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The State and Future of Soils

1.1. Soils as a key component of the critical zone

1.1.1. Definitions

The critical zone extends from the lower atmosphere to unweathered rocks [NAT 01, LIN 10]. It therefore includes vegetation, fauna, soils and water tables. Without it, humanity could not survive, hence the term critical [LIN 10, NAT 01].

According to the Larousse dictionary, soil is the surface layer of crust of a telluric planet (like Earth and Mars). In French, the term "soil" and/or "ground" also has many other meanings such as "surface", "ground staff"1, etc. The plural of the term, "soils", is often preferred by soil specialists to emphasize the diversity of soil natures and properties that constitute a continuum referred to as "soil cover".

1.1.2. Soil functions and services

The first book in this series, named Soils as a Key Component of the Critical Zone 1: Functions and Services, deals with the functions and services of soils. The functions relate to ecosystems, and the services relate to humanity. However, this distinction is questionable since ecosystem functions, for the most part, are also services. Conversely, the priority given to a single service (intensive agricultural production, for example) may affect certain functions (water purification, for example). In 20152, as part of the International Year of Soils, the FAO drew up a list of eleven functions and services:

• - regulation of biogeochemical cycles (C, N, O, Al, Si, P, S, Mn, Fe, Cu, etc.) and nutrient cycling3;
• - carbon sequestration4;
• - climate regulation (see the volume Soils as a Key Component of the Critical Zone 1: Functions and Services);
• - regulation of the water cycle5 and flood regulations;
• - water purification6 and soil contaminant reduction;
• - habitat for soil organisms7, some of which can be pathogenic such as the soil bacillus Burkholderia pseudomallei, which is responsible for melioidosis, an often-fatal disease [MAN 17];
• - provision of food, fiber and fuel8;
• - source of pharmaceutical and genetic resources [BER 06, NES 15];
• - foundation for human infrastructures9;
• - provision of construction materials10;
• - cultural heritage11, particularly in terms of archaeological archives.

This list is far from comprehensive, as soil renders many other services. For example, it is also involved in air quality (see Chapter 3 of this volume). For tens of thousands of years, it has offered mankind a place of burial, constituted an element of myths and entered into rites12.

1.1.3. Soil and land degradation, desertification

Soil degradation is defined as a change in the soil's state that results in a decrease in its ability to provide goods and services13. The FAO refers to soil health, a term that reflects an anthropomorphic view. If soil is a living environment, soil cover is not an individual who could be "sick" or "dying", given that it is an evolving continuum. In contrast, soil can indeed undergo degradation; its soft horizons can even disappear under the effect of erosion. It seems more correct, and indeed more frequent, to refer to soil quality. Moreover, even artificialized soil can provide services, as shown in Chapter 8 of this volume. As a result, deeply transformed soil, such as urban soil, may not be considered "very degraded" if it has been able to sufficiently maintain or restore several important properties (bacterial and mesofaunal activities, enzymes, sufficient porosity for infiltration, nutrients, etc.) that are likely to provide ecosystem services.

Land degradation covers a broader concept, but is also more fuzzy, since this term refers to both the solid part of the Earth's surface (as opposed to liquid surfaces) and the soil or all of the resources in the critical zone.

Desertification is the process of land degradation in arid and semiarid areas. It is also a term used for other climatic zones if they undergo irreversible change of the land to such a state that it can no longer be recovered for its original use.

1.2. The difficult assessment of the state and kinetics of soil degradation or enhancement

While it has become relatively easy to globally monitor atmospheric parameters such as air temperature or CO2 content, or even to characterize soils [EHL 14] and gullies [HAR 15] on Mars, no global system has yet really been put in place to determine and monitor the state of soil degradation. One of the difficulties comes from the very definition of soil degradation, which is tainted with a certain relativity, since it refers to goods and services whose expectations vary according to populations and eras. Furthermore, it is difficult to rely on a baseline: what soil has never been subjected to a degradation agent (fires, acid rain14, radionuclide fallout15 such as 137C)? Moreover, the many forms of degradation prohibit any use of a single, universal indicator of degradation that would simply have to be monitored periodically, as is the case, for example, for the CO2 content in the atmosphere. Can we be satisfied with only taking the sealed surfaces by constructions and infrastructures into account and, therefore, only basing our land degradation assessment on urban sprawl16, or surfaces that are so eroded17 that no agricultural, pastoral or forest production is possible anymore, or even on surfaces abandoned by agriculture [FIE 08]?

In addition to this essentially spatial approach, often linked to the assessment of areas considered to be "arable", there is a more qualitative approach to soil properties or "quality" in terms of permeability (Chapter 2 of this volume), biological and chemical fertility (Chapter 9), pH (Chapter 4), salt content (Chapter 5) and biological and chemical contaminants (Chapters 6 and 7).

1.2.1. Global assessment

Despite these difficulties, three types of approaches have been adopted in order to assess the degree and extent of soil degradation on a global scale.

1.2.1.1. Expert assessment

The first attempt, coordinated by United Nations Environment Program (UNEP; Global Assessment of Soil Degradation [GLASOD] [OLD 90]), was based on expert assessment from all countries. This approach has the advantage of field knowledge - something that is too often lacking in spatial remote sensing and modeling approaches. Moreover, it is the data from this international effort that continue to be referred to due to lack of a more recent practice of the same type. However, such an approach is not without its flaws. It stumbled on the issue of the standardization of criteria and the homogenization of assessments. The other difficulty arises from the hidden agendas of some countries that have declared their soils to be fully degraded, probably in the hope of increasing a better share of international aid, while it is evident that some of their soils under forests are not degraded or only lightly degraded, particularly in protected areas.

1.2.1.2. Satellite-derived primary productivity

Another approach (The FAO Global Assessment of Land Degradation and Improvement, GLADA, [BAI 08]) was aimed more at assessing land degradation than soil degradation. It is based on primary production, estimated from the Normalized Difference Vegetation Index (NDVI) and calculated from satellite data. This quantified objective index can be obtained regularly across the globe. However, this is more of a vegetation cover assessment than a soil degradation status assessment. Although lack of cover does promote erosion processes, not all vegetation cover has the same soil conservation suitability, and some tree plantations may even be related to severe erosion (see Chapter 3).

1.2.1.3. Modeling

Combining these spatial remote sensing data with databases and different models, the FAO followed an even broader approach (Global Land Degradation Information System) [NAC 10], combining vegetation, soil, water and human pressures. It has thus drawn up several maps of the state of soil degradation and trends. Despite their undeniable value, these maps have several inherent flaws regarding the unequal quality of the data, the models used and the lack of confrontation with the ground truth. These are closer to risk maps than to actual degradation maps.

1.2.1.4. Uncertainties that are still too great

Depending on the approach adopted, the global estimate of the total degraded area thus varies from 1 to more than 6 billion hectares [GIB 15], which is a difference of more than 50 million km2. There is therefore a significant risk of overestimating available land, particularly for non-food agricultural uses (biofuels, green chemistry). Moreover, these approaches do not all agree on the geographical distribution of degraded land, which raises the issue of the location of priority efforts to be made in terms of soil protection or rehabilitation.

1.2.2. Forms of degradation

Among the ten major types of...