A new study, "Evaluating the two-pool decay model for biochar carbon permanence" by Sanei, Petersen, Chiaramonti, and Masek, examines the widely used two-pool decay model and identifies areas for improvement to better reflect biochar’s stability and degradation processes. These insights could contribute to advancing biochar’s role in climate policies and carbon markets.
The Two-Pool Woolf Model is a mathematical approach used to predict how long biochar locks away carbon in soils. The model assumes that biochar has two "pools" or phases of carbon:
- The labile carbon fraction (C1), which is more easily broken down over time.
- The recalcitrant fraction (C2), which is stable and remains in the soil for centuries, contributing to long-term carbon sequestration.
Here is a breakdown of the key findings:
- Overestimation of biochar decay rate: The first issue is that the model assigns an incorrectly low percentage to the labile carbon fraction (C1), which represents the more reactive carbon in biochar. This is in relation to the recalcitrant fraction (C2), which represents the more stable carbon. As a result, the model ends up overly focusing on the stable (C2) fraction and treats the biochar like a single-pool system, ignoring the distinction between the labile and recalcitrant carbon.
- Mismatch with biochar chemistry: The model fails to correctly account for key factors like production temperature and hydrogen-to-carbon ratios, which affect the stability of biochar’s carbon. This results in an inaccurate representation of the chemistry and carbon sequestration potential of biochar.
- The issue is with the parameterisation, not the structure: While the research acknowledges that the multi-pool decay model is a suitable framework for predicting biochar's permanence, the key problem lies in how the parameters have been set up in Woolf’s model. The model needs a more accurate and scientifically aligned parameterisation to correctly represent the carbon dynamics of biochar and to provide more reliable predictions about its ability to sequester carbon over time.
- Implications for Carbon Dioxide Removal (CDR) models: These shortcomings in the current decay model lead to a significant underestimation of biochar's carbon sequestration potential. Since biochar is thermodynamically stable and can persist in geological environments, the current model’s assumptions about its degradation do not align with observed carbon behaviour, impacting its credibility in carbon accounting systems and biochar certification programs.
The authors recommend a new multi-pool decay model, based on empirical data and accelerated aging studies, to better capture biochar's carbon persistence. Incorporating the right data will help align the model with biochar’s actual stability in different environmental conditions.
Authors: Prof. Hamed Sanei, Prof. Henrik Ingermann Petersen, Prof. David Chiaramonti, Prof. Ondrej Masek
Publication: Biochar Journal (Springer), January 8, 2025.
