Soil Carbon Cycle and Laboratory Measurements of Carbon Related to Soil Health

Introduction

Courtesy: Canva

Carbon is one the most-important elements in agricultural soils. The following article takes an in-depth look at how carbon is understood, measured, and managed to benefit soil health in agricultural production.

Article Topics

• Soil Carbon Cycle
• Organic Carbon Cycle
• Inorganic Carbon Cycle
• Carbon Measurements
• Collection of Samples to Measure Soil Carbon
• Soil Carbon Stocks and Carbon Credits
• Management to Improve Carbon Indicators of Soil Health
• Summary and References

Soil Carbon Cycle

Carbon is one the most-important elements in agricultural soils. Carbon in the soil is found in forms created by life (organic) and by mineralization and dissolution of chemicals (inorganic). Carbon in soils is important for the formation of soil structure, nutrient retention and cycling, and agricultural production. Soils with a greater organic carbon concentration generally have better agricultural productivity. Increasing soil organic carbon content can improve soil structure, nutrient retention and microbial activity in the soil. Policymakers and many others are interested in the soil’s role in the environmental carbon cycle.

The element carbon is contained in the atmosphere, biosphere and geosphere. The atmosphere is the gas containing layer of earth. The biosphere is the layer of earth where organisms are living, including near-surface geologic materials and organisms living on the surface of the earth. The geosphere is the layers of minerals and rocks below the biosphere. Carbon found in the atmosphere is primarily in a gaseous form such as carbon dioxide (abbreviated as CO₂) and methane (abbreviated as CH₄). The carbon in the biosphere consists of carbon found in soil, fresh and salt waters, and living organisms within or on the surface of the biosphere. The lithosphere contains carbon that is stored in minerals, such as limestone, coal and oil. Carbon can be moved from one pool to another by organisms or chemical reactions. An example of the global carbon cycle is shown in Figure 1. Soils are at the interface between the atmosphere and lithosphere and play a large component in the biochemical carbon cycle.

Figure 1. Simplified geochemical carbon cycle. Adapted from E. Johnston and M. Price, UCAR, 2003 [CC BY-NC 4.0]

Carbon is a very important component of the soil matrix. Carbon in the soil can be living, dead or in inorganic forms in the soil. Living sources of carbon include growing plants, fungi, bacteria and other organisms that reside in the soil. Dead organic carbon sources include dead plants and other organisms and organic compounds that are contained in the soil. Organic compounds found within the soil include sugars, carbohydrates, proteins, lignin, cellulose, and fulvic and humic acids. The soil organic carbon pool is made up of three pools, which are the active pool, slow pool and recalcitrant pool. Pools are different fractions or reservoirs of carbon compounds that are contained within the biospheres or other earth layers. The active carbon pool is soil organisms and labile soil carbon, such as dead plant matter which may be broken down in a short time (months to a few years). The slow pool is organic carbon that will be broken down in over a decade’s time. Recalcitrant carbon is carbon that will remain in the soil for a long period of time and is comprised of complex carbon compounds, such as humic substances. Inorganic carbon is carbon that is precipitated in chemicals, such as calcium carbonate, or is part of the mineral parent material of the soil. The most-common soil parent material that contains carbon is limestone or dolomite. However, through biochemical processes, precipitated inorganic carbon can be formed in the soil.

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Organic Carbon Cycle

Photosynthesis occurs from plants taking carbon dioxide from the atmosphere and turning it into an organic form of carbon. An example of the soil carbon cycle is in Figure 2. Plants absorb light and use chlorophyll to transform carbon dioxide into carbohydrates, then release oxygen back into the air. Plants store the carbon that they fix in both above-ground and below-ground biomass. Both above and below ground carbon that is fixed by the plant can be involved in the microbial decomposition process in the soil. However, some bacteria, such as cyanobacteria, also are able to fix carbon from the atmosphere and convert CO₂ into sugars.

Figure 2. An example of the soil carbon cycle. Adapted from D. Naylor et al., 2020 [CC BY-NC 4.0]

Plants also need to respire to remove excess CO₂ gas through a process called respiration. Plants take in oxygen and use sugars in the plant for energy. When processing sugars for energy, CO₂ is produced. This occurs in both the above-ground and below-ground portion of the plant. Macro and microorganisms living above and below the soil surface also respire, which releases CO₂ into the atmosphere. Additionally, livestock operations also release carbon into the atmosphere as CO₂ from livestock that are respiring and the decomposition of the manures.

Mineralization /decomposition is the breakdown of organic carbon into CO₂ in the soil. Types of carbon that can be mineralized include above and below ground plant materials, fungi, bacteria, above and below ground biota, and stabilized organic carbon in the soil. Soil macro and microbiota eat plants or other carbon sources present in the soil and break it down into smaller organic carbon compounds to use carbon as a source of energy. Types of carbon that are easily used by microorganisms for food and mineralized in the soil include sugars, carbohydrates, lipids, and amino acids. Some of the carbon sources that are used by microbes are broken down and converted to CO₂, whereas others are converted into complex carbon compounds that can be transformed into the slow or recalcitrant carbon pool. During mineralization, organic carbon sources are converted to CO₂, and the gas is released into the atmosphere. However, some of the carbon compounds formed by soil biota can also be stabilized to sources of carbon that remain in soil for a long period of time in the soil (recalcitrant pool). Carbon sources that are stabilized soil organic carbon can be mineralized into CO₂ also and the CO₂ gas returns back to the atmosphere. Mineralization also occurs from decomposition of manures from livestock, which can return carbon in gaseous forms to atmosphere.

Stabilization of organic carbon present in the soil into compounds that will remain in the soil for long durations of time is what many people are interested in. This is due to the ability of the soil to remove carbon in the gaseous form from the atmosphere and convert it to a form that is stable for a long period of time in the soil. Some carbon sources that form during the microbial decomposition of carbon compounds in the soil are complex compounds, such as fulvic and humic compounds, which can remain in the soil for a long period of time. Carbon can be stabilized physically, chemically, and biologically in the soil. Physical stabilization is storage of carbon containing compounds into soil aggregates. The sources of carbon that can be stabilized into soil aggregates include chemical compounds formed from the mineralization of carbon and particulate organic matter from above and below ground organism biomass. Organic carbon also can be chemically stabilized by being absorbed to chemical compounds in the soil, such as clays. This process generally occurs with more complex carbon compounds that are formed through the mineralization of organic carbon sources. Soils with higher clay content generally can contain a higher quantity of soil organic carbon that is chemically stabilized. Soil biology processes fresh carbon sources in the soil. The carbon sources formed during microbial decomposition are then transformed into compounds that can diffuse into soil aggregates and be stabilized (Kravchenko et al., 2019). Climate, carbon sources present, and the soil microbial community present affect the rate that carbon is processed in the soil (Bhattacharyya et al., 2022). Stabilized carbon sources present in the soil can be mineralized/decomposed by soil microorganisms also. This will cause stabilized (recalcitrant pool) organic carbon to be converted to CO₂. Agricultural producers and others interested in carbon sequestration in soils want to increase carbon that is stabilized in the soil and reduce the amount of carbon released to the atmosphere as CO₂ by mineralization.

In anaerobic conditions, certain microorganisms use inorganic chemicals as an electron source instead of oxygen. The chemicals that are used by microorganisms depend upon the oxidation-reduction potential of the soil. When soil is not saturated, the redox potential is high, and oxygen is used as the electron source. As the redox potential decreases, different chemical compounds are used as an electron source. The order of chemicals being used with decreasing redox potential in the soil are Oxygen (abbreviated as O₂)> Manganese (abbreviated as Mn⁴⁺)> iron (abbreviated as Fe³⁺) > Sulfate (S⁶⁺) > carbon dioxide (C⁴⁺). During this process, these chemicals are transformed from a more-oxidized form (>positive electron charge) to a more-reduced form (more-negative electron charge). For example, during the formation of methane (methanogenesis), carbon is transformed from an electron charge of +4 (abbreviated as C⁴⁺) to an electron charge or -4 (abbreviated as C^4-). There are 8 electrons released during this process, which are used by microorganisms to break down carbon and water, and methane (abbreviated as CH₄) is created. Methanogenesis is a cycle that occurs during anaerobic conditions. The microorganisms that participate in the methane formation are archea. This process most commonly occurs in areas that are saturated with water, and oxygen is not present for microbes to use. This commonly occurs in wetlands and after rainstorms in fully saturated soils. Organisms in soils also can use methane as an energy source when it moves to a soil environment where there is oxygen present. Organisms also can release methane into the atmosphere during digestion. Ruminant animals release methane during digestion, which is commonly referred to as enteric methane emissions. Some people are concerned about methane being released from the soil and agriculture and think that it is a worse gas to have in the environment than carbon dioxide.

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Inorganic Carbon Cycle

Inorganic carbon is also contained in soil and is formed by chemical reactions and not by living organisms. Inorganic carbon is more common in semi-arid and arid soils and is most commonly in the form of calcite. However, producers commonly add inorganic carbon to soils in the form of lime to increase the soil pH, which is mostly calcium carbonate. Bicarbonate (abbreviated as HCO₃^-) and hydrogen in the form of H⁺ are formed in the soil from carbon dioxide and water. The amount of hydrogen and carbon dioxide formed depends upon the concentration of carbon dioxide present in the atmosphere/soil (Guo et al., 2016). The equations for the chemical reaction that forms carbonic acid in the water/soil and the precipitation of calcite are outlined in Equations 1 and 2.

Figure 3. An example of a chamber used to measure soil gas emissions. Courtesy: Hans Klopp

Soil gas emissions can be measured by sensors or by collecting gas emitted into a chamber over a duration of time. Sensors can be purchased that can measure specific gases emitted from soil, such as CH₄ and CO₂. Static chambers also are used, which can determine how much gas is released in a period of time in the area of the chamber. Some of these chambers can measure certain gases that are released into the chamber, such as the Licor in Figure 3. Samples of a certain volume also can be taken from the chamber throughout a period of time to determine how much of a certain gas is released during a period of time. The samples are then run on a gas chromatograph. An example of a chamber used to measure soil gas emissions is shown in Figure 3.

Carbon Compounds

Some researchers also are measuring certain types of carbon compounds that are contained in soils. Some of the carbon compounds that can be measured include sugars, carbohydrates, fulvic and humic acids, amino acids, bacteria, and fungi. Different extraction techniques can be used to separate these compounds from the soil. These organisms are then identified by techniques used to identify organic chemicals or markers of certain organisms. One way that is used to identify certain organisms in the soil is fatty acid methyl ester profiles (Cavigelli et al., 1995). Another way to identify certain soil organisms is by soil DNA extraction.

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Collection of Samples to Measure Soil Carbon

Much of the organic carbon that is contained in the soil is near the soil surface. However more inorganic carbon is deeper in the soil profile. Soil carbon related properties also can vary spatially throughout the field. If measuring soil carbon stocks, you may want to sample to a deeper depth to find carbon concentration that is stored in the entire soil profile. Samples to analyze properties indicative of labile or more-active carbon sources are mostly done near the surface. Soil organic carbon is higher near the soil surface. In the darker area in the soil pit pictured in Figure 4, there is more organic carbon, whereas deeper in the soil profile where there are white spots, there is more inorganic carbon. The white appearing spots are calcium carbonate. Most of the labile carbon, such as the straw on the surface and near surface roots, is where most of the carbon that is easily used by microbes is located.

Figure 4. A soil pit near Garretson, South Dakota. Units shown on the tape are in centimeters. Courtesy: Hans Klopp.

Changes in agricultural management practices will initially cause soil properties to change near the soil surface, so if sampling to monitor changes in soil carbon properties from management, samples should be taken near surface, such as from the 0 to 6 and 6 to 12-inch depths. If sampling to determine the soil total and organic carbon stocks, samples should be taken to a deeper depth, such as 0 to 36 inches. Sampling can be done by collecting a point sample, a composite sample, or as grid sampling. If collecting a composite sample, multiple samples should be collected throughout the field of interest to a desired depth. The samples collected from a given depth range then should be homogenized together prior to the analysis of soil carbon indicators. If grid sampling, the field is set up into a grid, and samples are collected at certain points in the field. Collecting a composite sample and sending it to the lab is cheaper, but it will not account for variation in soil carbon throughout the field. Collecting a point sample may not be able to show how soil properties are changing throughout the entire field. If determining soil carbon stock, bulk density of the soil also should be collected. When collecting samples for soil carbon analysis, they should be kept cool and sent to the desired lab as soon as possible to be processed. If they cannot be sent to a lab to be processed as soon as possible they, should be air-dried prior to sending to the lab.

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Soil Carbon Stocks and Carbon Credits

Policymakers and companies are interested in the soil's ability to store organic carbon and remove carbon from the atmosphere that is in the form of CO₂ and CH₄. Soil carbon stock is the tons of carbon per-acre stored in a certain soil depth. Carbon stock can be calculated by multiplying the percentage of carbon × bulk density × soil depth × area in an acre. For example, a soil that is 1.96 % carbon in the 0-2.5 in. (0-0.164 ft.) depth and has a bulk density of 60.4 lbs. ft^-3 has 0.19 lbs. carbon ft^-2, or 8464 lbs. (4.23 tons) of total carbon per acre in the 0-2.5 in depth. An example of soil total, organic and inorganic carbon concentration versus depth and stocks of carbon at the depth ranges sampled is found in Figure 5 and Figure 6 below. This data is of a Houdek soil, which has 283 tons of total carbon, 57 tons of organic carbon, and 226 tons of inorganic carbon per-acre in the 0-78 in. depth range (Figure 6). There is more organic carbon near the soil surface, whereas inorganic carbon is highest in the 23.5-31.5 in. depth (Figure 5). Organic carbon concentration is usually higher near the soil surface, whereas inorganic carbon values vary depending upon soil parent materials and climate. In older soils and where annual precipitation is higher, there generally is little inorganic carbon near the soil surface, except when lime (CaCO₃) is added near the soil surface. In arid or semi-arid areas, carbonates generally are not leached out of the soil profile in the top 6 ft.

(Courtesy: U.S. Department of Agriculture)

Carbon is the most-essential element in the soil for maintaining soil health. As previously mentioned, some management practices that can increase soil organic carbon are planting cover crops, reducing tillage intensity, planting perennial vegetation, and retaining crop residues (Lal 2013). Also, applying organic materials, such as animal manures or compost, to the soil can increase soil organic carbon and active carbon in the soil (Li et al.2018). In relatively short durations of time, these management practices generally only increase soil organic carbon near the surface. Many of the soil carbon indicators measured in soil health are indicators of active carbon, which are affected by amount of carbon being annually returned to the soil and microbial activity occurring in the soil. To improve the soil properties that are indicators of active or labile carbon, producers should use management practices that return more carbon to the system, such as cover crops, animal manure application, diversifying the cropping system,  leaving straw or stover in field, or using practices that minimize disturbance to soil aggregates, which slows the mineralization rate of carbon in the system. The carbon-to-nitrogen ratio of the soil and materials being applied to the soil may affect the percent makeup of certain carbon compounds in the total soil carbon concentration. Management practices that may lead to a reduction in soil pH, such as applying nitrogen fertilizers or chemicals that reduce soil pH, may lead to a reduction in soil inorganic stock. However, irrigation with water high in carbonate and bicarbonate can increase soil inorganic carbon stock.

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Summary

Carbon is an essential element to life and is the most important chemical to soil health. Carbon is contained in the atmosphere, biosphere, and geosphere and can be moved to another layer by biochemical processes. Within the soil, carbon can be compromised of many types of organisms and different chemical forms. There are many different laboratory procedures available to measure types of carbon in the soil. A certain procedure can be used if you are interested in a certain pool of carbon that is contained in the soil. A composite sample should be collected only if you want to send in one sample from a field, but grid sampling should be conducted to see if there is variation in soil carbon related properties across the field. There is interest in the soils ability to store carbon in the form of carbon stocks. Carbon credits are what the cost of a ton of CO2 is worth and what companies will pay agricultural producers to conduct practices that will increase soil carbon and remove CO2 from the atmosphere. Management practices that improve indicators of soil health that are related to carbon are retaining crop residues, reducing tillage, adding animal manure, and diversifying cropping systems. Future articles will be added to SDSU Extension's soil health library discussing soil carbon and the parts of the carbon cycle mentioned in this article.

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