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Soil carbon sequestration is a natural process of transferring atmospheric CO2 into soils, on a long-term basis. The transfer consists of capturing the CO2 and storing it as soil carbon. Long-term carbon storage is related to the several factors including climate, soil physical characteristics, soil biology and land-management practices.

Photosynthesis is the primary mediator for capturing and transferring carbon into biomass, as organic carbon. The storage of this organic carbon within the soil is dynamic and can be in different forms (see: soil organic matter). The amount and the form of carbon storage arise from several processes including decomposition, organic matter addition/incorporation and, plant and animal excretions.

While most carbon in soil is sequestrated as soil organic carbon, it can also be stored as soil inorganic carbon (i.e. Carbonates), whose rate and extent are considered as relatively low. This process consists of the reaction of soil minerals with atmospheric CO2, that forms secondary minerals (i.e. Carbonates).

Soil carbon sequestration is an effective climate change mitigation method through promoting carbon fluxes from the atmosphere to the soils. It also plays a major role in improving soil quality.

Overview
Carbon is continuously cycling around the globe, through carbon fluxes between different carbon pools (i.e. reservoirs); in its different forms. Among these carbon pools; soils represent the largest terrestrial carbon reservoir.

Soil carbon sequestration is likely to occur when:


 * the rate and/or the amount of the carbon fluxes to the soils increase (e.g. from the atmosphere through photosynthesis; from added organic matter through microbial activity);


 * the rate and/or the amount of soil carbon losses decrease (i.e. decreased rates of carbon fluxes to other reservoirs); and
 * the length of carbon storage within the soil increases (i.e. through physical, chemical and/or biochemical stabilization of soil organic matter ).

In these cases, carbon would accumulate within the soil on a long-term basis.

The amount of carbon stored (carbon balance) is dependent on the soil carbon inputs and outputs; whose rate and extent are affected by various factors including climate, soil characteristics, organic matter quality and management practices.

Table 1 shows several carbon inputs/outputs ways to/from the soil.: Considering that there is a constant carbon dioxide output due to soil respiration; maintaining continuous soil carbon input is crucial for the sequestration, especially for the managed ecosystems. A common indicator of soil carbon sequestration ability is the relationship between annual carbon inputs and annual carbon accumulation rate (soil carbon sequestration efficiency).



Factors affecting soil carbon sequestration
Factors affecting the amount and length of carbon storage within the soil are as following:

Climate
Temperature and precipitation highly affect the rate of photosynthesis as well as soil temperature and moisture; which are the two factors that are highly correlated with microbial activity. Microbial activity and decomposition rates are generally highest at moderate temperature and moderate soil moisture, i.e. warm, moist and aerobic conditions. In these cases, carbon dioxide emission to the atmosphere increases; causing a carbon output from the soil.

Accordingly, soil carbon sequestration is generally positively correlated with higher precipitation rates and negatively correlated with higher temperature.

For example, in colder and wetter climates (e.g. northern latitudes), rate of carbon sequestration is generally the highest (approximately 1000kg carbon/ha annually ) whereas arid and hot climates have the lowest values for sequestration rates (zero or negative values ). In tropical zones, due to high precipitation and high temperature levels, the rate of carbon sequestration is intermediate. Whereas in temperate climates; the rate is likely to vary seasonally due to the seasonal fluctuations of temperature and precipitation.

Additionally, even though the net carbon sequestration is generally the lowest in arid and semi-arid climates, soil inorganic carbon sequestration is more likely to occur in these climates (i.e. stored in the form of secondary carbonates).

Soil physical characteristics
Soil texture, structure (i.e. aggregates and aggregate stability) and organic matter content are highly related to the process of soil organic carbon stabilization within the soil matrix. The more stable the soil organic carbon is; the longer time it will stay within the soil; without being decomposed by microorganisms or without being leached out or washed away with runoff/erosion.

If the soil texture is with relatively high amount of clay (%); it is more likely to form bonds with organic matter particles (i.e. chemical stabilization ). On the other hand, a sandy and/or single grained structure which would have a relatively lower water and nutrient holding capacity; would stabilize soil organic matter to a much lesser degree than finer textured soils. Clay and organic matter percentages are positively correlated with soil aggregation; due to their higher specific surface areas and surface charges. Aggregate formation provides a physical stabilization by holding the soil particles and soil organic matter together. Aggregates might also limit their contact with soil organisms by trapping the soil organic matter within the soil macropores. A soil with a strong structure would also limit soil carbon outputs by erosion and/or sediment transport.

Studies found that the soil organic carbon content has a negative relationship with carbon sequestration efficiency. The rate of sequestration is expected to decrease when approaching to the saturation level; which varies according to the soil's carbon stabilization capacity. Thus, for example, rehabilitating degraded lands with very low organic matter content, would sequestrate carbon at a relatively higher rate.

Quality of organic matter additions
The carbon content of organic matter added to soils, cycles at different rates, with turnover times ranging from months to hundreds of years depending on the quality of the organic matter.

If the turnover rates are high (days to years) (e.g. fresh leaves), then the soil carbon sequestration efficiency would expected to be low; the carbon added would rapidly be lost to the atmosphere through decomposition. However, if turnover rates are lower (decades to millenia) (e.g. compost) then the efficiency of soil carbons sequestration would be much higher.

For example, simple carbon compounds such as sugars and starches decompose quickly an would not stay in the soil for a long time, whereas other more complex and larger C compounds such as cellulose and lignin are more likely to stored within the soil on a long-term basis since they would be degraded slowly. Relative proportions of the simple/complex organic compound types and their amount within an organic matter (i.e. plant litter) would affect the carbon sequestration.

Soil organisms
Soil organisms play crucial roles in soil carbon sequestration. Their roles include:


 * Shredding, mixing and burrowing soil organic matter (mainly by larger organisms such as earthworms). Incorporated organic matter into the deeper layers of the soil generally has longer residence time.
 * Promoting aggregation through gluing effect of their secretions as well as fungal hyphea and through formation of fecal pellets (i.e. casting). Their casting are a more stabile form of organic matter than their feed.
 * Formation of humic substances (complex molecule structures) through decomposition process, which have higher mean residence time; higher recalcitrance (i.e. biochemical stabilization ).

The rate and the extent of these roles are highly related to the abundance and the community structure of soil organisms. As different groups of soil organisms (i.e. meso, macro and microfauna) perform various functions, it is important for the soils to have high biodiversity.

Land management practices
Management practices have a direct impact on the rate and the amount of soil carbon sequestration; for managed ecosystems (i.e. croplands, rangelands, forest lands and urban areas). Research indicates that cultivated soils have lost 25 to 50% of their soil organic carbon. As the majority of the soil organic matter is found in the top layers in the soil profile, it is vulnerable to external disturbances.

Accordingly, land management practices promoting soil carbon sequestration might include the following objectives:


 * Decreasing decomposition rates: Practices that increase soil aeration (e.g. tillage) and/or soil temperature (e.g. summer fallow) would enhance microbial activity. Thus, practices promoting covering soils (e.g. cover crops, mulching), with less soil disturbance (e.g. conservation tillage) would decrease C emission to the atmosphere.
 * Adding enough organic matter to the soil: Crop and/or timber harvesting reduces organic carbon input substantially. Accordingly, organic matter should be added to the soil in the form of crop residues; mulch; compost etc. for balancing/increasing the soil organic carbon. It is common to use inorganic/chemical fertilizers for plant nutrition, which don't add any carbon to the soil.
 * Promoting soil biodiversity: Any external disturbance to the soil might negatively affect soil organisms' population and/or community structure (e.g. overgrazing, tillage, inorganic fertilizers, pesticides...). Practices with less soil disturbance and more organic matter addition would promote soil biota.
 * Enhancing soil structure/aggregate formation: For limiting erosion and increasing soil organic matter stabilization.
 * Water management: For reducing the amount of runoff and discharge of dissolved carbon and/or transport of sediments.

Mitigating climate change
Soils represent the largest terrestrial carbon pool (i.e. reservoir) and play an important role within the global carbon cycle. The carbon cycle consists of continuous carbon fluxes between different carbon pools (i.e. reservoirs), with a large range of spatial and temporal scales. This cycle has been highly manipulating by humans mainly by burning fossil fuels, land-use changes (e.g. deforestation, urbanisation) and poor land-use practices. Through these activities, carbon fluxes to the atmosphere in the form of carbon dioxide and methane (which are both greenhouse gases) have increased.

Even though the terrestrial carbon pool is smaller than the ocean and the Earth's crust (i.e. fossil fuels); there is a stronger interaction between terrestrial ecosystems and the atmospheric pool, mainly through respiration and photosynthesis. Accordingly, soil carbon sequestration is an efficient method for capturing and storing atmospheric carbon; since it's relatively cheaper among carbon sequestration techniques, responsive to the integrated management practices, applicable to everywhere and by everyone, and it's also beneficial for soil quality.

Additionally, since organic matter amendments would increase plant nutrient availability, the use of synthetic fertilizers would be expected to decrease. Synthetic fertilizers (i.e. inorganic or chemical fertilizers) are the major source of nitrous oxide emission; which is 300 times stronger form of greenhouse gas than carbon dioxide.

Improving soil quality
Soils have many critical functions including; supporting plant growth, cycling nutrients, regulating water flow and sequestrating atmospheric carbon. Soil organic matter content (i.e. organic carbon %) is positively correlated with the soil quality; since it improves soil physical, chemical and biological characteristics. An increase in soil carbon (i.e. through soil carbon sequestration) would increase soil water and nutrient holding capacity; promote soil life and biodiversity, increase nutrient cycling and nutrient availability, filtering/immobilizing pollutants and enhance soil structure by promoting aggregation. All these functions would lead an increased soil fertility/crop yield; various environmental benefits (e.g. increased water quality, decreased erosion rates) and restoration of degraded ecosystems. Considering the increasing population, food/energy demand and environmental issues throughout the world; the ecosystem services that soils might provide have an important place in the current situation that is faced.