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EUROPEAN BUREAU  RESEARCH REPORT NO. 7

Soil Taxonomy: Provisions for Anthropogenically Impacted

WILDING Larry P. 1 and AHRENS Robert J.2 1 Department Soil and Crop Sciences, Texas A&M University, College Station, TX 77843, USA Email: [email protected] 2 USDA-NRCS-National Center, USDA-NRCS, Lincoln, NE 68508-3866, USA

Abstract Soil Taxonomy accommodates anthropogenically-impacted soils in a variety of ways, but there are still pending questions about the adequacy of these provisions. For this reason we initiated the International Committee on Anthropogenic Soils (ICOMANTH) to address this matter. Diversity of opinion exists among US soil scientists regarding the best manner to accommodate anthropogenic impacts on soils. Recommendations include a new soil order for Anthrosols, revisions in current taxa to accommodate anthropogenically-impacted soils, development of new taxa in Soil Taxonomy based on prior knowledge of history and/or genesis of humanly-managed soils, and finally better design and description of mapping units in soil surveys to transfer knowledge about anthropogenically-impacted soils. The human factor is one of the biological components of soil formation but is not considered a process per se. Commonly anthropogenic effects are difficult to calibrate consistently with morphogenetic markers as diagnostic differentiae. Then the question is posed how much change in a soil is needed before one recognizes anthropogenic impacts (e.g. surface plowing, addition of agricultural chemicals, irrigation, drainage, accelerated , strongly disturbed soils, etc.). In considering this question, it is important to review basic tenets in rationale used to construct Soil Taxonomy, namely: (1) soils are classified on the basis of observed or measured properties that are expressed contemporaneously; (2) cultivated and non- cultivated soils should be placed in the same taxa whenever possible; (3) preference should be given to taxa differentiae representing more permanent subsurface properties rather than temporal surface conditions; (4) preference should be given to taxa differentiae representing properties that carry the greatest number and most important accessory properties; (5) Soil Taxonomy should continue to evolve as a dynamic system, and (6) Soil Taxonomy should be fully bifurcated so all known soils within the landscape can be placed within the system. In this spirit, Soil Taxonomy has accommodated anthropogenic impacts on soils by application of the differentiae at the order, suborder, great group, subgroup, family, and series categories and with the following diagnostic horizons and features: anthropic and plaggen epipedons, agric and sulfuric horizons, aquic conditions (anthric saturation), and densic materials. This paper does not provide solutions for all the questions raised above, but does set the framework of how Soil Taxonomy functions in regard to anthropogenically- influenced soils. Undoubtedly with time and knowledge, further revisions in Soil Taxonomy will be needed, as we better understand how to manage, interpret and classify anthropogenically-influenced soils.

Keywords: human factor, soil diversity, diagnostic properties, Anthrosols, ICOMANTH

Introduction Soil Taxonomy (1999) accommodates anthropogenically-impacted soils in a variety of ways but there is still pending questions about the adequacy of these provisions. For this reason formal activities of the International Committee Anthropogenic Soils (ICOMANTH) began in August, 1995. The committee has had limited activity, partly because of competing activities of other international committees but perhaps even more important was the diversity of philosophy about how to best accommodate anthropogenically- impacted soils. The committee would welcome inputs from the soils community via its web site:

Soil Taxonomy: Provisions for Anthropogenically Impacted Soils. Wilding & Ahrens 35 EUROPEAN SOIL BUREAU  RESEARCH REPORT NO. 7 hppt://wwwscas.cit.cornell.edu/icomanth. ICOMANTH is charged with defining appropriate classes in Soil Taxonomy for human-altered soils (Bryant, et al., 1999). As a part of this charge the committee is requested to establish criteria for long-term changes in morphogenetic properties and soil behavior. New classes were to be defined that would provide for improved understanding, classification, management and interpretations of these soils. To date, the committee has developed two circular letters and hosted an International Workshop on Classification, Correlation, and Management of Anthropogenic Soils (Kimble, et al., 1999).

Several difficulties in accomplishing the charges given to ICOMANTH are entrained in questions like how much difference in soil properties does it take before one recognizes anthropogenic effects (Bryant et al., 1999)? Does it require simply plowing the soil or is more drastic soil disturbance required? Will a few years of irrigation be sufficient or will hundreds to thousands of years be required to recognize significant effects of such management practices? Under highly variable spatial and temporal soil conditions, how does one recognize and categorize the effects due to anthropogenic activities from those due to other non- anthropogenic impacts? The human factor is certainly one of the biological components but is human activity really a pedogenic process that can be differentiated from other pedogenic processes that are not humanly induced? Commonly the pedogenic processes associated with anthropogenically-impact soils are similar to ones which occurred prior to human intervention. To further confound the challenges of ICOMANTH, the proposals to this committee varied from establishing a new soil order of Anthrosols, to revising current Soil Taxonomy provisions, designing new taxa based on prior historic knowledge, and improving the design and description of mapping units as soil complexes (Kimble, et al., 1999).

The human impact on soils is recognized in a number of taxa, especially and , but in general the soils community is still striving to improve Soil Taxonomy’s capability to accommodate this human factor. Transformations of soils that have been impacted by humans is common. For example, can be transformed to by erosion and vice versa by heavy liming and manure applications; paddy soils can undergo secondary salinity and waterlogging under long-term rice culture; urban/industrial environments can be markedly modified by landfills, landfarming, earth movement, and heavy metal contamination; and drastically disturbed soils are common in regions where precious metals, rock aggregate, and fossil fuels have been mined. ICOMANTH is charged to address these issues, but solutions and revisions of Soil Taxonomy will be deliberate and debated by a larger community of soil scientists.

Sencindiver and Ammons (2000) have recently summarized information on minesoil genesis and classification which touches on many of the points raised above. Fanning and Fanning (1989) also have shared their philosophy about genesis and classification of anthropogenetically-impacted soils. Burghardt and Dornauf (2000 a,b,c) have edited three volumes of proceedings dealing with humanly-influenced soils with a focus on urban, industrial, traffic, and mining areas. These contributions deal with , mapping, spatial variability, genesis, classification, and problems encountered in accommodating anthropogenically-impacted soils. All these sources provide excellent background to the topic covered herein but general solutions are not offered or warranted.

The purposes of this paper are: (i) to discuss the philosophy of Soil Taxonomy construction relative to anthropogenically-impacted soils; (ii) to illustrate ways in which the system does accommodate anthropogenically-impacted soils; and (iii) to consider limitations and constraints of the system for this purpose. It will draw heavily on several published literatures for this information including Smith (1963, 1983, 1986), Cline (1949, 1963), Soil Survey Staff (1999), Ahrens and Engel (1999), Ahrens and Arnold (2000), and Wilding (2000a,b).

Philosophy of Soil Taxonomy Before presenting ways in which Soil Taxonomy accommodates anthropogenically-impacted soils it is worthwhile as a prelude to remember some of the principles and philosophy of the system. These signposts provide rationale as to how the system handles humanly impacted soils. Smith (1983) provides a classical summary of construction of Soil Taxonomy. Cline (1963) described the logic of Soil Taxonomy and key requirements of the system.

36 Soil Taxonomy: Provisions for Anthropogenically Impacted Soils. Wilding & Ahrens EUROPEAN SOIL BUREAU  RESEARCH REPORT NO. 7

Soil Taxonomy serves the Soil Survey Program in US The architects behind Soil Taxonomy agreed that the classification system must serve the soil survey program of the United States. The system was utilitarian in scope with its primary purpose to identify and locate soils, and to predict the consequences of alternative uses of soils. In countries where no classification system existed or in which the system did not adequately serve the needs of the community, Soil Taxonomy was available to be tested, modified and used as deemed appropriate. Numerous countries took advantage of this opportunity and use Soil Taxonomy or some derivative of it.

The founders of Soil Taxonomy believed the system should provide a basis for developing principles of soil genesis, and predictions of soil behavior and responses to management and manipulation. The system should help elucidate cause and effect relationships, understand reasons for landscape soil patterns, provide a basis to extrapolate spatial soil knowledge from one area to the next, and assist in interpreting soils for use and management (Smith, 1963). It was clear that classes of the system must have real counterparts in soil bodies. The differentiae used as class criteria for grouping similar soil conditions should not be applied independently of the spatial diversity within landscape bodies. This link with identifiable geographic bodies separates Soil Taxonomy from hypothetical organization of soil property information (Cline, 1949). Soil Taxonomy is hierarchical The categorical levels in Soil Taxonomy are order, suborder, great group, subgroup, family and series. Phases of series lie outside the taxonomic system. It was believed that the system should be hierarchical in construction where a few generalized statements could be made about taxa at higher categories, and more specificity, precision, and number of statements could be made at lower levels of abstraction. This facilitates remembering properties and relationships among the classes at each categorical level. The logic of the hierarchy represented by Soil Taxonomy is one of its major strengths, yet at the same time it retains the weaknesses of such organizational frameworks. The efficiency with which accessory characteristics are carried along with the differentiating ones from higher to lower categories is very powerful indeed. It follows the principle of “superposition of differentiating and accessory characteristics” outlined by Cline (1949). On the other hand, the uncertainty of the degree of accessory property relationships and the rigidity of class structure of the system limit the precision of statements that can be made for soils in the field and the correspondence of theoretical soil taxa with real soil-landscape cartographic units.

Soil Taxonomy was constructed in an ascending manner by grouping soils with similar properties and behavior into successively more generalized categories (Smith, 1983). It is used as a key in a descending manner where major limitations and restrictions of soil taxa are keyed out first. Classes separated at a higher categorical level remain separated throughout the lower categories of the system. For ease of operation the classes are mutually exclusive, that is, without overlap. Operationally, the listing of the categories and the listing of the classes within each category need to be prioritized because only a few of the characteristics of individuals are used to obtain placement within the scheme (Smith, 1983). This means that the scheme can be presented as a key or set of keys for consistent application of the definitions and differentiating criteria. Soil Taxonomy is dynamic Soil Taxonomy is a dynamic system constantly subject to change. This is best illustrated by the International Committees that have been developed since Soil Taxonomy was first published (Soil Survey Staff, 1999). These committees have resulted in numerous revisions of established orders, development of new orders, and many changes in class differentiae at all categorical levels in the system. Many examples of modifications have occurred, especially in the last seven years. Cline (1963) noted that a classification system that does not change is a stagnant and dead system. The definitions must be continually tested by the nature of functioning of the soils grouped within a taxon. A taxonomy for the use of soil survey needs to be tested by the nature of the interpretations that could be made (Smith, 1983). These interpretations include those for genesis and the mapping of soils as well as for the behavior of soils under multiple uses. However, changes need to be made with a minimum of disturbance to the rest of the system, if possible. Changes made must justify the efforts expended and not be cosmetic in scope.

Soil Taxonomy: Provisions for Anthropogenically Impacted Soils. Wilding & Ahrens 37 EUROPEAN SOIL BUREAU  RESEARCH REPORT NO. 7

Soil Taxonomy is fully bifurcated All known soils can be classified by Soil Taxonomy. This is because soil conditions which would normally fall through the system because they do not fit within any other rigidly defined class are picked up in Inceptisols, the “junk basket” taxon (Wilding, 2000b). One of the criticisms of early classification systems developed in the US was that the system could not classify all soils without ambiguity. A system with mutually exclusive classes that is fully bifurcated classifies all soils, albeit whether the soils are placed in most meaningful taxa. Soil Taxonomy must account for all soils on the landscape and all soils that are known to exist. The "Keys" are designed so that every known soil can be classified. The question is whether the classification is the best or the most appropriate. This is one of the innovations of Soil Taxonomy and has proven to be a good test of the usefulness of the classes provided. Soil Taxonomy is definitive with operational Morphogenetic Differentia The definition of a category suggests criteria that may be appropriate to separate the classes of that category. The criteria are then recognized by properties or features which are called differentiating characteristics. These characteristics are observable or measurable properties of individuals that are grouped into a class. The criteria are not based on speculative genetic theories, land use history, nor simply morphology. Diagnostic horizons and properties were intended to be definitive and operational such that individuals with different training and experiences would be able to identify them consistently, preferable under field conditions. An ardent attempt was made to couple morphology and genesis to understand cause and effect relationships of soils to landscapes so extrapolation of this body of knowledge was possible. It was well recognized that many soils with similar morphology have polygenetic history, so care is taken that morphogenetic differentiae are real properties of soils or measurements reflecting known behavior, and not genesis concepts per se. Soil Taxonomy gives preference to less Temporal Subsurface Properties Soil Taxonomy gives subsurface differentia preference over surface attributes. This was intentional so temporal changes that are common in surface epipedons did not result in changes of the placement over short periods of time. It was believed that differentia subject to change, such as base saturation percentage, should be measured at sufficient depths in the soil where anthropogenic-impacts, such as liming amendments, would not effect the data. Similarly, and for the same reasons, more permanent (static) soil properties such as texture, mineralogy, , soil depth, CEC, and OC content were given preference as differentiae over the more dynamic soil properties such as , soluble salt content, hydraulic conductivity, and water content, etc. (Wilding et al., 1983, 1992, 2000a). Differentia are sometimes a combination of surface and subsurface characteristics in which preference was given to those properties that were more stable and at a depth in the soil where use and management, such as plowing or liming, would not result in wholesale changes of classification over short time periods. However, some diagnostic horizons and properties were subject to cultural impacts, and thus, the combination of both historical-static-stable properties and temporal-dynamic ones results in confounding spatial and temporal variability. This is a problem that will need to be addressed in the future. Soil Taxonomy places cultivated and non-cultivated analogues together Soil Taxonomy strives to keep undisturbed soils and their cultivated or human-modified equivalents in the same taxa insofar as possible. The reason is so the classification of a soil and the geometric representation of the cartographic unit is not changed by a single tillage operation at the soil surface. This is accomplished by the definition of the epipedon whereby it is was determined after mixing the soil to a depth of 18 cm, or by mixing the whole soil if the depth to bedrock was < 18 cm. However, surface layers of some soils were so modified by humans that Soil Taxonomy defined two anthropogenic horizons, anthropic and plaggen.

Most will agree that a plowed and equivalent unplowed soil should be classified the same. However, in addition to plowing, humans have added soil amendments to increase crop production, irrigated soils in drylands, made terraces to combat erosion, drained lands, etc. At what point do we wish to consider a soil humanly-influenced? When should we stop classifying a cultivated soil the same as its non-cultivated equivalent? Certainly it is known that plowing results in accelerated soil erosion, alters soil structure,

38 Soil Taxonomy: Provisions for Anthropogenically Impacted Soils. Wilding & Ahrens EUROPEAN SOIL BUREAU  RESEARCH REPORT NO. 7 decreases OM content, increases soil compaction, decreases biological activity, and decreases nutrient cycling. In some instances for example, Mollisols on unstable landform positions can change to Alfisols or Inceptisols in a matter of 10’s to 100’s of years of cultivation because the mollic epipedon is truncated by erosion. In other cases, liming an umbric epipedon can convert an to a (Yli-Halla and Mokma, 2002, this issue) or a Spodosol to an . Hence, this amplifies the reason why Soil Taxonomy has opted in most soil orders to utilize subsurface differentia as preference over surface differentia. It also justifies why the philosophy of many pedologists is to classify the soil observed at the present time rather than trying to infer what the soil was in the past and classifying what was thought to be present at some former time.

Diagnostic Horizons and Properties In developing Soil Taxonomy through a series of early approximations it became apparent that scientists with different backgrounds, training and experiences lacked general agreement relative to horizon concepts, designations and definitions (Soil Survey Staff, 1960, p. 32). To overcome this limitation, it was necessary to assign names to proposed diagnostic horizons and properties used for class differentia. This permitted scientists to focus on concepts for class differentiae rather than be caught up in disagreements on definitions of horizons. Further, it was the philosophy of Soil Taxonomy to classify soils on the basis of their own properties rather than presumed concepts of genesis or conditions that were believed to have been present at some former time. Diagnostic horizons and properties served this goal and provided morphogenetic class differentiae to group soils at different levels of categorical generalization. Diagnostic horizons represented morphological threads of genesis formed under multiple pedogenic pathways (Smith 1986).

Diagnostic horizons provide for groupings of soils whose properties are the result of, and reflect, major soil forming processes (suborder criteria), and additional subordinate controls on these processes (great group criteria). The characteristics selected were those that produced the desired taxa for a specified classification objective. While "all" soil characteristics were used in making this choice, only a few characteristics were used so the definitions would be easily comprehended and remembered. The characteristics decided upon were not necessarily the most important in themselves, but reflected differences in the dominance of one or more combinations or sets of genetic processes and carried with them many accessory characteristics (Smith, 1983).

In forming and defining taxa in Soil Taxonomy, those properties selected as diagnostic at the higher levels were ones most important to plant growth and that resulted from or influenced soil genesis. Other properties important to plant growth (for example soil temperature, soil thickness, texture, mineralogy, pH, etc.), environmental quality, and engineering interpretations, but without pedogenic inferences, were used at the lower categories. Examples of diagnostic horizons selected included those that reflected translocated clay, iron, organic matter, silica, gypsum and carbonates (illuvial subsurface zones); thick, dark-colored surface horizons (mollic, pachic, umbric, and anthropic); physical and chemical pans that limit root growth and water movement (root-limiting layers including fragipan, , duripan, placic, petrogypsic, petrocalcic, and petroferic); low activity clay systems (kandic); and horizons with an abundance of high surface area, amorphous or short-range order minerals (andic).

Additional properties important to use, management and that are diagnostic in Soil Taxonomy but not considered as diagnostic horizons per se include: soil reaction (euic and dysic), and temperature classes, mineralogy classes, particle size classes, the occurrence of redoximorphic segregations (aquic conditions), base status (nutrient cycling potential), bedrock (lithic and paralithic), shrink-well potential (vertic conditions), shear failure (slickensides), soil strength (n-value), rupture-resistance classes (densic materials), irreversible hardening (petroferric contact), and interfingering (penetrations of eluvial horizons into argillic).

In summary several innovations that accrued to Soil Taxonomy as a result of using diagnostic horizons and properties as class differentiae are the following: 1. Horizon ABC nomenclature gave way to diagnostic horizons as a basis for classification; 2. Properties of whole soil were used, (for example, temperature and moisture conditions), in addition to nature of horizons; 3. Diagnostic criteria were developed for defining families and subgroups that closed the gap between series and great soil groups;

Soil Taxonomy: Provisions for Anthropogenically Impacted Soils. Wilding & Ahrens 39 EUROPEAN SOIL BUREAU  RESEARCH REPORT NO. 7

4. Class definitions were applicable to both natural and disturbed soils without splitting soils arbitrarily due to use and management; and 5. Criteria for differentiation of soils were operational definitions.

Differentia used for Anthropogenically-Impacted Soils Soil Taxonomy has accommodated anthropogenic impacts on soils by application of the following differentiae at the order, suborder, great group, subgroup, family, and series categories and the following diagnostic horizons and features: anthropic and plaggen epipedons; agric and sulfuric horizons; anthric saturation (anthraquic conditions); fragments of diagnostic horizons in strongly disturbed soils; densic materials (natural or anthropogenic); drainage, irrigation and summer fallow accommodations; and cumulic (over-thickened mollic epipedons by accretion). Undoubtedly with time and knowledge, further revisions in Soil Taxonomy will be needed to better understand how to manage, interpret and classify anthropogenic- influenced soils. The following discussion will briefly describe the nature of the horizons or features that have been incorporated as provisions in Soil Taxonomy (Soil Survey Staff, 1999) to accommodate anthropogenically-impacted soils and indicate where in the system such differentia have been used. Anthropic Epipedon (Gr. Anthropikos, human) The anthropic epipedon is similar to the mollic epipedon. It is rich in bases and organic matter and forms under base-rich parent materials on the steppes of America, Europe and Asia. It has some evidence of disturbance by humans and conforms to all the requirements of a mollic epipedon except the limits on acid- soluble P2O5, or the duration of available moisture. In the first form of the anthropic epipedon, the high levels of phosphorus are the result of disposal of bones and shells near sites of human occupation. The second form of the anthropic epipedon is the result of long-term irrigation by humans. These soils have developed an epipedon similar to the mollic epipedon, but occur in environments that are too dry for the requirements of a mollic epipedon. Plaggen Epipedon (Ger.Plaggen, sod) The plaggen epipedon is a human-made surface layer 50-cm or thicker and is formed by long-term manuring. This epipedon occurs in Western Europe where in medieval time sod and other earthy materials were used for bedding livestock. The earthy material was spread on cultivated fields producing a thickened epipedon. The plaggen epipedon contains artifacts such as pieces of pottery or brick. Some plaggen epipedons have spade marks. Many of the mapped soil areas with plaggen epipedons are in straight-sided, rectangular elevated bodies. This epipedon is not common in the US.

The anthropic and plaggen epipedons are used as taxa differentia at the suborder and great group level in Inceptisols (Anthrepts and Plaggananthrepts), at the great group level in (Anthracambids), and at the subgroup level in Entisols (Plaggeptic Udipsaments), Spodosols (Plaggeptic Fragiaquods, Plaggeptic Haplohumods, Plaggeptic Fragiorthods, and Plaggeptic Alorthods), and (Anthropic Kandihumults and Anthropic Kanhaplohumults). Agric Horizon (L. ager, field) The agric horizon is an illuvial horizon that has formed under cultivation and contains significant amounts of illuvial , clay, and humus. This generally occurs after soils have been cultivated for long periods of time. Development of large pores in the plow layer and the absence of vegetation immediately after plowing encourage a turbulent flow of muddy water. The water enters wormholes and other macropores depositing silt, clay, and organic matter on the walls or holes or faces of peds. The macropores become darkened by the debris and in some instances filled. In some soils the flow of muddy water leaves behind lamellae. These lamellae are a second form of the agric horizon. No classes of this soil have been recognized in the US to date. Sulfuric Horizon (L. sulfur) The sulfuric horizon is 15 cm or more thick and is composed of either mineral or organic soil material that has a pH value of 3.5 or less and shows evidence that the low pH value is associated with sulfuric acid. Such evidence is characteristic yellow jarosite segregations and nodules (iron potassium sulfate compounds), water soluble sulfate contents of 0.05% or more, and a directly underlying layer of sulfidic materials. Most sulfuric horizons form as a result of drainage, natural or artificial, and oxidation of sulfide- rich mineral or organic materials. These horizons are common in coastal and tidal marsh regions. Some

40 Soil Taxonomy: Provisions for Anthropogenically Impacted Soils. Wilding & Ahrens EUROPEAN SOIL BUREAU  RESEARCH REPORT NO. 7 also form in places where sulfidic materials have been exposed as a result of surface mining, road construction, dredging, or other earth-moving equipment. The sulfuric horizon is a good example of a diagnostic horizon with both anthropogenic and non-anthropogenic polygenic origins. Unless one invokes past history and land use, it would be difficult to determine whether the sulfuric horizon formed by human intervention or not.

The sulfuric horizon is used at the great group level in (Sulfosaprists and Sulfohemists) and at the subgroup level in Inceptisols (Sulfudepts and Sulfaquepts). Anthraquic Conditions (Anthric Saturation) Anthric saturation is a form of aquic conditions. Aquic conditions require saturation, reduction, and redoximorphic features. Anthric saturation is a variant of episaturation and characterizes the saturation of cultivated soils forming under humanly-induced flooding, such as rice paddies and cranberry bogs. This variant of episaturation is associated with controlled flooding which causes reduction in the saturated, puddled surface soil and oxidation of iron and manganese in the unsaturated soil. The morphological responses are gleyed surface horizons overlying less permeable subsurface horizons containing redoximorphic features. Specifically, the definition of an anthraquic condition states that the surface soil is saturated and reduced for 3 months and has a chroma of 2 or less in the matrix, the subsurface horizons contain redox depletions and concentrations of iron, or they contain at least two times as much sodium dithionite-extractable iron as in the tilled surface horizon. Unless the flood history and land use of the area are known, from field evidence alone it may often be difficult to differentiate anthraquic and aquic conditions. Even with laboratory data this may sometimes be problematic. While the concept appears appropriate, differentiating human induced wetness parameters and those that are of natural causes continues to be a challenge.

Anthaquic subgroups have been established for great groups where known anthric saturation occurs. These include anthraquic subgroups of Hapludalfs, Paleudalfs, Hapludands, Melanudands, Ustifluvents, Ustorthents, Eutrudepts, Haplustepts, Haplustolls, and Paleudults. Others may be established as needed. Fragments of Diagnostic Horizons Humans affect soils in many ways. Some Entisols have been created where humans have deposited soil material by earth-moving operations including highway construction, surface mining, landfill construction and dredging. When these operations result in fragments of natural diagnostic horizons being incorporated with the spoil materials, then Soil Taxonomy recognizes a suborder of Entisols, Arents. Specifically, Arents are defined as strongly disturbed soils which have, in one or more layers between 25-100 cm from the mineral surface, 3% or more by volume of fragments of diagnostic horizons that are randomly oriented. The Arents were originally established to recognize the soils from mine reclamation. In the past several years great groups of Arents were added based on soil moisture regimes. Examples where Arents have been recognized in California where duripans have been ripped by heavy equipment to improve orchard production (Duric Xerarent), in Tennessee where gullied land has been leveled to improve soils for agricultural production (Alfic Udarents), in Texas where reclamation of lignite surface mines resulted in overburden with fragments of argillic horizon, especially near the edge of the mine where the overburden was thin (Alfic Ustarents), and in Jamaica where reclamation of bauxite surface mines resulted in fragments of the oxic and subjacent limestone bedrock to be mixed with the reclamation topsoil materials (Haplic Ustarents).

While the Arents provide provisional placements for many of the human activities associated with strongly disturbed lands, many of the subgroups are yet to be fully developed and/or modified. This is particular concern for the Ustarents in Jamacia which should be classed as Oxic Ustarents rather than Haplic Ustarents. Densic Materials Densic materials are relatively unaltered materials that have a rupture-resistance class of non-cemented. Roots cannot enter densic materials except along cracks spaced at intervals of 10 cm or greater. Densic materials don’t meet criteria of other diagnostic horizons or properties. They would include natural earthy materials and mechanically compacted ones yielding a polygenetic origin. Densic materials include till, flows, non-cemented bedrock, and mechanically compacted layers such as tillage pans from plowing, mine-spoil reclamation, and other land uses if the materials have been altered to restrict entry by roots and

Soil Taxonomy: Provisions for Anthropogenically Impacted Soils. Wilding & Ahrens 41 EUROPEAN SOIL BUREAU  RESEARCH REPORT NO. 7 movement of water. The densic differentia are used at the great group and lower categorical levels throughout Soil Taxonomy. Drainage and Irrigation Modifications of soil moisture conditions by human intervention that would effect identifcation of morphogenetic properties and placement of soils in Soil Taxonomy have been considered throughout many taxa. For example, the exclusion “or artificially drained” is used in aquic suborder definitions to permit drained and undrained wet soil equivalents to be placed in the same class. The philosophy of including artificially drained soils with non-drained equivalents was that tile subsurface drainage is a temporary provision and if the drain was not maintained or failed the drained soil would once again be subjected to aquic soil conditions.

In the case of cracks used as differentiae for soil moisture regimes in , there is an exclusion “if the soil is cultivated” that accommodates strongly mulched surfaces where the cracks tend to fill with granular materials when tillage occurs while cracks are open. Likewise, for many of the seasonal dry Vertisols there is an exclusion for suborders “unless irrigated” that permits cracks to remain open for a shorter period than defined in the class definition. In Aridisols, aquic suborders allow either the soil to be saturated by “irrigation” or by natural causes. Sediment Accretion Associated with post-cultural erosion is sediment accretion in lower topographic positions. Soil Taxonomy recognizes this human impact by over-thickened mollic epipedons (cumulic) that form cumulic subgroups in Mollisols of Epiaquolls, Endoaquolls, Haploxerolls, Haplustolls, and Hapludolls. This serves as a useful morphogenetic marker of accelerated erosion and accretion, but does not necessarily differentiate these conditions from long-term geological erosion. Therefore, here is another example of a morphogenetic marker with multiple genetic pathways.

Limitations and Constraints of Soil Taxonomy for Anthropogenetic-Impacted Soils Soil Taxonomy continues to be a dynamic system. Undoubtedly with time and knowledge, further revisions in Soil Taxonomy will be needed, as we better understand how to manage, interpret and classify anthropogenic-influenced soils. Herein we will pose a few examples where current placements in Soil Taxonomy may not adequately accommodate anthropogenically-impacted soils. Permanent Drainage Changes When levees are built along major drainage courses, such as the Mississippi River in the US and consequent soil hydrology is permanently changed for relatively long periods of time (e.g. 100’s of years), then perhaps it would be better for Soil Taxonomy to classify the soils we find at these locations at the present time rather than what was presumed to be present at some former time. This situation is uniquely different than the establishment of subsurface tile drainage where if not maintained on a short-term basis will fail and the former aquic conditions will return.

Other examples of permanent changes in hydrology caused by human impacts include deforestation and cultivation. For example, in the Midwestern sector of the US the forests were cleared and replaced with annual croplands. Similar land use changes occurred in Australia and many other parts of the world. Consequences of this were permanent changes in hydrology. This led to decreased depths to ground water and secondary salinity. Similar hydrology and salinity changes occurred in Texas when tall grass prairies of coastal regions were replaced by annual crops. Yet another illustration of vegetation/drainage changes is the deforestation of tropical steeplands which has resulted in destabilizing the soils by increasing seasonal wetness and decreasing the deep rooting of perennial plants welding soils to the subjacent bedrock. Waste Disposal Areas It is still a challenge how to classify soils in areas where municipal and industrial wastes have been deposited either above or below ground. These materials may or may not be earthy in nature, have high spatial variability, and give rise to no logical morphogenetic differentiae that appear suitable for provisional revisions in Soil Taxonomy. Likewise, landfarming of sludges and biosolids may result in heavy metal concentrations that need to be recognized in Soil Taxonomy. Because of the inadequacies of the present

42 Soil Taxonomy: Provisions for Anthropogenically Impacted Soils. Wilding & Ahrens EUROPEAN SOIL BUREAU  RESEARCH REPORT NO. 7 classification system, new suborders and/or subgroups of Entisols have been proposed for soils in human- deposited or human-exposed parent materials (Fanning and Fanning, 1989; Sencindiver and Ammons, 2000). Urbents have been proposed for soils in miscellaneous urban fill that had object artifacts such as brick or concrete in the particle size control section (generally between a depth of 25 and 100 cm). Garbents have been proposed for soils with garbage of an organic nature within 2 m of the soil surface which would readily subside and generate methane under anaerobic conditions. These proposals and others for drastically disturbed lands have not been fully developed because of the difficulty in finding morphogenetical markers that can be applied to such heterogeneous substrate without prior knowledge of land use and history. Formation of Mollic Epipedons Yli-Halla and Mokma (2002, this issue) provide an excellent example of the problem of deep plowing of sandy surface epipedons over high-base clayey . Prior to liming, manuring and plowing these native soils of Finland, they had an umbric epipedon (or anthropic epipedon when the horizons had more than 250 mg/kg P2O5 in pre-1988 Keys to Soil Taxonomy) and were placed as Eutrochrepts. With the higher P2O5 requirements (1500 mg/kg P2O5) for anthropic epipedons, the current Soil Taxonomy places these soils as Haplocryolls with a mollic epipedon. This is not a new problem but could perhaps be resolved by not permitting sandy epipedons, which meet the requirements of mollic by deep plowing, to have an abrupt lower boundary. The abrupt boundary would be the morphogenetic marker indicating human impact. Precedence for such rationale is available in Soil Taxonomy where epipedons are not recognized in recent alluvial or eolian deposits that retain rock structure in the form of fine stratifications (Soil Survey Staff, 1999, p22). Here an Ap horizon directly underlain by such stratification is not included in the epipedon concept. Hence, a mollic epipedon could not be formed by plowing recent alluvium that otherwise meets all the requirements of a mollic. Strongly Disturbed Soils In many soils with human-deposited or human-exposed parent materials, fragments of diagnostic horizons are not identifiable; thus a classification of Arents would not be appropriate. Such soils commonly would be classified in the suborder of Orthents (Soil Survey Staff, 1999). In other cases fragments of a diagnostic horizon are recognized but the zone with this morphogenetic marker is too thin to be recognized as Arents (Greenberg, 2001). This is the case for many reclaimed bauxite mine lands in Jamaica where pre-mined and post-mined lands are placed in the same taxa. For the Ustarents only one suborder is provided, Haplic, and this does not convey much useful meaning about reclamation of oxic materials. Hence, a suborder of Oxic Ustarents is needed to represent low activity clays with low available retention conditions. These soils like many other strongly disturbed soils have extreme variability over short distances, and hence, soil complexes are appropriate for mapping unit design.

Many disturbed soils in materials deposited by bulldozers or other earth-moving equipment have an irregular distribution of organic C with depth. It is assumed in this case that the organic C is not associated with indigenous paralithic materials (such as shales) or fossil in organic C such as lignite fragments. In theory, these soils would classify as Fluvents at the suborder. In contrast to Fluvents, however, human- disturbed soils are not stratified by natural processes, are not flooded in most years, and are more dense. Because these soils behave more like Orthents than Fluvents, they have been grouped as Orthents ignoring their irregular organic-C depth distribution (Smith,1976). Additional provisions in Soil Taxonomy are needed to accommodate these conditions.

With very coarse-textured deposits that have been drastically disturbed by surface mining, gravel extraction, or gold mining placers, it is commonly very difficult to find morphogenetic markers of the soil that would separate these soils from undisturbed equivalents without the prior knowledge of land use history. This, of course, violates the principles of Soil Taxonomy.

Major land sculpturing activities such as in parts of China and in other countries of East Asia are probably not given appropriate attention in Soil Taxonomy. Many of these soils contain brick and artifacts of ancient cultures, may have irregular organic C distributions, and sometimes have densic materials. Many would likely be placed with Fluvents or Orthents without specific recognition of human impact. If they contained fragments of diagnostic horizons they would be classed as Arents. Those with anthropic or plaggen epipedons would be placed in appropriate anthropic subgroups of Soil Taxonomy. In the Chinese taxonomy and World Reference Base these soils would be classed as Anthrosols (Spaargaren, 2000).

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Soil Erosion Perhaps this concern is best illustrated in the US where accelerated soil erosion due to agricultural plowing practices over the past 50-100 years has been reported to change Mollisols to Alfisols or Inceptisols. While this may be the case, these unstable landform positions were also subject to geological erosion to the extent that mollic epipedons may or may not have ever been present. This is precisely the dilemma. Are we going to classify the soils we think were present prior to anthropogenic-impact or the soils we find in the landscape today? In the case of Mollisols we have not been able to find subsurface morphogenetic markers to use in place of the mollic epipedon. Many attempts have been made to use organic C distribution patterns, distributions of calcium phosphates, chemical fractions of organic matter, and other markers of genesis, but to date none are completely satisfactory. Hence, we find some of the soil science community in favor of recognizing eroded Mollisols while others prefer to call such soils Alfisols. The authors of this paper opt for classifying soils as currently found in the landscape, hence, in this case Alfisols and not Mollisols.

In tropical steeplands there are many situations where forest cover has been removed and the soils are subsequently cultivated. Accelerated erosion by block slumping and mass wasting are common consequences. The landforms are a complex series of slope scars, buried soils and over-thickened soils that Soil Taxonomy does not accommodate well. In general Soil Taxonomy does not handle in a very constructive manner and this is the case with this example (Wilding, 2000b). The spatial diversity in these steeplands can be handled with better mapping unit designs, but the naming of component composition taxa, some with paleosols and some without, is still wanting and not resolved with current provisions in Soil Taxonomy.

Conclusions Soil Taxonomy does accommodate many of the anthropogenically-impacted soils in all taxa with a variety of special diagnostic horizons and class differentia specific to human interventions. Recent field trips to evaluate this situation have concluded that the system works reasonably well for many humanly-impacted soils but there are still opportunities to improve and revise the system to better accommodate constraints (Kimble, et al., 1999). These improvements would include new morphogenetic criteria, new or revised taxa definitions, and better quantification of spatial diversity. Little impetus has been generated for wholesale changes in Soil Taxonomy for a new Anthrosol order.

A major constraint in developing new differentia to classify anthropogenically-impacted soils is the lack of clearly recognized and defined morphogenetic markers, the basic ingredient in formulating classes for Soil Taxonomy. Further, morphogenetic properties that might be chosen as differentia are commonly temporal surface properties subject to short-term changes. These are not considered very useful. Several limitations and constraints of Soil Taxonomy are identified in this paper and proposals are made in a few instances to better accommodate these concerns. However, the authors reject the notion of utilizing historical land use as a morphogenetic criteria because it violates the principle that morphogenetic differentia are observable and/or measurable properties. Likewise, the human factor is certainly one of the biological components but it is not considered a pedogenic process per se. Many of the processes associated with humanly-modified soils also occur under natural conditions. Hence, even some of the diagnostic horizons and properties used in Soil Taxonomy to accommodate anthropogenic effects are polygenic in nature and have formed under alternate pathways, some humanly induced and others not.

Hence, with time, technology, creativity, knowledge, and new proposals, Soil Taxonomy will strive to better accommodate anthropogenically-impacted soils. It is expected that ICOMANTH will continue to solicit and assimilate this information for the soil community to evaluate and scrutinize. The goal is that Soil Taxonomy will continue to evolve to better classify soils where the human factor has been significant so we may better understand the genesis, management and behavior of these anthropogenically-impacted soil systems.

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