Soil carbon sequestration has been noted to be a potentially powerful climate mitigation tool with the IPCC estimating that by 2030, up to 5.3 GtCO2 can be mitigated per year by soils (Lutz & Welsh, 2021). Soil organic carbon is one component of soil organic matter (which consists of litter, accumulation of dead organisms, and organic residues such as faeces or eggshells). Soil carbon sequestration occurs when plants capture or "sequester" atmospheric carbon dioxide in soils. The decaying plant matter with the carbon contains becomes integrated into soils for a long time before being broken down by microbes which release the carbon back into the atmosphere (Lutz & Welsh, 2021).
However, for the reduction in global atmospheric carbon dioxide to be long-lasting, organic matter would need to be in a more stable and resistant fraction known as "resistant organic matter" (ROM). This is a relatively inert material that contains organic carbon more resistant to further change. It forms as organic matter transforms from one form to another over time, through decomposition and cycling by microorganisms into different soil fractions. The turnover time for ROM can be 100s to 1000s of years (Government of Australia, 2021).
Figure 1. Organic matter transforms from one form to another through decomposition and nutrient cycling. 1 - inputs, 2 - transformation, 3 - nutrient release, 4 - stabilizing organic matter. Source: Government of Australia
The carbon that enters soil as organic matter and is hence sequestered in the soil depends on a variety of factors such as climate, soil composition, and structure that are in turn impacted by anthropogenic activities, including farming methods and soil pollution. For example, the accumulation of soil contaminants into plant tissues alters litter quality and leads to avoidance by decomposers, contributing to the reduction of incorporation of soil organic carbon within the soil profile. Research has also shown that a decrease in litter decomposition activity was observed in soils contaminated by radionuclides in Chernobyl after the nuclear disaster as high levels of radiation led to the accumulation of undecomposed litter (FAO, 2021).
Another compelling reason to act on soil pollution, apart from public health and environmental concerns is its role in slowing down the carbon sequestration potential of soils. However, scientists have raised their apprehension about the political, economic and physical constraints affecting the feasibility of the potential for soils as a large-scale climate mitigation tool (Amundson & Biardeau, 2018).
For example, climate change could lead to a positive feedback loop where soil carbon is converted to carbon dioxide by soil microbes due to increased temperatures. Farmers may also be more concerned about the economics i.e. increasing crop yields rather than adopting regenerative agricultural methods such as no-till farming.
Therefore, a "carbon tunnel vision" with a strong emphasis on "climate-friendly" soil farming methods is probably not the best approach. Instead, a multifaceted strategy is needed, one that includes countering soil pollution, improving food security (especially for developing nations) and water management in soil harmonized with better soil carbon management.
References
Amundson, R., & Biardeau, L. (2018). Soil carbon sequestration is an elusive climate mitigation tool. Proceedings of the National Academy of Sciences, 115(46), 11652–11656. https://doi.org/10.1073/pnas.1815901115
FAO and UNEP (2021). Global assessment of soil pollution: Report. Chapter 4: Environmental, health and socio-economic impacts of soil pollution.
Government of Australia - Department of Primary Industries and Regional Development. (n.d.). What is soil organic carbon? | Agriculture and Food. https://www.agric.wa.gov.au/measuring-and-assessing-soils/what-soil-organic-carbon
Lutz, J., & Welsh, C. (2021, August 26). Soil Carbon Sequestration: Myths, Realities, and the Biden. Centre for Strategic & International Studies. https://www.csis.org/analysis/soil-carbon-sequestration-myths-realities-and-biden-administrations-proposals