The alternative meaning of pedoderm and its use for soil surface characterization

Posted by Dan Yaalon 20 Dec 2004 15:34:37

M.V. Fey[1]*, A.J. Mills1 & D.H. Yaalon2

1Department of Soil Science, University of Stellenbosch, Private Bag X01, Stellenbosch, 7602, South Africa
2Institute of Earth Sciences, Hebrew University, Givat Ram Campus, Jerusalem 91904, Israel

Abstract

A recently proposed meaning of pedoderm to describe the thin layer of mineral soil at the interface with the atmosphere is here more formally defined so as to contrast it with an older definition that is used infrequently in soil stratigraphy. It has implications for the understanding of lateral and vertical fluxes of water, air and nutrients; soil erosion; crust formation and ecosystem function; and in the use of remote sensing for soil evaluation and management.

Keywords: pedoderm; soil stratigraphy; geosol; physical crust; biological crust

Preamble
While assessing the effects of vegetation burning on soil properties, Mills & Fey (2004) proposed the term pedoderm to describe those first few centimetres of soil in which certain properties are often expressed much more strongly than in the remainder of the soil surface horizon. The implication is that many types of topsoil have a “micro-topsoil” of their own, in which the essence of topsoil character exhibits maximal expression at the soil-atmosphere interface. When proposing this term for the “soil skin” in the same sense that geoderm connotes “earth skin”, Mills and Fey (2004) were unaware that the pedoderm had previously been defined in a soil stratigraphic context (Brewer et al. 1970). A means of rectifying and even capitalising on this oversight was subsequently suggested by D H Yaalon, and the objective of our note is jointly to suggest that both definitions of ‘pedoderm’ can be accepted and used, even though the soil stratigraphic term may well have fallen into disuse as explained below. Anticipating that the newly published meaning may gain currency, we believe that the ambiguity should be brought to the attention of readers. (Duality of meaning has many acceptable precedents - for example, pedology also means the study of children). Furthermore, the definition of pedoderm as proposed by Mills & Fey (2004) needs to be sharpened and the implications of its use need to be considered.

Pedoderm as a stratigraphic unit
Brewer et al. (1970) proposed the pedoderm as a new formal soil-stratigraphic unit, defined as “a mappable unit mantle of soil, entire or partially truncated, at the earth’s surface or partially or wholly buried, which has physical characteristics and stratigraphic relationships that permit its consistent recognition and mapping”. This definition included subsolum features and weathered parent rock. Pedoderm was redefined by Butler (1982) as “the soil mantle unit which has continuity, possibly interrupted, and characteristic soil, sediment and stratigraphic attributes and relationships that permit consistent recognition and mapping“. Later, Walker et al. (1984) rejected this definition on the grounds that the inclusion of the term ‘sediment’ required pedologists to resolve both soil stratigraphy and rock stratigraphy. They recommended that the original definition of the pedoderm by Brewer et al. (1970) be retained.
In a review of soil stratigraphy Finkl (1980) noted that ‘geosol’ (originally introduced by Morrison in 1964) and ‘pedoderm’ were the only soil-stratigraphic terms that had been formally proposed for regional use: ‘geosol’ in the North American Stratigraphic Code and ‘pedoderm’ for the Australian Stratigraphic Code. Geosol was formally accepted in the USA and is now in common use in many countries, defined as ‘a buried pedostratigraphic unit - a traceable, three-dimensional body of rock that consist of one or more differentiated pedologic horizons’ (article 55 and 56 in the North American Stratigraphic Code, 1983). On the other hand ‘pedoderm’ as a soil stratigraphic unit has never been included officially in the Australian Stratigraphic Code and is rarely used even in Australia, where buried paleosols are common.

Pedoderm as a thin mineral surface layer
Mills and Fey (2004) defined pedoderm as the “top few centimetres of mineral soil, i.e. the interface between the atmosphere and the soil body” and noted that it “affects ecosystem function in a disproportionate manner as it controls water and air entry into the soil”. We refine this definition here as a thin zone of mineral soil at the interface with the atmosphere, a few millimeters to centimeters thick, within which several properties exhibit an ecologically and hydrologically meaningful change in intensity with depth. The pedoderm hosts or serves as substrate for the formation of both biological and physical crusts of the kind described, respectively, by Belnap and Lange (2003) and Singer and Shainberg (2004). It is not synonymous with such crusts, however, and might even in some instances be less coherent or compacted than the soil beneath it. Pedoderm should also not be confused with the A horizon or epipedon for which there are conventional definitions based on a darkening from the accumulation of organic matter within a mineral matrix. The pedoderm is that upper portion of the A horizon within which there are certain properties showing greatly enhanced expression relative to the bulk of the A horizon, such as the content of organic matter and plant nutrients, the activity of microorganisms and the stability of soil aggregates, and/or there is the appearance of features not evident in the remainder of the A horizon, such as physical crusting, colonization by a biological crust, the presence of a vesicular layer (Dan et al. 1982), bleaching or salt crystals.

Some implications of recognising the pedoderm
The ecological and hydrological consequences of the thin, excited skin of the soil are numerous and include soil aeration, water infiltration, development of biocrusts, erosion and surficial flow of water (Belnap and Lange 2003; Singer and Shainberg 2004), root growth and nutrient cycling. The exponential change of properties with depth on a scale of centimetres is particularly apparent in arid and semi-arid regions (Woods 1989; Purnomo et al. 2000), but on a scale of millimetres it is likely to be apparent in any region where vegetation creates surface litter. The pedoderm tends to have more soil organic matter than deeper layers within the conventionally defined A horizon and consequently mineralisation is often disproportionately greater in this surface layer than below it (Woods 1989; Purnomo et al. 2000). It is also the membrane that receives, adsorbs and transfers nutrients from decomposing litter above the mineral soil. The ecological literature has recently emphasised the important role of boundaries in controlling fluxes of energy, water and nutrients between different systems (Belnap et al. 2003; Cadenasso et al. 2003). The concept of pedoderm serves as a good example of such a boundary. Some surface horizons can be expected to lack obvious expression of a pedoderm because of pedoturbation which may be biotic, cryic (freeze-thaw) or argillic (cracking clays).
Biological crusts in deserts are dynamic, variable, and rarely cover more than 40 percent of the surface (Dougill and Thomas 2004) while physical crusts are equally dynamic and spatially heterogeneous. Both kinds of crusts form on or within the pedoderm and derive part of their character from its properties. When remote sensing is used for the characterization, evaluation or mapping of soil properties (Ben-Dor 2002; Dematte et al. 2004), pedodermal heterogeneity (and that of the crusts which the pedoderm sustains) may not necessarily reflect similar subsurface heterogeneity.
Manifestations of pedodermal expression are probably much more diverse than we realize. Formal definition of the earth-atmosphere boundary could help focus attention on soil surface horizons and foster a research paradigm in which a much finer vertical resolution is applied when examining and sampling the uppermost part of soil profiles. Clearer pedological, ecological and hydrological understanding should result.

References

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Belnap, J., Lange, O.L., eds., 2003. Biological soil crusts: structure, function and management. Springer Verlag, Berlin.

Ben-Dor, E., 2002. Quantitative remote sensing of soil properties. Advances in Agronomy 75, 173-2543.

Brewer, R., Crook, K.A.W., Speight, J.G., 1970. Proposal for soil-stratigraphic units in the Australian Stratigraphic Code. Journal of the Geological Society of Australia 17, 103-111.

Butler, B.E., 1982. A new system for soil studies. Journal of Soil Science 33, 581-595.

Cadenasso, M.L., Pickett, S.T.A., Weathers, K.C., Bell, S.S., Benning, T.L., Carreiro, M.M., Dawson, T.E., 2003. An interdisciplinary and synthetic approach to ecological boundaries. BioScience 53, 717-722.

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Finkl, C.W., 1980. Stratigraphic principles and practices as related to soil mantles. Catena 7, 169-194.

Mills, A.J., Fey, M.V., 2004. Frequent fires intensify soil crusting: physico-chemical feedback in the pedoderm of long-term burn experiments in South Africa. Geoderma 121, 45-64.

North American Stratigraphic Code, 1983. Bulletin of the American Association of Petroleum Geologists 65, 841-875.

Purnomo, E., Black, A.S., Smith, C.J., Conyers, M.K., 2000. The distribution of net nitrogen mineralisation within surface soil. 1. Field studies under a wheat crop. Australian Journal of Soil Research 38, 643-652.

Singer, M.J., Shainberg, I., 2004. Mineral soil surface crusts and wind and water erosion. Earth Surface Processes and Landforms 29, 1065-1075.

Walker, P.H., Beckmann, G.G., Brewer R., 1984. Definition and use of the term 'pedoderm'. Journal of Soil Science 35, 505-510.

Woods, L.E., 1989. Active organic matter distribution in the surface 15 cm of undisturbed and cultivated soil. Biology and Fertility of Soils 8, 271-278.

[1] Corresponding author. Tel.: +27-21-808-4794; Fax: +27-21-808-4791
E-mail address: fey@sun.ac.za (M.V. Fey).