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Soils retrogression and degradation

2007 Schools Wikipedia Selection. Related subjects: Geology and geophysics

   Soils retrogression and degradation in the French school of pedology
   are two regressive evolution processes associated with the loss of
   equilibrium of a stable soil. Retrogression is primarily due to erosion
   and corresponds to a phenomenon where succession reverts back to
   pioneer conditions (such as bare ground). Degradation is an evolution,
   different of natural evolution, related to the locale climate and
   vegetation. It is due to the replacement of the primitive vegetation
   (known as climax) by a secondary vegetation. This replacement modifies
   the humus composition and amount, and impacts the formation of the
   soil. It is directly related to human activity.

Soil evolution cycle

   The soil represents the surface layer, of the earth's crust, resulting
   from the transformation of the bare rock, enriched by organic input.

   At the beginning of a soil formation, only the bare rock outcrops. It
   is gradually colonized by pioneer species (lichens and mosses), then
   herbaceous vegetation, shrubs and finally forest. In parallel a first
   humus-bearing horizon is formed (the A horizon), followed by some
   mineral horizons (B horizons). Each successive stage is characterized
   by a certain association of soil/vegetation and environment, which
   defines an ecosystem.

   After a certain time of parallel evolution between the ground and the
   vegetation, a state of steady balance is reached; this stage of
   development is called climax by some ecologists and "natural potential"
   by others. Succession is the evolution towards climax. Regardless of
   its name, the equilibrium stage of primary succession is the highest
   natural form of development that the environmental factors are capable
   of producing.

   The cycles of evolution of soils have very variable durations, between
   a thousand-year-old for soils of quick evolution (A horizon only) to
   more than a million of years for soils of slow development. The same
   soil may achieve several successive steady state conditions during its
   existence, as exhibited by the Pygmy forest sequence in Mendocino
   County, California. Soils naturally reach a state of high productivity
   from which they naturally degrade as mineral nutrients are removed from
   the soil system. Thus older soils are more vulnerable to the effects of
   induced retrogression and degradation.

Ecological factors influencing soil formation

   There are two types of ecological factors influencing the evolution of
   a soil (through alteration and humification). These two factors are
   extremely significant to explain the evolution of soils of short
   development.
     * A first type of factor is the average climate of an area and the
       vegetation which is associated ( biome). This factor allows one to
       define the world major areas of vegetation and soils.
     * A second type of factor is more local, and is related to the
       original rock and local drainage. This type of factor explains
       appearance of specialized associations (ex peat bogs).

Biorhexistasy theory

   The destruction of the vegetation implies the destruction of evoluted
   soils, or a regressive evolution. Cycles of succession-regression of
   soils follow one another within short intervals of time (human actions)
   or long intervals of time (climate variations).

   The climate role in the deterioration of the rocks and the formation of
   soils lead to the formulation of the theory of the biorhexistasy.
     * In wet climate, the conditions are favorable to the deterioration
       of the rocks (mostly chemically), the development of the vegetation
       and the formation of soils; this period favorable to life is called
       biostasy.
     * In dry climate, the rocks exposed are mostly subjected to
       mechanical disintegration which produces coarse detrital materials:
       this is referred to as rhexistasy.

Perturbations of the balance of a soil

   When the state of balance, characterized by the ecosystem climax is
   reached, it tends to be maintained stable in the course of time. The
   vegetation installed on the ground provides the humus and ensures the
   ascending circulation of the matters. It protects the ground from
   erosion by playing the role of barrier (for example, protection from
   water and wind). Plants can also reduce erosion by binding the
   particles of the ground to their roots.

   A disturbance of climax will cause retrogression, but, if given the
   opportunity, nature will make every effort to restore the damage via
   secondary succession. Secondary succession is much faster than primary
   because the soil is already formed, although deteriorated and needing
   restoration as well.

   However, when a significant destruction of the vegetation takes place
   (of natural origin such as an avalanche or human origin), the
   disturbance undergone by the ecosystem is too important. In this latter
   case, erosion is responsible for the destruction of the upper horizons
   of the ground, and is at the origin of a phenomenon of reversion to
   pioneer conditions. The phenomenon is called retrogression and can be
   partial or total (in this case, nothing remains beside bare rock). For
   example, the clearing of an inclined ground, subjected to violent
   rains, can lead to the complete destruction of the soil. Man can deeply
   modify the evolution of the soils by direct and brutal action, such as
   clearing, abusive cuts, forest pasture, litters raking. The climax
   vegetation is gradually replaced and the soil modified (example:
   replacement of leafy tree forests by moors or pines plantations).
   Retrogression is often related to very old human practices.

Influence of human activity

   Erosion is the main factor for soil degradation and is due to several
   mechanisms: water erosion, wind erosion, chemical degradation and
   physical degradation.

   Erosion is strongly related to human activity. For example, roads which
   increase impermeable surfaces lead to streaming and ground loss.
   Agriculture also accelerates soil erosion (increase of field size,
   correlated to hedges and ditches removal). Meadows are in regression to
   the profit of plowed lands. Spring cultures (sunflower, corn, beet)
   surfaces are increasing and leave the ground naked in winter. Sloping
   grounds are gradually colonized by vine. Lastly, use of herbicides
   leaves the ground naked between each crop. New cultural practices, such
   as mechanization also increases the risks of erosion. Fertilization by
   mineral manures rather than organic manure gradually destructure the
   soil. Many scientists observed a gradual decrease of soil organic
   matter content in soils, as well as a decrease of soil biological
   activity (in particular, in relation to chemical uses). Lastly,
   deforestation, in particular, is responsible for degradation of forest
   soils.

   Agriculture increases the risk of erosion through its disturbance of
   vegetation by way of:
     * overgrazing of animals
     * planting of a monoculture
     * row cropping
     * tilling or plowing
     * crop removal
     * land-use conversion

Consequences of soil regression and degradation

     * yields impact: Recent increases in the human population have placed
       a great strain on the world's soil systems. More that 5.5 billion
       people are now using about 10 % of the land area of the Earth to
       raise crops and livestock. Many soils suffer from various type of
       degradation, that can ultimately reduce their ability to produce
       food resources. Slight degradation refers to land where yield
       potential has been reduced by 10%, moderate degradation refers to a
       yield decrease from 10-50 %. Severely degraded soils have lost more
       than 50% of their potential. Most severely degraded soils are
       located in developing countries such as Asia and Africa.

     * natural disasters: mud flows, floods ... responsible for the death
       of many living beings each year

     * deterioration of the water quality: the increase in the turbidity
       of water and the contribution of nitrogen and of phosphorus can
       result in eutrophication. Soils particles in surface waters are
       also accompanied by agricultural inputs and by some pollutants of
       industrial, urban and road origin (such as heavy metals). The
       ecological impact of agricultural inputs (such as weed killer) is
       known but difficult to evaluate because of the multiplicity of the
       products and their broad spectrum of action.

     * biological diversity: soil degradation may involve the
       disappearance of the climax vegetation, the decrease in animal
       habitat, thus leading to a biodiversity loss and animal extinction
       ...

Soil enhancement and rebuilding

   Problems of soil erosion can be fought, and certain practices can lead
   to soil enhancement and rebuilding. Even though simple, methods for
   reducing erosion are often not chosen because these practices outweigh
   the short-term benefits. Rebuilding is especially possible through the
   improvement of soil structure, addition of organic matter and
   limitation of runoff. However, these techniques will never totally
   succeed to restore a soil (and the fauna and flora associated to it)
   that took more than 1000 years to build up.

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