Pedosphere

right|thumb|Cross-section illustrating the soil layer, showing the [[topsoil (A); regolith (B); and saprolite, a less-weathered regolith (C).]]

The pedosphere () is the outermost layer of the Earth's crust that is composed of soil and subject to soil formation and erosion processes. It exists at the interface of the lithosphere, atmosphere, hydrosphere and biosphere. The pedosphere is the "stratum corneum" of the Earth's surface and only develops when there is a dynamic interaction between the atmosphere (air in and above the soil), biosphere (living organisms and associated organic matters), lithosphere (unconsolidated regolith and consolidated bedrock) and the hydrosphere (water in, on and below the soil). The pedosphere is the foundation of terrestrial ecosystems on Earth.

The pedosphere acts as the mediator of chemical and biogeochemical flux into and out of these respective systems and is made up of gaseous, mineralic, fluid and biologic components. The pedosphere lies within the Critical Zone, a broader interface that includes vegetation, pedosphere, aquifer systems, regolith and finally ends at some depth in the bedrock where the biosphere and hydrosphere cease to make significant changes to the chemistry at depth. As part of the larger global system, any particular environment in which soil forms is influenced solely by its geographic position on the globe as climatic, geologic, biologic and anthropogenic changes occur with changes in longitude and latitude.

The pedosphere lies below the vegetative cover of the biosphere and above the hydrosphere and lithosphere. The soil forming process (pedogenesis) can begin without the aid of biology but is significantly quickened in the presence of biologic reactions, where it forms a soil carbon sponge. Soil formation begins with the chemical and/or physical breakdown of minerals to form the initial material that overlies the bedrock substrate. Biology quickens this by secreting acidic compounds that help break rock apart. Particular biologic pioneers are lichen, mosses and seed bearing plants, but many other inorganic reactions take place that diversify the chemical makeup of the early soil layer. Once weathering and decomposition products accumulate, a coherent soil body allows the migration of fluids both vertically and laterally through the soil profile, causing ion exchange between solid, fluid and gaseous phases. As time progresses, the bulk geochemistry of the soil layer will deviate away from the initial composition of the bedrock and will evolve to a chemistry that reflects the type of reactions that take place in the soil.

Lithosphere

The primary conditions for soil development are controlled by the chemical composition of the rock on which the soil will be. Rock types that form the base of the soil profile are often either sedimentary (carbonate or siliceous), igneous or metaigneous (metamorphosed igneous rocks) or volcanic and metavolcanic rocks. The rock type and the processes that lead to its exposure at the surface are controlled by the regional geologic setting of the specific area under study, which revolve around the underlying theory of plate tectonics, subsequent deformation, uplift, subsidence and deposition.

Metaigneous and metavolcanic rocks form the largest component of cratons and are high in silica. Igneous and volcanic rocks are also high in silica, but with non-metamorphosed rock, weathering becomes faster and the mobilization of ions is more widespread. Rocks high in silica produce silicic acid as a weathering product. There are few rock types that lead to localized enrichment of some of the biologically limiting elements like phosphorus (P) and nitrogen (N). Phosphatic shale (<&nbsp;15% P₂O₅) and phosphorite (>&nbsp;15% P₂O₅) form in anoxic deep water basins that preserve organic material. Greenstone (metabasalt), phyllite, and schist release up to 30–50% of the nitrogen pool. Thick successions of carbonate rocks are often deposited on craton margins during sea level rise. The widespread dissolution of carbonate and evaporites leads to elevated levels of Mg²⁺, , Sr²⁺, Na⁺, Cl⁻ and ions in aqueous solution.

Weathering and dissolution of minerals

The process of soil formation is dominated by chemical weathering of silicate minerals, aided by acidic products of pioneering plants and organisms as well as carbonic acid inputs from the atmosphere. Carbonic acid is produced in the atmosphere and soil layers through the carbonation reaction. The Mg is soluble in water and is carried in the runoff, but the Fe often reacts with oxygen to precipitate Fe₂O₃ (hematite), the oxidized state of iron oxide. Sulfur, a byproduct of decaying organic material, will also react with iron to form pyrite (FeS₂) in reducing environments. Pyrite dissolution leads to low pH levels due to elevated H⁺ ions and further precipitation of Fe₂O₃ Lichen has long been viewed as the pioneers of soil development as the following 1997 Isozaki statement suggests:

However, lichens are not necessarily the only pioneering organisms nor the earliest form of soil formation as it has been documented that seed-bearing plants may occupy an area and colonize quicker than lichen. Also, eolian sedimentation (wind generated) can produce high rates of sediment accumulation. Nonetheless, lichen can certainly withstand harsher conditions than most vascular plants, and although they have slower colonization rates, they do form the dominant group in alpine regions.

Organic acids released from plant roots include acetic acid and citric acid. During the decay of organic matter phenolic acids are released from plant matter and humic acid and fulvic acid are released by soil microbes. These organic acids speed up chemical weathering by combining with some of the weathering products in a process known as chelation. In the soil profile, these organic acids are often concentrated at the top of the profile, while carbonic acid plays a larger role towards the bottom of the profile or below in the aquifer.

Decomposition in anoxic or reduced soils is also carried out by sulfur-reducing bacteria which, instead of O₂ use as an electron acceptor and produce hydrogen sulfide (H₂S) and carbon dioxide in the process: In most freshwater wetlands there is little sulfate () so methanogenesis becomes the dominant form of decomposition by methanogenic bacteria only when sulfate is depleted. Acetate, a compound that is a byproduct of fermenting cellulose is split by methanogenic bacteria to produce methane (CH₄) and carbon dioxide (CO₂), which are released to the atmosphere. Methane is also released during the reduction of CO₂ by the same bacteria. the high concentration of CO₂ and the occurrence of metals in soil solutions results in lower pH levels in the soil. Gases that escape from the pedosphere to the atmosphere include the gaseous byproducts of carbonate dissolution, decomposition, redox reactions and microbial photosynthesis. The main inputs from the atmosphere are aeolian sedimentation, rainfall and gas diffusion. Eolian sedimentation includes anything that can be entrained by wind or that stays suspended in air and includes a wide variety of aerosol particles, biological particles like pollen, and dust particles. Nitrogen is the most abundant constituent in rain (after water), as water vapor utilizes aerosol particles to nucleate rain droplets. Furthermore, carbon buildup in the soils is decreased due to slower decomposition rates. As a result, the rates of carbon circulation in the biogeochemical cycle is decreased.

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