Soil - an exchange and transfer medium. Consequences of soil use by humans: a French example
Soils perform a large variety of functions involved in regulation of biogeochemical fluxes between the atmosphere and the aquatic environment. Soil characteristics show significant variation spatially, in the diversity of physical, physico-chemical and biological processes. Soils also vary with respect to their location as an interface with the atmosphere, the geological substrate and aquatic environments. The time-scales governing the formation of soils are generally long, ranging from a thousands to millions of years and therefore, they are not a renewable resource from the human perspective. The functions involved in soil regulation can be modified by various forms of stress related to farming, industrial and urban activities. In soil management, the catchment slope or watershed represents the most appropriate spatial scale for the assessment and understanding of soil distribution.. Throughout history, soil use has changed drastically depending on different environmental constraints, the available technology and the requirements of the human population. In order to manage soil, it is necessary to acquire a complete understanding of the conditions under which it has been formed. The chemical evolution of soils is related to natural processes. For example, in temperate regions, the main factor dictating soil evolution is the gradual loss of cations. These reductions in cations affect soil carbonates, which are gradually dissolved. Following the dissolution of soil carbonates, there are changes in levels of exchangeable cations, initially with the calcium found on the surface of the soil components (clay, organic matter and oxides). The soil then becomes acidic, and when the acidity becomes sufficiently low, the mineral components of the soil can be partially dissolved. Free aluminum can therefore be present in the soil and water, thus creating toxic conditions for plants (cultivated soils, forests) and to fish (pond water and river water). Changes in soil use and the introduction of new practices often have a considerable impact on soil components and soil properties. In the soils of France today, the carbon and nitrogen balances are generally negative. This results in a widespread reduction in the levels of organic matter. Cultivated soils generally imply CO2 emission into the atmosphere, often related to grassland cultivation and to other intensive farming practices (Arrouays et al. 1994; Rivière, 1999). Measuring and modeling the environmental balance therefore represents a major challenge both with respect to the protection of the soil environment and also with respect to understanding the global cycle of greenhouse gases such as nitrogen and carbon compounds. The presence of contaminants and the study of their bioavailability also represent major challenges in soil research. Certain parent rocks contain a appreciable quantities of potentially toxic trace elements (Pb, Cd, Ni, Co, Cu, Zn, As). The continuous supply of substances containing pollutants results in greater inflows of trace elements than outflows. In France, there are an estimated 36,000 t/yr of copper applied to French vineyards. Regions with intensive indoor husbandry, such as Brittany, are also concerned with the addition of copper and zinc in animal diets. The supply of phosphates as fertilizers has also been a source of cadmium pollution. Urban waste is also a recent source of pollution, and the effects of air-based pollution around industrial zones needs to be taken into consideration. Human activity affects not only the storage and transfer of water and dissolved elements, but also the aeration and temperature conditions that could impact biological activity. Although the organization of solid particles and pores is partially a result of conditions under which the soil has been formed, human activity both directly (cultivation methods, soil drainage) and indirectly (influence of plant and fauna activity) influences the physical properties of soil. For example, the general trend is for the organic matter content to decrease with an increase in soil use (ploughing of grassland). This is particularly true in the case of silty soils, which contain small quantities of clay and are thus less stable structurally and are susceptible to physical erosion. Soil chemistry also affects physical properties. For example, under slightly acidic conditions, soil particles can become highly mobile in the environment. The migration of clay from the upper soil layers to the lower layers (translocation) should not be ignored. This relative loss of clay from the surface can be observed over time as the soil becomes particularly sensitive to the effects of water (incrustation, particle suspension) and, finally, erosion and run-off occur. Soil science must provide a reservoir of knowledge in order to increase the understanding of soil functions and to meet the needs of agriculture and the environment. The study of soil must incorporate both spatial and temporal trends, as changes are often slow and difficult to detect. This demonstrates the need to reassess soil with respect to its environmental functions, without limiting the assessment to merely the function of production.