is fragmented landscapes where soil-forming processes become controlled by constructed topographic, hydrologic, and ecologic factors. Thus, while the morphology of many of these soils do not show the dramatic changes of their downtown counterparts, their soil forming processes are considerably altered. This can be illustrated by considering two hypothetical adjacent yards; one growing turf that is heavily fertilized and irrigated while the other is planted to conifer shrubs and trees but receives no fertilizers and extra water. Over time, the properties of the soils under these two yards will diverge in response to their different microclimates (wet versus dry), biota, and chemical inputs. Like their constructed counterparts, they will be characterized by disorganized variability although in this case the disorganization occurs at the landscape (i.e., yard-to-yard) scale.

The impact of cities and their continued growth on soil formation is increasing as large areas of prime farmland as well as wild, often fragile, lands get converted to urban uses (Figure 5). In the USA the area of urban land increased from 6 million to 26 million hectares between 1945 and 1992, with some parts of California and New England developing into “100 mile cities” as urban areas became interwoven through express highways and suburbs. Urbanization is even more of a critical factor in developing nations because their populations are growing at a rate four-fold greater than the world population. The World Resource Institute estimates that nearly 500,000 hectares of rural soils are converted to urban land annually. For example, the urban core of São Paulo,

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Brazil expanded from 18,000 to 90,000 hectares between 1930 and 1988, with the overall metropolitan area now encompassing more than 800,000 hectares.

Industry, like cities, directly and indirectly affects soil formation. Land excavation and drainage are two important direct agents of change. Dredging of wetlands as well as sediments from beneath shallow waters to improve harbors, build canals and meet other commercial needs is an example of industrial excavation. The U.S. Army Corps of Engineers estimates that billions of cubic meters of sediment are dredged annually around the globe with over 300 million cubic meters being dredged annually just in the USA. Dredging has three direct impacts with respect to soils and their formation. First, the dredging itself destroys whatever natural soil existed. Second, the dredged material – or spoil – is piled somewhere and is later used in soil construction. Soil created from dredge spoil may be used for buildings, roads, parks, farms, or even wild areas. Over time it will develop a sequence of horizons in response to its environment. The third pedologic impact of dredging is oxidation of mineral and organic matter. This occurs because spoil that was excavated from below the water table is now directly exposed to the oxygen-rich atmosphere. In many cases, the oxidation is fairly benign in terms of soil properties and formation. A noteworthy problem occurs when the spoil contains sulfides. Exposure of sulfidic compounds to air results in the rapid formation of large quantities of sulfuric acid in these soils. Such acid sulfate soils are also created when naturally sulfidic soils are drained. It is estimated there are 24 million hectares of potential acid sulfate soils globally, with many located in prime settings for agricultural and urban development.

The indirect impacts of industry on soil formation are as manifold as the indirect impacts of urbanization. One of these is the addition to soil of contaminants such as lead, cadmium, and other heavy metals. In some locations these metals are deposited on soils as airborne contaminants that originated in factory or vehicle exhaust. In other cases they are added to soil directly and intentionally as constituents of fly ash and other wastes. Land farming refers to disposing of waste by applying and incorporating it into soil a few tons per hectare at a time. These soils are subsequently cropped, although the crops are normally not used for human consumption. Compounds that contain sodium are another class of industrial contaminant that changes soil morphology - ultimately creating sodic soils. This process is increasingly noted worldwide because of the mind-boggling number of industrial uses for sodium and sodium compounds (e.g., coolant in nuclear processes; fumigants; solvents in manufacturing of brass, paper, ceramic glazes, textiles, and fertilizers; and in food processing) and the frequency at which sodium wastes are applied to land. Radionuclides are another class of industrial contaminants that alter soil processes. For example, 8.4 million hectares around Chernobyl, Ukraine remains contaminated by 90Sr and 137Cs from the 1986 nuclear reactor disaster there.

The influence of most industrial processes on soil formation is proportional to the intensity of industrial activity and distance of the soil from the industry. This means that soils in areas having numerous factories, automobiles, and other industrial activities are more highly enriched in contaminants than soils where there is little industry. Likewise, the magnitude of impact radiates out with distance from the source of impact.

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Mining

Humans have mined the earth for metals, minerals, and fuel for thousands of years. Currently, 8 billion tons of mineral and oil are extracted from the earth annually. Since the 19th century, mining activities have radically transformed landscapes and altered natural soil development processes. Mining requires that the overburden of soil and its parent materials be displaced so that the product (e.g., ore, rock, or coal) can be removed, concentrated, and transported off-site as economically as possible. Mining often results in large pits, high walls, and large accumulations of unconsolidated overburden, smelting residue, and finely ground residual ore. When abandoned at the earth’s surface, these materials often pose significant environmental risks due to subsidence, settlement, and slope instability. Spoil may also be contaminated with soluble metals. In addition, when some types of spoil are exposed to air and percolating water, oxidation reactions produce strongly acidic water with high metal concentrations, severely restricting subsequent biological activity. In the U.S. alone, it has been estimated that surface mining has disturbed more than 23,000 square kilometers.

Concern about mines and their spoils prompted many nations to enact environmental quality laws beginning in the late 20th century. For example, before the Resource Conservation and Recovery Act of 1976 and the Surface Mining Control and Reclamation Act of 1977, many mines in the United States were simply abandoned when the product had been exhausted or was too expensive to extract from the remaining ore. Since that time, however, planning for and completing reclamation of mined land has been part of every mining operation in the U.S. Thus a number of practices are actively followed to allow the site to be used again for other purposes and to ensure that surface and ground water resources are protected. Construction of new soil at reclaimed mine sites is an example of the most radical of human impacts on soil formation.

Successful mine land reclamation must address a number of large-scale environmental issues while at the same time creating a near-surface environment that promotes vegetative growth and movement of water through the watershed. For example, a major problem in reclamation of coal and metallic ore mines is the oxidation of sulfide minerals and subsequent drainage and runoff of acidic water (Figure 6). Acidic spoil, surface water, and seeps must be treated with large amounts of calcite or hydrated lime to neutralize acidity before vegetation can be established. Organic matter and plant-available nutrients are commonly lacking in spoil, so application of sewage sludge or other organic amendments to the spoil surface is often helpful. Heavy applications of plant nutrients such as phosphorus and potassium may also be required.

Large-scale topographic reconstruction of mined land is expensive, but it is usually essential to ensure that reconstructed soils maintain ecological and environmental quality. In some reconstructions, it is possible to approximate the general contours of the pre-mining landscape (Figure 7). Throughout the process of consolidating, grading, and shaping the new landscape, control of settlement, slope instability, and surface erosion will minimize off-site damage of sediments and drainage as well as maintain optimum conditions for plant growth and development of natural soil horizons. Water management in reclaimed mine lands includes designing watersheds for directing both run-on and runoff water. The new landscape may be designed to retain or slow the movement of water by passing it through constructed wetlands.

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Where the original materials were not systematically stockpiled during mining, reclaimed soils often display extreme variability in particle size and composition. Problems of inhomogeneous mixtures of coarse and fine particles include both subsidence and inadequate water-holding capacity for plants to grow. For this reason, modern reclamation approaches usually call for restoration of the natural sequence of geological materials (e.g., bedrock-derived spoil placed below till-derived spoil placed below organic-matter-rich topsoil). Rates of soil development in reconstructed mine soils are slow. Reclaimed mine soils tend to have “AC” horizons sequences, in which incipient topsoil is underlain by unconsolidated layers lacking soil structure or other pedogenic features. Still, where reclamation has been well managed, the thickness of surface horizons in the reclaimed soil may exceed those of nearby unamended soils that have been degraded by farming or logging operations.

War

War, like mining, may have rapid and long-lasting impacts on soils. For example, nearly a century after an 11-month battle during World War I, extensive trench works and bomb craters remain near Verdun, France. Vietnam provides a documented example of the impacts of more recent wars. There, it has been estimated that bombs created some 21 million craters between 1965 and 1971, covering more than 50,000 hectares (Figure 8). The explosion of a 2400-kilogram bomb typically created a hole about 10 meters in diameter and 5 meters deep; metal fragments from each bomb could be spread over an area of 0.5 hectares. Some craters exposed soil horizons of iron-oxide accumulation that subsequently irreversibly hardened to petroplinthite.

Similarly, land-clearing programs in Vietnam altered soil processes. Widespread and concentrated application of defoliating herbicides as well as bulldozing of vegetation resulted in extensive erosion, loss of organic-matter-rich surface horizons, and increased flooding. Native mangrove forests have not returned to the drastically disturbed or herbicide-treated lands. More than thirty years after the application of herbicides ended, dioxin, a manufacturing contaminant in the herbicides, remains in the soil and sediments of the countryside. Thus soils are effectively beginning a new phase of soil genesis with topography, parent material, and vegetation altered from the previous state of dynamic equilibrium.

In addition to the persistence of war-related changes on soil properties and processes, there may be indirect effects that have a cascading environmental impact caused by displacement of civilian, agrarian populations. In rural Vietnam, bomb craters; soils contaminated with metal fragments, unexploded ordnance, and residual dioxin, and war-related destruction of water-diversion structures have made agricultural and forestry recovery very difficult for the inhabitants. In response, some farmers have moved to less productive, marginal land that is more susceptible to erosion. Others have moved to urban slums, increasing the environmental pressures on urban areas. Thus the long-term impacts of war on soil properties and the processes of soil formation can extend beyond the zone of immediate impact.

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Climate Change and Soil Formation

Soil formation is closely tied to climate, and to the extent that human activities alter local or regional climatic variables such as temperature and effective precipitation, soil properties will inevitably be changed as well. At the global scale, climatic change in the next century is likely to be driven by increasing atmospheric concentrations of “greenhouse gases” such as carbon dioxide (CO2) and methane (CH4). As a result, it is expected that the mean temperature of the earth's surface could rise as much as 1.5 to 4.5 °C over the next 100 years. Although the local impacts of global warming are difficult to predict with certainty, significant shifts in the mean and seasonal variation of air temperature as well as in the total and seasonal distribution of rainfall are likely to perturb both natural and agricultural ecosystems. The impact of a global increase in temperature will be unevenly spread in both space and time. Some of the impacts include: (1) new precipitation patterns in which today’s wet regions get wetter and dry regions get drier, (2) migration northward of boreal and hardwood forests, (3) melting of ice sheets, sea ice, and glaciers, with concomitant rise in sea level and flooding of coastal soils, and (4) melting of permafrost in northern latitudes and oxidation of organic matter now stored in cold-region soils. For example, the temperature increase has the potential to markedly shrink the extent of approximately 11 million square kilometers of Gelisols, the cold-region soils with permafrost where an estimated 13 percent of the world’s terrestrial carbon is stored.

Human activities, mainly burning of fossil fuels, will thus lead to some global and regional climate change over the next century. Both regionally and locally, climate change will directly and indirectly impact soil processes by altering vegetation patterns, encouraging increased water and wind erosion, favoring mass movement, increasing the leaching of nutrients and organic matter through soils, and favoring microbial oxidation of organic matter in surface horizons. Although we are not now able to predict how fast soil properties will be altered as a result of global and regional climate change, ultimately some types of soil horizons will be lost (for example, surface horizons composed entirely of organic matter) and some types will be newly developed where they did not previously exist (for example, spodic horizons under new boreal forests). As human populations adapt agricultural practices to new climate conditions, further changes in soils may be accelerated. For example, salinization may increase due to more extensive irrigation and landscape instability may become widespread as more forest land is cleared for production of crops and livestock.

Significance and Future of Human Impacts on Soil Formation

Soil is a critical, dynamic natural resource and vital component of ecosystems, but one that is often neglected. This is in part because soil lies beneath the surface and so is not as familiar as other resources like water, plants, or animals. As a natural resource, soil is crucial for food, fiber, and fuel production systems, for construction materials and foundations, for replenishing and maintaining the quality of surface and ground water, and for waste processing and containment. Natural variation in pathways of soil formation and soil properties make for a diverse range of suitabilities for different land

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uses and sensitivities to anthropogenic change. Conservation of soil resources and soil quality is a critical priority globally because soil is fundamentally nonrenewable on a human time scale.

Human activities dictate that soil change is inevitable and necessary. Anthropogenic soil change has a long history, but it has exponentially increased in intensity, spatial extent, and rate during the past century. Some changes are advantageous and improve soil functionality. However, many soil changes induced by past and present land use have resulted in environmental degradation. In developed and developing nations alike, accelerated erosion, compaction and disruption of structural aggregates, lowered fertility, and contamination by pollutants, continue to be sources of serious concern. Because soil serves as a filter, substrate, and reservoir linked to other land, water, and biological resources, degradation is not just detrimental to the soil internally and locally, but it extends to all parts of the larger hydrologic, geologic, and biological system. Some soils, like plants and animals and the habitats and ecosystems with which they are closely connected, have become endangered and in some cases extinct. Human-altered soil figures significantly in global environmental change, both as a cause and a recipient of change. Oxidation of soil organic matter is a source of increased atmospheric carbon dioxide, soil properties stand to be further altered by global climate change, and yet soil also has the potential help reverse the trend by sequestering more carbon. Soils also play a key role in other forms of global change such as desertification, acid precipitation, and loss of tropical forest ecosystems.

What can be done to counter the negative impacts of human activities on soils and environment and to support conservation and more sustainable land use? Improved knowledge of soil formation and soil change is an important starting point. By more clearly recognizing the extent of soil change and understanding its causes, mechanisms, and consequences, the better our chances become to develop and implement management practices that can sustain soil resources and restore damaged land. Traditionally, soil maps have portrayed soils in their relatively original, undisturbed state. However, because of the unprecedented scale of soil change and massive transformation, it is imperative that anthropogenic soil change be documented, monitored, and acted upon to a much greater degree. Efforts to deal seriously with the extent and significance of human impact on soil formation and distribution have lead to a proposal to include Anthrosols in the U.S. Soil Taxonomy, and to recognition of Anthrosols at the highest level in the current international soil classification system (World Reference Base for Soil Resources). Improved knowledge of soil formation processes in relation to natural and human-altered pathways is essential to the restoration of ecosystems and the development of sustainable land use. The future of human society and the Earth we inhabit depend on soil formation processes, and increasingly, to how we respond to the changes human actions cause in soils and the biosphere.

Further Reading

Amundson, R., and H. Jenny. 1991. The place of humans in the state factor theory of ecosystems and their soils. Journal of Soil Science 151:99-109.

Amundson, R., Y. Guo, and P. Gong. 2003. Soil diversity and land use in the United

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States. Ecosystems 6:470-482.

Bidwell, O.W., and F.D. Hole. 1965. Man as a factor of soil formation. Soil Science 99:65-72.

Bryant, R.B., and J.M. Galbraith. 2003. Incorporating anthropogenic processes in soil classification. p. 57-66. In H. Eswaran et al. (ed.) Soil classification: a global desk reference. CRC Press, Boca Raton, Florida.

Courty, M.A., P. Goldberg, and R.I. Macphail. 1989. Soils, micromorphology, and archaeology. Cambridge University Press, Cambridge, UK.

Craul, P.J. 1999. Urban soils: applications and practices. John Wiley & Sons, Inc., New York.

Dudal, R., F.O. Nachtergaele, and M.F. Purnell. 2002. The human factor of soil formation. Transactions of the 17th World Congress of Soil Science, Bangkok, Thailand.

Effland, W.R. and R.V. Pouyat. 1997. The genesis, classification, and mapping of soils in urban areas. Urban Ecosystems 1:217-228.

Gong, Z.T. 1983. Pedogenesis of paddy soil and its significance in soil classification. Soil Science 135:5-10.

Goudie, A. 1994. The human impact on the natural environment. 4th ed. MIT Press, Cambridge, Massachusetts.

Jenny, H. 1984. The making and unmaking of a fertile soil. p. 42-55. In W. Jackson et al. (ed.) Meeting the expectations of the land. North Point Press, San Francisco.

Johnson, D.L., and L.A. Lewis. 1995. Land degradation: creation and destruction Blackwell Publishers, Oxford, UK.

Lal, R., (ed.) 1999. Soil quality and soil erosion. CRC Press, Boca Raton, Florida.

Lal, R., T.M. Sobecki, T. Livari, and J.M. Kimble. 2003. Soil degradation in the United States. CRC Press, Boca Raton, Florida.

Ruddiman, W.F. 2001. Earth’s climate: past and future. W.H. Freeman and Co., New York.

Sandor, J.A., and N.S. Eash. 1991. Significance of ancient agricultural soils for long-term agronomic studies and sustainable agriculture research. Agronomy Journal. 83:29-37.

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Sencindiver, J.C., and J.T. Ammons. 2000. Minesoil genesis and classification, In R.I. Barnhisel et. al (ed.) Reclamation of drastically disturbed lands. Soil Science Society of America, Madison, Wisconsin.

Westing, A.H. and W. Pfieffer. 1972. The cratering of Indochina. Scientific American 226:20-29.

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