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Carbon, as it relates to the organic matter of soils, is a major component of soil and catchment health. Several factors affect the variation that exists in soil organic matter and soil carbon; the most significant has, in contemporary times, been the influence of humans and agricultural systems.
The pool of organic carbon exists in dynamic equilibrium between gains and losses; soil may therefore serve as either a sink or source of carbon through carbon sequestration or greenhouse gas emissions, respectively, depending on exogenous factors.
Organic carbon held in soil is eventually returned to the atmosphere through the process of respiration carried out by heterotrophic organisms, but a substantial part is retained in the soil in the form of soil organic matter; tillage usually increases the rate of soil respiration, leading to the depletion of soil organic matter. [24]
When the organic component of a soil is substantial, the soil is called organic soil rather than mineral soil. A soil is called organic if: Mineral fraction is 0% clay and organic matter is 20% or more; Mineral fraction is 0% to 50% clay and organic matter is between 20% and 30%; Mineral fraction is 50% or more clay and organic matter 30% or ...
Particulate organic matter (POM): is the organic matter that retains evidence of its original cellular structure, [18] and is discussed further in the next section. Humus: is usually the largest proportion of organic matter in soil, contributing 45 to 75%. Typically it adheres to soil minerals, and plays an important role structuring soil.
Humins make up about 50% of the organic matter in soil. [ 1 ] Due to their very complex molecular structure, humic substances, including humin, do not correspond to pure substances but consist of a mixture of many compounds that remain very difficult to characterize even using modern analytical techniques.
Stocking and destocking of soil carbon are under strong climate influence. [68] They are normally balanced through an equilibrium between production and mineralization of organic matter, but the balance is in favour of destocking under present-day climate warming, [69] and more especially in permafrost. [70]
In general, organic matter contacting soil has too little nitrogen to support the biosynthetic needs of the decomposing soil microbial population. If the C:N ratio of the decomposing organic matter is above circa 30:1 then the decomposing microbes may absorb nitrogen in mineral form as, e. g., ammonium or nitrates. This mineral nitrogen is said ...