Measuring what matters. How much carbon can soil sequester?

In the first article of this series, Hamish Macdonald , Managing Director of Carbon Asset Solutions , explained the different pathways in which plants catch and release or retain carbon. One of those pathways led to soil carbon sequestration for the long-term. In this article Hamish dives into how much carbon can be sequestered and how to measure this credibly.??

Key Take Aways

• Traditionally, the percentage of soil organic matter is determined by an ignition test.
• This test does not differentiate between decomposing crop residues, Glomalin and humus, despite their different rate of sequestering and releasing carbon.
• There are more accurate soil carbon tests, such as Mobile Inelastic Neutron Scattering (MINS).
• Agricultural practice changes, such as reduced tillage and increased and diversified crop covers, can increase the level of humus and hence carbon.

#1 What to measure??

Due to the different pathways with which plants catch and release or retain carbon, the amount of carbon respired or sequestered varies for humus, organic matter and soil organic matter. Soil organic matter contains the wide spectrum from non-degraded and unstable organic matters to the degraded and most stable humic substances [1].

A soil ignition test to measure soil carbon is inaccurate.

Traditionally, the percentage of soil organic matter is determined by an ignition test in the lab where the oven-dried soil sample is weighed before and after it is heated to burn off all organic matter. The difference in weight, and hence the combusted fraction of the soil, is the total organic soil matter. This test does not distinguish between decomposing crop residues, and Glomalin and humus: they will all burn off and be counted equally despite their different ability to release or retain carbon. E.g. the organic matter of crop residue will be digested by microbes whereby its carbon content is respired into the atmosphere as CO2; this carbon won’t be sequestered. Moreover, the amount of decomposing crop residues is more variable or unstable than the steadier state content of humus. Therefore, a soil ignition test to measure soil carbon is inaccurate. In contrast, new technological advancement measures soil organic carbon at the atomic level by using Mobile Inelastic Neutron Scattering (MINS) technology) [2].??

#2 From humus to carbon?

In ‘catch and release’, you could read how humus is the part of the soil that sequesters carbon for the long-term. Humus constitutes 1 to 10% of the soil mass [3] depending on various variables, such as climate, humidity, temperature and disturbance [4]. Overall, this humus contains 60% carbon by weight [5].??

While soil analysis laboratories measure in % of carbon in soil, carbon accounting measures in tons of carbon per (hect)acre. Therefore, we must transfer the percentage of carbon into tons. On an average agricultural soil (compared over 12 soil types), the volume of 30cm deep soil over an acre is considered to weight 4,481,494 kg or ca. 4,500 tons. For example, in a ploughed farm soil in a more humid area, the carbon content could be 2%. Hence, the carbon content in this layer of soil would be 2% of 4,500 tons, or 90 tons.

While this is very optimistic, we use at Carbon Asset Solution the more conservative estimation of 0.08% projected carbon sequestration in a soil. However, specific cases, such as the Eden Pond farm at Kyogle (NSW, Australia) we worked together with, has shown that high sequestration rates are possible: they sequestered 14.8 tons of CO2eq per hectare in 12 months.??

#3 From carbon to carbon dioxide?

However, carbon in the soil does not equal carbon dioxide in the atmosphere: soil sequesters the carbon atom, not the carbon dioxide gas. However, based on the weight of the sequestered carbon atoms, we can calculate the removed carbon dioxide [6]. A CO2 molecule has a total atomic weight of 44g/mol, of which carbon weighs 12 g/mol and oxygen 16 g/mol. As such, the ratio between CO2-gas and one carbon atom is 44 units CO2/12 units C, or 3.6666 or ca 3.7. Based on the measured carbon in the soil, we can now measure the tons of sequestered CO2: for every ton of soil carbon that was once part of CO2, 3.7 tons of that greenhouse gas was removed from the atmosphere. For example, if 3 tons of carbon per acre are sequestered, 11 tons of CO2 were removed from the atmosphere (3.7 times 3) per acre in that time interval.??

?Facts & Figures?

While the rate of humus accumulation, and subsequent carbon sequestration, varies per farm, climate region, farmer practice (especially application of no-till and cover crops), specific weather circumstances (moisture, temperature, …), our experience taught that:?

As farms see an increase in humus and soil carbon, they will generally see an increase in the fertility and productivity of their soils.

• Under decent no-till with cover crop management, with decent weather and precipitation, an agricultural soil of decent fertility should see an increase of 0.5% humus per year over six to ten years, and plateau at 4-5%.? This will represent a median of about 3 tons of elemental carbon per 11 tons of CO2 removed from the atmosphere?per acre per year until plateau in six to ten years in very good conditions.??

• Poor soils with poorer application of no-till with cover crops will average 30% lower rates of carbon accumulation than the above. Richer soils in best climates with skilled application of no-till, cover crops and good weather will average 30% higher.??

• Most soil measurements, measure up to 30cm depth, however humus, roots and Glomalin go deeper. While technically challenging, the deeper the measurement could reach, the more tons of carbon could be measured.??

• As farms see an increase in humus and soil carbon, they will generally see an increase in the fertility and productivity of their soils, with other cost-cutting, efficiency and positive environmental changes accruing [7].?Therefore, applying regenerative agricultural practices to comply with carbon credit schemes will not only result in climate change mitigation but also improved farm resilience, crop yields etc. Measuring other micronutrients alongside carbon could help making informed decisions to improve both, leading to the importance of accurate data measurement, verification and monitoring underpinning precision agriculture.?

• This?is easier to achieve in northern climates like the corn belt of the US, Europe and the Ukraine than it is in hotter climates like Australia and the Tropics where the heat retards the retention of humus.?

Hamish Macdonald is the managing director of Carbon Asset Solutions (CAS). He is an accomplished executive with 25 years of experience in public and private company directorships. He has been instrumental in taking companies to IPO in biotechnology, telecommunication, health care, and food industries.

Carbon Asset Solutions is a climate technology and data company dedicated to bringing accuracy, transparency, and integrity to the carbon market. We believe that agriculture holds the potential to mitigate climate change while supporting global food security. Our fully integrated digital Measurement, Reporting, and Verification (MRV) platform, driven by patented Mobile Inelastic Neutron Scattering (MINS) technology, provides a reliable, scalable, and trustworthy system for quantifying, verifying, and monetizing soil carbon sequestration. This innovative approach empowers farmers to play a pivotal role in climate action and offers corporations a credible, transparent way to offset their carbon footprint.

Sources

[1, 4] Berg, B, McClaugherty, C, SpringerLink & SpringerLink 2014, Plant Litter: Decomposition, Humus Formation, Carbon Sequestration 3rd ed. 2014., Springer Berlin Heidelberg, Berlin, Heidelberg. https://link-springer-com.ezproxy.usc.edu.au/book/10.1007/978-3-642-38821-7

[2] https://carbonassetsolutions.com/our-technology/

[3] Andrés, P, Doblas-Miranda, E, Rovira, P, Bonmatí, A, Ribas, à, Mattana, S, Romanyà, J 2022, Research for AGRI Committee – Agricultural potential in carbon sequestration-Humus content of land used for agriculture and CO2 storage. European Parliament, Policy Department for Structural and Cohesion Policies, Brussels. https://www.europarl.europa.eu/RegData/etudes/STUD/2022/699655/IPOL_STU(2022)699655_EN.pdf

[5] The editors of Encyclopedia Brittanica 2024, Humus. Soil Component,? https://www.britannica.com/science/humus-soil-component

[6] Environmental Protection Agency (EPA), 2024, Greenhouse Gas Equivalencies Calculator - Calculations and References, https://www.epa.gov/energy/greenhouse-gas-equivalencies-calculator-calculations-and-references

[7] Sharma, P, Sharma, P & Thakur, N 2024, ?Sustainable farming practices and soil health: a pathway to achieving SDGs and future prospects. Discov Sustain 5, 250, https://doi.org/10.1007/s43621-024-00447-4, https://link.springer.com/article/10.1007/s43621-024-00447-4#citeas