Anthropogenic Soils

Anthropogenic soils (anthrosoils) are soils that have been influenced, modified or created by human activity, in contrast to soils formed by natural processes. They are found worldwide in urban and other human-impacted landscapes. Anthrosoils are formed by: (1) sealing a natural soil beneath pavement or other artificially manufactured impervious material, (2) transformation of a natural soil by human action, or (3) development of a new soil profile in parent materials created and deposited by human action.

Since ancient times, major impacts of human culture on the landscape have been generally associated with the growth of agriculture and cities. Hence, the nature and global extent of anthropogenic soils is linked to the sociological and geographical aspects of civilization (geocultural setting). The history of anthrosoils can be traced back to the early days of agriculture in the “fertile crescent,” and the first city-based civilization in ancient Mesopotamia about 5000 years ago. Here some of the first anthrosoils were formed as a consequence of long-term horticultural activities and salinization resulting from irrigation. Anthropogenic soils were widely produced as a result of agriculture-induced soil degradation, and partly contributed to the demise of ancient civilizations. The rate of anthrosoil formation began to increase rapidly on a global scale during the 18th century when the Industrial Revolution initiated an ongoing period of exponential population growth and urban expansion. The rates have increased even faster since the end of WWII as a consequence of globalized economies, and the explosive growth in population, industrialization and urbanization.

Anthropogenic landforms are created either directly by artificial processes (e.g., strip mining), or indirectly by natural processes triggered by human activity (e.g., accelerated soil erosion). They are commonly produced by the building up of the land with artificial fill materials (aggradation), or as a result excavation (degradation). Anthropogenic landforms vary as a function of geocultural setting, and are generally recognized by their deviation from the natural landscape. Anthropogenic landforms can be classified physically into six basic types: (1) flats, (2) benches, (3) terraces, (4) convexomorphic, (5) concavomorphic, and (6) plateau. The NRCS Geomorphic Description System used in the U.S. classifies landforms hierarchically according to landscape, landform, microfeature and anthropogenic feature.

Pedogenesis (soil formation) generally results from weathering and horizonation processes. Weathering includes processes that change the physical and chemical characteristics of soil particles, aggregates and artifacts. Horizonation results from additions, losses, translocations, and transformations of solid and chemical soil constituents. Both weathering and horizonation are affected by the five soil forming factors: climate, landscape position, organisms, parent material and time.

Artifacts are objects >2 mm, whereas microartifacts are 0.25–2.0 mm, in size that were produced, modified, or transported from their source, by human activity. Artifacts are typically coal-related wastes (coal, cinders, etc.), waste building materials (brick, mortar, etc.), industrial wastes (coked coal, slag, etc.), and objects of archaeological significance (pottery, bone, etc.).

In U.S. Soil Taxonomy, anthropogenic soils are classified into two basic categories depending on type of soil parent material: (1) human-altered material (HAM), and (2) human-transported material (HTM). Generally speaking, a surface layer of HTM or HAM ≥50 cm thick must be present in order for a pedon to be classified using the anthropogenic soil profile.

Anthrosols are the characteristic type of anthropogenic soil found wherever people have practiced agriculture for many centuries.

In addition to the Plaggic and Pretic Anthrosols discussed in Chap. 7, four other types of anthropogenic soil can be distinguished at archaeological sites.

Mine-related anthropogenic soils are primarily associated with modern landscapes created by the surface mining of coal. They are classified as Spolic Technosols having been formed in HTM containing technogenic artifacts in the form of mine spoil. Mine spoils are comprised of sedimentary rock fragments that have been fractured and rapidly exposed by blasting and excavation. Spoils are composed predominantly of coarse fragments, and are characterized by unusually large voids not seen in natural soils. Some mine spoils contain significant amounts (30–60%) of fine earth fraction soil-sized depending on blasting intensity.

Anthropogenic soils found in urban areas vary as a function of land use and city structure. They are primarily classified as Technosols, the central concept of which is a soil formed in HTM characterized by an abundance of technogenic artifacts. Urban soils in residential areas are Urbic Technosols characterized by artifacts indicative of human habitation, whereas those associated with industrial land are Spolic Technosols with an artifact assemblage comprised of industrial wastes. Urban soils typically have an ^Au-^Cu profile, although cambic-like horizons may form in 18– 70 years. They are characteristically heterogeneous and often suffer from excessive compaction, excessive artifact content, diminished biological activity, and reduced infiltration due to surface crusting or water repellency. They generally have elevated levels of pH, exchangeable bases, and carbonate content, but levels of organic C, N and P tend to be very low in recently deposited HTM. Melanization results in the formation of ^A horizons in as little as 12 years under an udic moisture regime with a cover of grass. Under such conditions, urban soils are resilient and may reestablish properties similar to those of natural soils in 30–100 years.

It seems clear that even after anthropogenic activities have ceased, the impacts of humans on soils can persist for hundreds and perhaps thousands of years. This is shown by the fact that ancient Hortic Anthrosols (e.g., garden and midden soils) and Pretic Anthrosols (i.e. Amazonian Dark Earth soils) have strongly influenced the colonization pattern of native plant species, and modern day crop production. On the other hand, soils created by human disturbance can be quite resilient. For example, certain urban and mine-related anthropogenic soils have rebounded to a quasi-natural state in ~60 years even without the application of reclamation methods.