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Soil organic carbon (SOC) is a vital indicator of soil health and can be negatively impacted by the removal of biomass for the use of biofuels

Summary​

Soil organic carbon (SOC) is a vital indicator of soil health and is especially relevant for sustainable agriculture and second-generation biofuels. It represents the largest terrestrial carbon pool, estimated at 2,344 gigatonnes. SOC comes from organic sources and is distinct from inorganic carbon that is found in minerals and rocks. The removal of crop residues, such as corn stover, for biofuel production can negatively impact SOC and soil stability, with removal rates needing careful management. Conversely, deep-rooted perennial energy crops like switchgrass can increase SOC, though this carbon storage is reversible and dependent on land use practices.

Soil organic carbon (SOC) is the measure of the carbon content found in soil. High levels of soil organic carbon are an indicator of healthy soil, and maintaining these levels is a vital aspect of sustainable agriculture. Soil organic carbon is especially relevant for second generation biofuels, as one of the primary feedstock sources is from agricultural residue. Additionally, due to the proposed use of energy crops as a major second generation feedstock, changes to soil organic carbon, whether positive or negative, become relevant in calculating the sustainability of the biofuel.

Soil Organic Carbon Basics​

Soil organic carbon is essentially any carbon that is found in the soil that has come from an organic source such as plants or animals. This is in contrast to soil inorganic carbon (SIC) that can still be found within the soil, but originated from an inorganic source such as minerals and rocks. Soil organic carbon is the single largest terrestrial (above ground) pool of organic carbon, estimated to be approximately 2,344 Gt (2,344,000,000,000 tonnes) of carbon. In comparison, the earth's oceans contain approximately 38,000 Gt of carbon and terrestrial biotic carbon (the carbon contained in living biomass) is estimated to be approximately 560 Gt 1. Measures for soil organic carbon include all forms of carbon found within the soil, including fresh organic material such as fallen leaves or fully decomposed matter that has been breaking down for years. Soil organic carbon is also described as the elemental carbon (C) content that is found in Soil Organic Matter (SOM), which is estimated to be made up of 58% carbon 1. Soils that have a high organic carbon content tend to be dark in color whereas soils with low organic carbon tend to appear more mineral-like or sandy. It should be noted that although extremely vital to soil health, soil organic matter (and the soil organic carbon that it contains) makes up a very small portion of the soil, generally no more than 6% by weight 2.

Crop Residues​

Crop residues are the plant materials that remain after the valuable parts of a plant have been removed. In the United States, the most significant source of crop residue comes from corn cultivation, known as corn stover. Throughout the late 2000s, when excitement for lignocellulosic second generation biofuels was at its peak, significant focus was placed on corn stover as a potential feedstock due to its immediate availability. However, the removal of crop residue can have a wide range of negative impacts on soil organic carbon, water retention, erosion resistance, and nutrient cycling 3. The majority of studies into the impact of crop residue removal have focused on corn stover, showing that removal rates as low as 25% (meaning 75% is left on the field) can have a negative impact on soil stability 3. Effects of corn stover removal on soil organic carbon depend on the soil type, with a 50% removal rate resulting in significant soil organic carbon losses in high productivity loam soils, while showing no significant changes in clay soils 3. The United States Department of Agriculture (USDA) states that as a rule, no more than 60% of agricultural residue should be removed from a field 4. However, this is highly dependent on the location, existing soil health, and form of tillage. For example, it is recommended that no residue be removed from fields that utilize conventional tillage, and up to 45% can be removed from fields that utilize no or reduced tillage 5.

Energy Crops​

Energy crops are plants that are grown with the primary purpose of producing energy, whether that be for biofuels or other bioenergy applications such as direct combustion. One interesting trait of some types of energy crops such as deep-rooted perennial grasses is their ability to improve or rehabilitate soil health by increasing soil organic carbon. This rehabilitation is achieved due to the deep root systems that essentially bring matter that was created via photosynthesis and respiration of CO₂ (carbon and other nutrients) deeper into the soil where it can be stored or broken down through microbial decomposition. A large number of studies have definitively shown that deep-rooted perennial grasses such as switchgrass and miscanthus increase soil organic carbon concentrations 4 6 7. However, it should be noted that the form in which this carbon is stored is not considered permanent and is dependent on how the soil and land are managed and used. This is because the form of soil organic carbon that is stored is susceptible to decomposition where it can return to the atmosphere as CO₂, meaning that maintaining the land usage as a perennial crop may be required to lock in the benefits 6. It is also acknowledged that there is still very little research into the long-term effects of cultivation histories and soil properties over time in relation to energy crop soil carbon fixing 6.

Sources​

Footnotes​

  1. Stockmann, U., Adams, M. A., Crawford, J. W., Field, D. J., Henakaarchchi, N., Jenkins, M., Minasny, B., McBratney, A. B., de Remy de Courcelles, V., Singh, K., Wheeler, I., Abbott, L., Angers, D. A., Baldock, J., Bird, M., Brookes, P. C., Chen, J., Jastrow, J. D., Lal, R., Lehmann, J., O'Donnell, A. G., Parton, W. J., Whitehead, D., & Zimmermann, M. (2013). The knowns, known unknowns and unknowns of sequestration of soil organic carbon. Agriculture, Ecosystems & Environment, 164, 80–99. https://doi.org/10.1016/j.agee.2012.10.001 ↩ ↩2

  2. Troeh, F. R., & Thompson, L. M. (2005). Soils and soil fertility (Vol. 489). Iowa: Blackwell. ↩

  3. Blanco-Canqui, H. (2010). Energy crops and their implications on soil and environment. Agronomy Journal, 102(2), 403–419. https://doi.org/10.2134/agronj2009.0333 ↩ ↩2 ↩3

  4. Langholtz, M. H. (Lead). (2024). 2023 Billion-Ton report: An assessment of U.S. renewable carbon resources (ORNL/SPR-2024/3103). Oak Ridge National Laboratory. https://doi.org/10.23720/BT2023/2316165 ↩ ↩2

  5. Downing, M., Eaton, L. M., Graham, R. L., Langholtz, M. H., Perlack, R. D., Turhollow, A. F., ... & Brandt, C. C. (2011). U.S. billion-ton update: Biomass supply for a bioenergy and bioproducts industry (No. ORNL/TM-2011/224). Oak Ridge National Laboratory. ↩

  6. Min, K., Nuccio, E., Slessarev, E., Kan, M., McFarlane, K. J., Oerter, E., Jurusik, A., Sanford, G., Thelen, K. D., Pett-Ridge, J., & Berhe, A. A. (2025). Deep-rooted perennials alter microbial respiration and chemical composition of carbon in density fractions along soil depth profiles. Geoderma, 455, 117202. https://doi.org/10.1016/j.geoderma.2025.117202 ↩ ↩2 ↩3

  7. Sands, R. D., Malcolm, S. A., Suttles, S. A., & Marshall, E. (2017). Dedicated energy crops and competition for agricultural land (Economic Research Report No. 223). United States Department of Agriculture, Economic Research Service. https://doi.org/10.22004/ag.econ.252445 ↩