ReviewThe anthropogenic sealing of soils in urban areas
Introduction
Soils perform a number of crucial functions which make them environmentally, economically and socially important. Which soil functions can be distinguished? The production, the carrier, the filter, the resource, the habitat, and the cultural function are usually recognized. Some of these functions are exclusive and in competition (EC, 2006). In earlier times, with less available technology, land use was largely determined by the functions that could be performed by the natural soil. This relation between soil functions and land use has been lost to a certain extent in the course of the last century because of technological developments. Consequently, the cover of land is not simply an attribute but a concrete set of features that result largely from its use (Bouma, 2006). The use of land implies that almost always different and contrasting interests that span from agriculture to recreation, from infrastructure construction to environment preservation are in competition. A given land cover can be modified, consumed or degraded and a new type generated. As such, the consumption and formation of land cover is very similar to the transformation of capital goods. Since land cannot, in general terms, be created or destroyed (with the exceptions of coasts and polders), land cover change can generally be characterized in terms of different types of flows between land cover types. A key focus of land cover accounts is the understanding of the way in which the stocks of different land covers and uses are transformed over time in the connection between human societies and ecological systems (e.g. Kareiva et al., 2007, van Kamp et al., 2003, Various Authors, 2008). One area of needed knowledge is the sealing of soils, where human infrastructures and activities interrupt the connection of the soil with other ecosystem compartments.
Soil seal (Duley, 1939) refers to a thin layer which limits infiltration through the (wet) soil. Various causes have been identified that can lead to the impermeabilisation of the soil surface and they include the loss of structure due to the impact of rain or soil laboring, the dispersion of colloids, the compaction. All causes impact the porosity of the soil by either reducing its amount or by modifying its pattern. The modification of macropore patterns negatively influences water infiltration, as they are fundamental in determining the rate of water intake in the soil (Bouma, 1992, Rousseva et al., 2002). The natural sealing is commonly followed by transport and deposition of detached particles (Panini et al., 1997, Singer and Shainberg, 2004, Singer, 2006).
A plethora of studies on soil compaction have demonstrated that the main human activities that are responsible for soil compaction are agro-forestry, because of the large areas they affect (Van den Akker and Canarache, 2001).
Contrary to natural sealing, artificial sealing is generally extensive and permanent, and entails a modification of the neighboring ecosystems (e.g., Burghardt, 2006). The significance of sealing must be extended to describe the covering of its surface by impervious materials such as, for example, concrete, metal, glass, tarmac and plastic. Soil sealing is then a common consequence of urbanization and infrastructure construction (Fig. 1).
In Europe, the sprawling nature of many cities is critically important because of the consequent increased energy and soil consumption (Fig. 1, III). A report of the European Environment Agency describes the detrimental effects of expanding urban areas on the environment and an impact analysis carried out for the European Commission suggests that soil degradation may cost up to US$ 56 billion per year (EEA, 2006).
The degree of sealing is related to the type of land use and to the population density. Commonly, populations exceed the carrying capacity of their ecosystems, and experience a rapid decline until conditions for growth are restored. Estimates for the next 50 years indicate that Humankind is tending to a global density of 1 person for each 0.01 km2 of reasonably biologically productive land (Certini and Scalenghe, 2006).
Changes in the size of human population as well as changes in the activity of sectors such as transport and tourism may lead to urban expansion and infrastructure construction. This may not be solely the result of an increase in population but it can be the result of a change in behavior (as with urban sprawl, more extensive urban patterns are preferred). As a consequence, a certain amount of land is consumed and built-up areas increase (due to the lack of more precise information, the built-up areas increase is used as a proxy to quantify the land taken by urban expansion although, by definition, built-up areas also include land which is not actually sealed) at the expense of other types of land use (EEA, 2001). The soil being covered will no longer be able to perform the range of environmental functions associated with it as it will be separated from the other environmental compartments (EEA, 2001) (Table 1). In addition, the proximity of unsealed areas to pollution sources such as vehicular traffic expose them to pollution (e.g., Biasioli et al., 2006, Scalenghe and Fasciani, 2008, Wolf et al., 2007).
Positive effects of sealing are less obvious. Sealing landfills, waterproofing adobe, mitigating radionuclides contamination or the effects of magnetic fields are examples. The preservation of cultural heritage is another example. Ancient Troy, previously known only through the text of Homer (?)’s Iliad, or Pompeii were kept safe by the sealing.
In Europe, a decade after the Swiss legislation on soil protection (Swiss Confederation, 1998), an effort has been made by the European Commission (2006) that released the Thematic Strategy for Soil Protection, in which soil sealing is identified as one of the threats.
To have an idea of the amount of existing information on this subject one can put the words ‘soil’ and ‘sealing’ into Google™. But as over two millions of pages are received back, to tackle the problem of overload in searches, Web 2.0 use the so-called ‘clustering’. A new search with one of these search engines (e.g., Clusty) over 400 clustered results is retrieved. The first seven categories are ranked in the following order (largest first): Soil Surface > European > (Soil) Protection > Method/Patent > Workshops > Stabilization > Land. This confirms that, in Europe, the threat of soil loss by impermeabilisation attracts nowadays more attention than the engineering aspects.
This study presents an overview of the current issues of artificial soil sealing in Europe and reviews the recent literature on the threat that sealing poses to the conservation and functioning of the soil resource in an attempt to identify the current approaches to the problem and to highlight the needs of research.
Section snippets
Geography of soil sealing in Europe
Europe is a continent where three quarter of its population live in cities of small and medium-sized. The European territory is characterized by 1595 functional urban areas (FUAs) and 76 Metropolitan European Growth Areas (MEGAs) have been identified in 29 countries (ESPON, 2006). There is a dense urban structure in the central part of Europe, stretching from the UK via the Netherlands, Belgium, western Germany and northern France, and continuing into Italy, Czech Republic, South Poland,
Effects of sealing on soil functions
The effects of soil-sealing on the major environmental component concern the kinetic of chemical reactions and the exchange of water, gas, particles, and energy between the soil and other environmental compartments thereby affecting the proper functioning of the soil (Effland and Pouyat, 1997, Flores et al., 1998, Kaye et al., 2006, Pickett and Cadenasso, 2008).
Reducing soil sealing
In paved areas, semipervious pavement systems could be adopted to limit the consequences of sealing. They are constructed to be highly conductive by using a material which is weak in retention, but its upper layer, the dark seam material, which is rich in organic carbon, may act as a filter. Weaknesses are that with increasing age, the original seam filling becomes less conductive due to accumulations of different materials (foliage, dust, oil, etc.). So, compared to impervious soil sealing,
Voluntary sealing
There are different types of voluntary sealing of soil: (i) physical, (ii) chemical and (iii) biological. Physical soil sealing is the clogging and plugging of soil pores to reduce the hydraulic conductivity. Chemical sealing is the changing in the soil structure caused by chemical reactions. Biological sealing is the clogging of soil pores with microbial products or by-products.
Sealing landfills. Soil chemical treatments based on the dispersion and flocculation of clay minerals, as well as the
Conclusive remarks
The future of most part of Humankind will be urban and sealing will go along with urbanization at a scale unprecedented in human history. Is the solution Eutropia or Olinda, invisible cities envisioned by Italo Calvino (1972)? Eutropia, made up of many cities, all but one of them empty, and that its inhabitants periodically travel to the next; Olinda that contains the Olinda-yet-to-be in embryo which, as a meristem, grows out. In our visible cities, it is impossible to think a development of
Acknowledgements
We thank Jens Wiegand and Bernd Schott. We are extremely indebted to Eric Sanderson who kindly provided the sorted database of the Human Footprint.
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