Soil carbon
Encyclopedia
Soil carbon is the generic name for carbon
held within the soil
, primarily in association with its organic
content. Soil carbon is the largest terrestrial pool of carbon. Humans have, and will likely continue to have, significant impacts on the size of this pool. Soil carbon plays a key role in the carbon cycle
and thus is important in global climate model
s.
; about 60 Gt annually becomes various types of soil organic matter including surface litter; about 60 Gt annually is respired or oxidized from soil.
Soil carbon is the last major pool of the carbon cycle
. The carbon that is fixed by plants is transferred to the soil via dead plant matter including dead root
s, leave
s and fruiting bodies. This dead organic matter creates a substrate which decomposer respire back to the atmosphere as carbon dioxide or methane depending on the availability of oxygen in the soil. Soil carbon can also be oxidized by combustion and returned to the atmosphere as carbon dioxide.
Soil carbon is primarily composed of biomass and non-biomass carbon sources. Biomass carbon primarily includes various bacteria and fungi. Non-biomass carbon sources or substrates reflect the chemical composition of plant biomass and primarily include cellulose
, starch
, lignin
and other diverse organic carbon compounds. Some of the substrate carbon will bind to the mineral soil becoming encapsulated in soil aggregates (singular masses of coherent soil particles, or peds) or chemical complexing.
The biomass feeds off of the substrate carbon compounds at different rates.
Some of the carbon compounds are easily digested and respired by the microbes resulting in a relatively short residence time. Others, like lignin
, humic acid
or substrate encapsulated in soil aggregates, are very difficult for the biomass to digest and have very long residence times.
-holding capacity of sandy soil and it contributes to the structural stability of clay soils by helping to bind particles into aggregates. Soil organic matter
, of which carbon is a major part, holds a great proportion of nutrients, cations and trace elements that are of importance to plant growth. It prevents nutrient leaching and is integral to the organic acids that make minerals available to plants. It also buffers soil from strong changes in pH
. It is widely accepted that the carbon content of soil is a major factor in its overall health.
and drainage both expose soil organic matter to oxygen and oxidation. In the Netherlands, East Anglia, Florida, and the California delta, subsidence of peat lands from oxidation has been severe as a result of tillage
and drainage.
Grazing management that exposes soil (either excessive or insufficient recovery periods) can also cause losses of soil organic carbon.
It has long been encouraged that farmers adjust practices to maintain or increase the organic component in the soil—on one hand, practices that hasten oxidation of carbon, such as burning crop stubbles or over-cultivation are discouraged; on the other hand, incorporation of organic material, such as manuring has been encouraged. Increasing soil carbon is not a straightforward matter—it is made complex by the relative activity of soil biota, which can consume and release carbon and are made more active by the addition of nitrogen fertilizer
s.
to offset climate change. Despite this emphasis, a much wider range of soil and catchment
health aspects are improved as soil carbon is increased. These benefits are difficult to quantify due to the complexity of natural resource systems and the interpretation of what constitutes soil health; nonetheless, several benefits are proposed in the following points:
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
held within the soil
Soil
Soil is a natural body consisting of layers of mineral constituents of variable thicknesses, which differ from the parent materials in their morphological, physical, chemical, and mineralogical characteristics...
, primarily in association with its organic
Organic compound
An organic compound is any member of a large class of gaseous, liquid, or solid chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of carbon-containing compounds such as carbides, carbonates, simple oxides of carbon, and cyanides, as well as the...
content. Soil carbon is the largest terrestrial pool of carbon. Humans have, and will likely continue to have, significant impacts on the size of this pool. Soil carbon plays a key role in the carbon cycle
Carbon cycle
The carbon cycle is the biogeochemical cycle by which carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and atmosphere of the Earth...
and thus is important in global climate model
Global climate model
A General Circulation Model is a mathematical model of the general circulation of a planetary atmosphere or ocean and based on the Navier–Stokes equations on a rotating sphere with thermodynamic terms for various energy sources . These equations are the basis for complex computer programs commonly...
s.
Overview
Although the figure is frequently being revised upwards with new discoveries, over 2700 Gt of carbon is stored in soils worldwide, which is well above the combined total of atmosphere (780 Gt) or biomass (575 Gt), most of which is wood. Carbon is taken out of the atmosphere by plant photosynthesisPhotosynthesis
Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...
; about 60 Gt annually becomes various types of soil organic matter including surface litter; about 60 Gt annually is respired or oxidized from soil.
Soil carbon is the last major pool of the carbon cycle
Carbon cycle
The carbon cycle is the biogeochemical cycle by which carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and atmosphere of the Earth...
. The carbon that is fixed by plants is transferred to the soil via dead plant matter including dead root
Root
In vascular plants, the root is the organ of a plant that typically lies below the surface of the soil. This is not always the case, however, since a root can also be aerial or aerating . Furthermore, a stem normally occurring below ground is not exceptional either...
s, leave
Leave
Leave may refer to:* Leave , a 2002 Mandopop album* "Leave ", a 2004 R&B song* "Leave" , a 2000 pop rock song...
s and fruiting bodies. This dead organic matter creates a substrate which decomposer respire back to the atmosphere as carbon dioxide or methane depending on the availability of oxygen in the soil. Soil carbon can also be oxidized by combustion and returned to the atmosphere as carbon dioxide.
Soil carbon is primarily composed of biomass and non-biomass carbon sources. Biomass carbon primarily includes various bacteria and fungi. Non-biomass carbon sources or substrates reflect the chemical composition of plant biomass and primarily include cellulose
Cellulose
Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to over ten thousand β linked D-glucose units....
, starch
Starch
Starch or amylum is a carbohydrate consisting of a large number of glucose units joined together by glycosidic bonds. This polysaccharide is produced by all green plants as an energy store...
, lignin
Lignin
Lignin or lignen is a complex chemical compound most commonly derived from wood, and an integral part of the secondary cell walls of plants and some algae. The term was introduced in 1819 by de Candolle and is derived from the Latin word lignum, meaning wood...
and other diverse organic carbon compounds. Some of the substrate carbon will bind to the mineral soil becoming encapsulated in soil aggregates (singular masses of coherent soil particles, or peds) or chemical complexing.
The biomass feeds off of the substrate carbon compounds at different rates.
Some of the carbon compounds are easily digested and respired by the microbes resulting in a relatively short residence time. Others, like lignin
Lignin
Lignin or lignen is a complex chemical compound most commonly derived from wood, and an integral part of the secondary cell walls of plants and some algae. The term was introduced in 1819 by de Candolle and is derived from the Latin word lignum, meaning wood...
, humic acid
Humic acid
Humic acid is a principal component of humic substances, which are the major organic constituents of soil , peat, coal, many upland streams, dystrophic lakes, and ocean water. It is produced by biodegradation of dead organic matter...
or substrate encapsulated in soil aggregates, are very difficult for the biomass to digest and have very long residence times.
Soil carbon and soil health
Soil carbon improves the physical properties of soil. It increases the cation exchange capacity (CEC) and waterWater
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
-holding capacity of sandy soil and it contributes to the structural stability of clay soils by helping to bind particles into aggregates. Soil organic matter
Soil organic matter
Organic matter is matter that has come from a once-living organism; is capable of decay, or the product of decay; or is composed of organic compounds...
, of which carbon is a major part, holds a great proportion of nutrients, cations and trace elements that are of importance to plant growth. It prevents nutrient leaching and is integral to the organic acids that make minerals available to plants. It also buffers soil from strong changes in pH
PH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...
. It is widely accepted that the carbon content of soil is a major factor in its overall health.
Losses of soil carbon
Although exact quantities cannot be documented, human activities have caused massive losses of soil organic carbon. First was the use of fire, which removes soil cover and leads to immediate and continuing losses of soil organic carbon. TillageTillage
Tillage is the agricultural preparation of the soil by mechanical agitation of various types, such as digging, stirring, and overturning. Examples of human-powered tilling methods using hand tools include shovelling, picking, mattock work, hoeing, and raking...
and drainage both expose soil organic matter to oxygen and oxidation. In the Netherlands, East Anglia, Florida, and the California delta, subsidence of peat lands from oxidation has been severe as a result of tillage
Tillage
Tillage is the agricultural preparation of the soil by mechanical agitation of various types, such as digging, stirring, and overturning. Examples of human-powered tilling methods using hand tools include shovelling, picking, mattock work, hoeing, and raking...
and drainage.
Grazing management that exposes soil (either excessive or insufficient recovery periods) can also cause losses of soil organic carbon.
Managing soil carbon
Natural variations in SOM occur as a result of climate, organisms, parent material, time and relief. The greatest contemporary influence has been that of humans; for example, historical SOM in Australian agricultural soils may have been twice the present range that is typically from 1.6 to 4.6 per cent.It has long been encouraged that farmers adjust practices to maintain or increase the organic component in the soil—on one hand, practices that hasten oxidation of carbon, such as burning crop stubbles or over-cultivation are discouraged; on the other hand, incorporation of organic material, such as manuring has been encouraged. Increasing soil carbon is not a straightforward matter—it is made complex by the relative activity of soil biota, which can consume and release carbon and are made more active by the addition of nitrogen fertilizer
Fertilizer
Fertilizer is any organic or inorganic material of natural or synthetic origin that is added to a soil to supply one or more plant nutrients essential to the growth of plants. A recent assessment found that about 40 to 60% of crop yields are attributable to commercial fertilizer use...
s.
Managing for catchment health
Much of the contemporary literature on soil carbon relates to its role, or potential, as an atmospheric carbon sinkCarbon dioxide sink
A carbon sink is a natural or artificial reservoir that accumulates and stores some carbon-containing chemical compound for an indefinite period. The process by which carbon sinks remove carbon dioxide from the atmosphere is known as carbon sequestration...
to offset climate change. Despite this emphasis, a much wider range of soil and catchment
Drainage basin
A drainage basin is an extent or an area of land where surface water from rain and melting snow or ice converges to a single point, usually the exit of the basin, where the waters join another waterbody, such as a river, lake, reservoir, estuary, wetland, sea, or ocean...
health aspects are improved as soil carbon is increased. These benefits are difficult to quantify due to the complexity of natural resource systems and the interpretation of what constitutes soil health; nonetheless, several benefits are proposed in the following points:
- Reduced erosionErosionErosion is when materials are removed from the surface and changed into something else. It only works by hydraulic actions and transport of solids in the natural environment, and leads to the deposition of these materials elsewhere...
, sedimentationSedimentationSedimentation is the tendency for particles in suspension to settle out of the fluid in which they are entrained, and come to rest against a barrier. This is due to their motion through the fluid in response to the forces acting on them: these forces can be due to gravity, centrifugal acceleration...
: increased soil aggregate stability means greater resistance to erosion; mass movement is less likely when soils are able to retain structural strength under greater moisture levels.
- Greater productivity: healthier and more productive soils can contribute to positive socio-economic circumstances.
- Cleaner waterways, nutrients and turbidityTurbidityTurbidity is the cloudiness or haziness of a fluid caused by individual particles that are generally invisible to the naked eye, similar to smoke in air. The measurement of turbidity is a key test of water quality....
: nutrients and sediment tend to be retained by the soil rather than leach or wash off, and are so kept from waterways.
- Water balance: greater soil water holding capacity reduces overland flow and recharge to groundwater; the water saved and held by the soil remains available for use by plants.
- Climate change: Soils have the ability to retain carbon that may otherwise exist as atmospheric CO2 and contribute to greenhouse warming.
- Greater biodiversityBiodiversityBiodiversity is the degree of variation of life forms within a given ecosystem, biome, or an entire planet. Biodiversity is a measure of the health of ecosystems. Biodiversity is in part a function of climate. In terrestrial habitats, tropical regions are typically rich whereas polar regions...
: soil organic matter contributes to the health of soil flora and accordingly, the natural links with biodiversity in the greater biosphere.