Accurately measuring soil organic carbon provides a valuable indicator for soil health and condition, supporting informed management decisions, improving productivity and sequestering carbon[1].

Measuring the carbon in a sample, is usually achieved by oxidizing the sample and measuring the carbon dioxide (CO2) released. Oxidization can be achieved through either a process of ‘wet digestion’ or ‘dry combustion’. Wet digestion makes use of chemicals, usually strong acids to ‘digest’ the carbon. Dry combustion uses controlled combustion, up to temperatures of 900c, to induce the thermal decomposition of carbon.

Dry combustion is increasingly the standard method for carbon assessment, for a number of reasons, including accuracy,  reliability and ease of use [2].  It’s for these reasons and more we us Dumas Combustion analysis, a method of dry combustion. The Dumas method is an internationally recognised method, with an associated ISO standard and a record as a reliable way to analyze soil carbon content across a wide range of soi types due to its versatility. [3]

Historically, to distinguish between organic/inorganic carbon, samples containing carbonates (such as limestone and chalk) would have required additional treatment to remove the carbonates, usually involving the application of an acid, which can impact the organic carbon. However, developments in the engineering behind dry combustion have led to temperature ramping protocols, where rather than heating a sample straight to 900c, the analyser will heat to and hold at set temperatures, typically 400c ,600c ,900c. At each step a different form of carbon burns off allowing for sample’s carbon to be fractionalized and better understood.[4]

Loss on Ignition (LOI) is sometimes offered as a measure of carbon, however it is more aptly described as test for organic matter, which can then be used to infer the organic carbon content. LOI is the process of heating a sample and measuring the weight lost, with the organic matter having converted into a gas. LOI is less reliable than Dumas, the methods are poorly standardized between labs, the test is reliant on a well implemented acid pre-treatment, the results must be adjusted using a clay content specific factor and combustible non-carbon element need to be accounted.[5]

With the growing focus measuring carbon, a number of new techniques are being developed and refined to measure carbon. One such technique is the near-Infrared scanning of a sample, which is then processed into a result based on a database of previously measured and scanned samples. Mechanically, this is similar to how we visually interpret darker soils as healthier and more carbon rich. Usually these systems are being built of the results of reliable dry combustion methods.
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References:
[1] Donovan, P. (2013) ‘Measuring Soil Carbon Change’. Soil Carbon Coalition. 
[2] Ramamoorthi, V. and Meena, S. (2018) ‘Quantification of soil organic carbon - comparison of wet oxidation and dry combustion methods’, International Journal of Current Microbiology and Applied Sciences, 7(10), pp. 146–154. doi:10.20546/ijcmas.2018.710.016.
[3] ISO 10694:1995 Soil quality — Determination of organic and total carbon after dry combustion (elementary analysis)
[4] BS EN 17505:2023 - Soil and waste characterization. Temperature dependent differentiation of total carbon (TOC400, ROC, TIC900)
[5] GREWAL, K.S., BUCHAN, G.D. and SHERLOCK, R.R. (1991), A comparison of three methods of organic carbon determination in some New Zealand soils. Journal of Soil Science, 42: 251-257. https://doi.org/10.1111/j.1365-2389.1991.tb00406.x

The soil microbial community (SMC) is an important factor in determining how much carbon is stored in the soil through soil carbon sequestration (the process in which CO2 is removed from the atmosphere and stored in the soil carbon pool) [1]. Soil microbial communities, also, play several important ecological and physiological functions such as soil organic matter decomposition, regulation of mineral nutrient availability and formation of mycorrhiza to stimulate plant growth [2]. Therefore, in general, the greater the soil microbial biomass, the greater the fertility of the landscape.

Fungi and bacteria are the two dominant groups of soil microbial communities worldwide. These organisms directly regulate the cycling of carbon between the soil and the atmosphere [3]. Fungi and bacteria differ in their responses to changes in agricultural management practices; with fungi usually being more sensitive to these changes. A change in SMC structure or reduction in SMC biomass and therefore nutrients available in the soil, has the potential to reduce the fertility and productivity of it. The fungal population will generally increase at a greater rate than that of bacteria, leading to a higher fungal-to-bacterial ratio in the soil. However, the use of fungicides can negatively affect the beneficial activities of the non-targeted organisms in the soil and change the composition of the SMC, altering the structure and function of soil microbial communities [4]. The fungal-to-bacterial ratio is therefore a good indicator of environmental changes in the soil and can have a significant impact on the structure and activity of the community [1].

The gradient effects of carbon and nitrogen in the soil can also cause significant effects upon the microbial community structure [5]. Previous studies have shown that soil depth is a fundamental environmental factor shaping the soil microbiome. The higher the soil compaction in the deeper layers reduces oxygen availability, therefore limiting the growth of many microbial taxa [6].

​Differences in the distribution and location of the soil microbial community can affect the turnover of soil carbon through its interaction with vegetation and various soil properties. Spatial variability arises because bacterial communities exist in distinct and different microenvironments within the architecture of the soil which directly affects their activity and therefore their ability to sequester carbon into the soil [7].

Citations:
[1] Bhattacharyya, S.S. et al. (2022) ‘Soil carbon sequestration – an interplay between soil microbial community and Soil Organic Matter Dynamics’, Science of The Total Environment, 815, p. 152928. doi:10.1016/j.scitotenv.2022.152928. 
[2] Sofo, A. et al. (2014) ‘Sustainable soil management in Olive Orchards’, Emerging Technologies and Management of Crop Stress Tolerance, pp. 471–483. doi:10.1016/b978-0-12-800875-1.00020-x. 
[3] Yu, K. et al. (2022) ‘The biogeography of relative abundance of soil fungi versus bacteria in surface topsoil’, Earth System Science Data, 14(9), pp. 4339–4350. doi:10.5194/essd-14-4339-2022. 
[4] Wainwright, M. (1977) ‘Effects of fungicides on the microbiology and biochemistry of soils — a review’, Zeitschrift für Pflanzenernährung und Bodenkunde, 140(5), pp. 587–603. doi:10.1002/jpln.19771400512. 
[5] Liu, M. et al. (2019) ‘Microbial community structure and the relationship with soil carbon and nitrogen in an original Korean pine forest of Changbai Mountain, China’, BMC Microbiology, 19(1). doi:10.1186/s12866-019-1584-6. 
[6] Hao, J. et al. (2021) ‘The effects of soil depth on the structure of microbial communities in agricultural soils in Iowa (United States)’, Applied and Environmental Microbiology, 87(4). doi:10.1128/aem.02673-20. 
[7] Nunan, N. et al. (2003) ‘Spatial distribution of bacterial communities and their relationships with the micro-architecture of Soil’, FEMS Microbiology Ecology, 44(2), pp. 203–215. doi:10.1016/s0168-6496(03)00027-8. 
 
 

Carbon is present in the soil as organic carbon (plants, bugs, and microbes) and inorganic carbon (rocks such as chalk). Numerous soil functions and ecosystem services depend on soil organic carbon (SOC). Improvements in soil health, along with increase in availability of water and nutrients, increases soil’s resilience against extreme climate change events and lends to disease-suppressive attributes [1], [2]. Carbon bonds within soil organic matter are the main determinants of biological activity because they provide the primary energy sources for living soil organisms [3]. The biological activity that is generated from this has a major influence on the physical and chemical properties of soil. Soil aggregation and stability of soil structure increases with increasing organic carbon. In turn, these factors increase the infiltration rate and available water holding capacity of the soil [4].

Soil carbon is also a food source for micro-organisms and an important bacteria metabolite, where microbial activity plays a key role in improving soil structure. Improved soil structure increases water infiltration and increases water holding capacity of the soil. 
High levels of organic carbon help to maintain agricultural production through its role in maintaining soil health, raising fertility, and reducing erosion.

Soil organic carbon therefore represents an important indicator for soil quality, for both agricultural functions and for environmental functions; with sequestration of carbon being an imperative way to mitigate climate change by reducing atmospheric carbon dioxide. On the Farm level, the presence (and increase) of soil carbon changes the structure of soils over time; providing increased resilience to physical degradation; slowing and storing water (to the benefit of plant life during drier spells); and creating space for beneficial microbes, such as fungi and bacteria, which provide nutrients through their processing of organic matter. 

​High SOC in mineral soils, is usually associated with higher biological productivity with positive implications for both, crop yields and local wildlife habitats [5]. Therefore, measuring and monitoring SOC levels can lead to a better understanding of the farm’s soil health, environmental impact, and the health of the landscape it is a part of. Measuring SOC as a response to changes in on-site activities can give you a vital insight into several important metrics and can help you to assess whether such activities will be useful going forward.
 
Citations:
[1] Lal, R. (2016) ‘Soil Health and Carbon Management’, Food and Energy Security, 5(4), pp. 212–222. doi:10.1002/fes3.96. 
[2] Singh, B.K. et al. (2023) ‘Climate change impacts on plant pathogens, food security and paths forward’, Nature Reviews Microbiology, 21(10), pp. 640–656. doi:10.1038/s41579-023-00900-7. 
[3] Jones, D. (2010) ‘Soil quality and biofuel production. by R. Lal and B. A. Stewart. Boca Raton FL, USA: CRC Press (2010), pp. 201. ISBN 978-1-4398-0073-7.’, Experimental Agriculture, 46(4), pp. 564–564. doi:10.1017/s0014479710000463. 
[4] Reicosky, D.C. (2003) ‘Conservation Agriculture: Global Environmental Benefits of soil carbon management’, Conservation Agriculture, pp. 3–12. doi:10.1007/978-94-017-1143-2_1. 
[5] Lal, R. (2020) ‘Soil Organic Matter Content and crop yield’, Journal of Soil and Water Conservation, 75(2). doi:10.2489/jswc.75.2.27a.

Soil carbon is the solid carbon stored in global soils; existing in organic and inorganic forms and is the key component of a soil’s ‘health’, affecting the soil’s chemical, physical and biological properties [1].

Soil organic carbon (SOC) refers only to the carbon component of organic compounds and constitutes the largest carbon stock in terrestrial ecosystems and is considered a fundamental building block of life. It is introduced into soils through decay, and processing by a soil microbiome evolved to make use of all the nutrients contained within it. Soil inorganic carbon (SIC), however, consists of mineral forms of carbon, either from weathering of parent material, or from the reaction of soil minerals with atmospheric CO2 [2].

SIC is often considered more securely sequestered in soils, than SOC, which will wax and wane with the seasons, hopefully in an upward trend. However, the processes that increase SIC happen far below the surface and are dependent on factors harder to directly manage such as precipitation rates and weathering of bedrock [3]. On the other hand, for SOC, we are seeing numerous farming practices (such as cover cropping and mob grazing), being deployed to improve the levels of SOC.

Soil carbon is also a fundamental component of soil organic matter (SOM), and commonly recognized as one of the key parameters of soil quality [4], with SOC often considered an important indicator of soil health. The quantity and quality of SOC are linked to vital soil functions including nutrient mineralization, permeability to air, water infiltration, and flood control.

Citations:
[1] Rice, C.W. (2005) ‘Carbon cycle in Soils | Dynamics and management’, Encyclopedia of Soils in the Environment, pp. 164–170. doi:10.1016/b0-12-348530-4/00183-1. 
[2] Lorenz, K. and Lal, R. (2018) ‘Soil Carbon Stock’, Carbon Sequestration in Agricultural Ecosystems, pp. 39–136. doi:10.1007/978-3-319-92318-5_2. 
[3] Ferdush, J. and Paul, V. (2021) ‘A review on the possible factors influencing soil inorganic carbon under elevated CO2’, CATENA, 204, p. 105434. doi:10.1016/j.catena.2021.105434. 
[4] Blanco, J.A. (2017) ‘Managing Forest Soils for Carbon Sequestration: Insights From Modeling Forests Around the Globe’, Soil Management and Climate Change [Preprint]. doi:10.1016/b978-0-12-812128-3.00016-1. ​