Explaining Random Reflectance

The recent paper by Carbon Direct has sparked discussions over the use of Random Reflectance as a measurement for determining the permanence of biochar for carbon crediting. 

Isometric currently includes Random Reflectance as an optional basis for crediting under the Biochar Production and Storage Protocol. It is also included in the European Union’s draft biochar methodology under the Carbon Removal and Carbon Farming Regulation. 

The Random Reflectance method—for quantifying the fraction of biochar that is stable over a geological timeframe (>1,000 years)—was incorporated into Isometric’s protocol following extensive scientific review and engagement with leading independent experts in biochar characterization and soil science.

In light of the recent discussion, we’d like to share our perspective on the use of Random Reflectance.

The key point is whether the Random Reflectance method has been sufficiently validated as a measure of biochar permanence to be considered conservative, particularly relating to storage in soils. 

Soils are highly diverse, with varying biological, chemical, and physical properties. Similarly, biochar characteristics—and its carbon stability—depend on the feedstock used and the pyrolysis conditions under which it is produced¹. As a general rule, biochar produced at higher temperatures is more stable due to increased aromatic condensation, which results in larger, more stable carbon structures^{2, 3}. 

Upon production, all biochar consists of a reactive organic carbon pool—the more easily degradable compounds—and a non-reactive fraction, with varying proportions. The scientific consensus is clear: the non-reactive fraction remains stable for centuries to millennia^{6, 7}. One of the most-cited and tangible examples of this is Terra Preta—soil enriched with biochar in the Amazon Basin, which has been radiocarbon-dated to up to 7,000 years⁸. This demonstrates the remarkable durability of biochar, even in some of the most extreme tropical soil environments that are known for their rapid breakdown and cycling of nutrients and organic matter. 

Permanence in carbon crediting

Understanding and predicting the stability of the non-reactive fraction of biochar is crucial to accurately determining its durability. Currently, there are two primary approaches to determining biochar durability in carbon crediting: the more widely used hydrogen-to-organic carbon ratio (H:C) and Random Reflectance. 

The current paradigm

The H:C ratio within biochar has long been established as a proxy for its chemical stability⁹. To date, biochar’s permanence has largely been estimated with the model set out in Woolf et al (2021)¹⁰, which uses H:C ratio and soil temperature at the storage location to estimate the fraction of biochar that remains stable over a given time period. 

When crediting biochar in soils with a durability of 200 years, Isometric applies this model conservatively—using values one standard deviation below the mean when calculating the durable fraction of the biochar to account for potential uncertainty. This is an appropriate application of the method based on published empirical scientific evidence. 

Random Reflectance as a basis for crediting

The most geologically stable forms of organic carbon in the natural environment are inertinites—a group of materials that remain unaltered when heated in the absence of oxygen. Methods for characterizing inertinites are well established in petrography, including Random Reflectance. 

Random Reflectance involves measuring reflectance—the proportion of light that bounces off a surface when illuminated—on specific particles within a material using a controlled process, to understand its structure. Sanei et al. (2024)¹¹ established a benchmark of 2% for inertinite biochar—meaning more than 2% of the light is reflected—identifying this as the threshold at which organic carbon reaches its most stable form.

Therefore, only material above the 2% threshold, resembling inertinite, is credited under Isometric’s 1,000-year crediting pathway. Isometric also requires multiple samples to be analyzed and accounts for variability by only crediting the amount of biochar equivalent to one standard deviation below the mean measurement.

The Random Reflectance method bases durability claims on direct measurement of the fraction of the biochar that resembles geologically inert organic carbon, providing a more accurate structural and chemical measure of its durability compared to H:C ratio.

This method has been rigorously evaluated in peer-reviewed scientific publications^12,13,14, and an expanding body of research demonstrates its reliability as a proxy for biochar stability and durability.

While some have suggested waiting for additional field experiments to validate these findings, doing so would require hundreds of years—time we do not have. However, alongside Terra Preta and other global analogues in Europe¹⁴, Australia¹⁵, and Africa¹⁶, there are numerous examples of char that persists for millennia in soils, including that produced by wildfires^{17, 18, 19}. It is clear that these chemical structures persist for millennia in environments similar to those where biochar is deployed.

We agree that more evidence comparing the different durability frameworks is important to further increase confidence in this method. To support this, Isometric requires suppliers using Random Reflectance to also measure the H:C ratio, allowing for cross-checking between the two methods. This will build an evidence base to inform the industry’s understanding of how the two frameworks interrelate. 

In addition to those cross-checks, we are actively engaging with the scientific community to explore other methods for assessing the durability of biochar in soil and other environments. These include methods such as Fourier Transform Infrared (FTIR) spectroscopy²⁰ and micro-Raman spectroscopy²¹ and Hydropyrolysis (HyPy)²². We believe that comparing a variety of methods and building a robust understanding of how they interrelate is the best way to comprehensively understand biochar’s durability and reduce reliance on direct testing over time. 

In the meantime, we remain confident that Random Reflectance can be applied conservatively to issue high-quality carbon removal credits. 

References:

1. Mašek, O., Brownsort, P., Cross, A., & Sohi, S. (2013). Influence of production conditions on the yield and environmental stability of biochar. Fuel, 103, 151-155. https://doi.org/10.1016/j.fuel.2011.08.044 
2. Wijitkosum, S., & Sriburi, T. (2023). Aromaticity, polarity, and longevity of biochar derived from disposable bamboo chopsticks waste for environmental application. Heliyon, 9(9), e19831. https://doi.org/10.1016/j.heliyon.2023.e19831 
3. Liu, Z., Niu, W., Chu, H., Zhou, T., & Niu, Z. (2018). Effect of the carbonization temperature on the properties of biochar produced from the pyrolysis of crop residues. BioResources, 13(2), 3429-3446.
4. Rombolà, A. G., Greggio, N., Fabbri, D., Facchin, A., Torri, C., Pulcher, R., Carlini, C., Balugani, E., Marazza, D., Zannoni, D., & Buscaroli, A. (2023). Changes of labile, stable and water-soluble fractions of biochar after two years in a vineyard soil. Environmental Science: Advances, 2, 1587-1599. https://doi.org/10.1039/D3VA00197K 
5. Jones, D. L., Murphy, D. V., Khalid, M., Ahmad, W., Edwards-Jones, G., & DeLuca, T. H. (2011). Short-term biochar-induced increase in soil CO2 release is both biotically and abiotically mediated. Soil Biology and Biochemistry, 43(8), 1723-1731. https://doi.org/10.1016/j.soilbio.2011.04.018
6. Kuzyakov, Y., Bogomolova, I., & Glaser, B. (2014). Biochar stability in soil: Decomposition during eight years and transformation as assessed by compound-specific 14C analysis. Soil Biology and Biochemistry, 70, 229-236. https://doi.org/10.1016/j.soilbio.2013.12.021 
7. Ernest, B., Yanda, P. Z., Hansson, A., & Fridahl, M. (2024). Long-term effects of adding biochar to soils on organic matter content, persistent carbon storage, and moisture content in Karagwe, Tanzania. Scientific Reports, 14, 30565.
8. Neves, E. G., Bartone, R. N., Petersen, J. B., & Heckenberger, M. J. (2001). The timing of Terra Preta formation in the central Amazon: New data from three sites in the central Amazon (p. 10). 
9. Leng, L., Huang, H., Li, H., Li, J., & Zhou, W. (2019). Biochar stability assessment methods: A review. Science of The Total Environment, 647, 210-222. https://doi.org/10.1016/j.scitotenv.2018.07.402
10. Woolf, D., Lehmann, J., Ogle, S., Kishimoto-Mo, A. W., McConkey, B., & Baldock, J. (2021). Greenhouse gas inventory model for biochar additions to soil. Environmental Science & Technology, 55(21).
11. Sanei, H., Rudra, A., Przyswitt, Z. M. M., Kousted, S., Sindlev, M. B., Zheng, X., Nielsen, S. B., & Petersen, H. I. (2024). Assessing biochar's permanence: An inertinite benchmark. International Journal of Coal Geology, 281, 104409. https://doi.org/10.1016/j.coal.2023.104409 
12. Rudra, A., Petersen, H. I., & Sanei, H. (2024). Molecular characterization of biochar and the relation to carbon permanence. International Journal of Coal Geology, 291, 104565. https://doi.org/10.1016/j.coal.2024.104565
13. Chiaramonti, D., Lotti, G., Vaccari, F. P., & Sanei, H. (2024). Assessment of long-lived carbon permanence in agricultural soil: Unearthing 15 years-old biochar from long-term field experiment in vineyard. Biomass and Bioenergy, 191, 107484. https://doi.org/10.1016/j.biombioe.2024.107484 
14. Wiedner, K., Schneeweiß, J., Dippold, M. A., & Glaser, B. (2015). Anthropogenic Dark Earth in Northern Germany — The Nordic Analogue to terra preta de Índio in Amazonia. CATENA, 132, 114-125. https://doi.org/10.1016/j.catena.2014.10.024
15. Downie, A. E., Van Zwieten, L., Smernik, R. J., Morris, S., & Munroe, P. R. (2011). Terra Preta Australis: Reassessing the carbon storage capacity of temperate soils. Agriculture, Ecosystems & Environment, 140(1-2), 137-147. https://doi.org/10.1016/j.agee.2010.11.020 
16. Solomon, D., Lehmann, J., Fraser, J. A., Leach, M., Amanor, K., Frausin, V., Kristiansen, S. M., Millimouno, D., & Fairhead, J. (2016). Indigenous African soil enrichment as a climate-smart sustainable agriculture alternative. Frontiers in Ecology and the Environment, 14(2), 71-76. https://doi.org/10.1002/fee.1226
17. Singh, N., Abiven, S., Torn, M. S., & Schmidt, M. W. I. (2012). Fire-derived organic carbon in soil turns over on a centennial scale. Biogeosciences, 9, 2847-2857. https://doi.org/10.5194/bg-9-2847-2012 
18. Reisser, M., Purves, R. S., Schmidt, M. W. I., & Abiven, S. (2016). Pyrogenic carbon in soils: A literature-based inventory and a global estimation of its content in soil organic carbon and stocks. Frontiers in Earth Science, 4, 80. https://doi.org/10.3389/feart.2016.00080 
19. Schiedung, M., Ascough, P., Bellè, S.-L., Bird, M. I., Bröder, L., Haghipour, N., Hilton, R. G., Lattaud, J., & Abiven, S. (2024). Millennial-aged pyrogenic carbon in high-latitude mineral soils. Communications Earth & Environment, 5, 177. https://doi.org/10.1038/s43247-024-01343-5 
20. McCall, M. A., Watson, J. S., Tan, J. S. W., & Sephton, M. A. (2025). Biochar stability revealed by FTIR and machine learning. ACS Sustainable Resource Management, 2(5), 842–852. https://doi.org/10.1021/acssusresmgt.5c00104
21.  Petersen, H. I., Stokes, M. R., Hackley, P. C., Rudra, A., Zhou, Z., & Sanei, H. (2025). Micro-Raman indicates biochar has similar stability and structural features as natural fusinite and semifusinite. International Journal of Coal Geology, 304, 104769. https://doi.org/10.1016/j.coal.2025.104769 
22. Meredith, W., Ascough, P. L., Bird, M. I., Large, D. J., Snape, C. E., Sun, Y., & Tilston, E. L. (2012). Assessment of hydropyrolysis as a method for the quantification of black carbon using standard reference materials. Geochimica et Cosmochimica Acta, 97, 131–147. https://doi.org/10.1016/j.gca.2012.08.037