Scientific research is showing new evidence that the carbon sink created by biochar could be staying much longer in soil than previously thought. A paper published in the Biochar Journal in 2022 named Permanence of soil applied biochar says that a large share of the biochar carbon sink persists over 1000 years.
Sited authors are Schmidt HP, Abiven S, Hagemann N, Meyer zu Drewer J. The paper states that stable carbon contains the largest percentage of biochar carbon. The finding could potentially change the durability grade of biochar and affect the carbon removal market, increasing biochar’s demand and supply.
The paper explains that biochar contains two distinct carbon pools – polycyclic aromatic carbon (PAC) and semi-persistent carbon (SPC). The polycyclic aromatic carbon is the so-called stable carbon and can persist after biochar soil application for more than 1000 years independent of the soil type and climate.
The semi-persistent carbon can stay in soil for 50 to 100 years depending on soil type and climate.
Biochar produced at pyrolysis temperatures above 550°C and presenting a molar H:C ratio below 0.4, holds a 75% fraction polycyclic aromatic carbon or stable carbon. PAC can also be considered to have a similar risk of reversal as DACCS, BECCS, and Enhanced Weathering. The remaining 25% of the biochar carbon may be considered semi-persistent carbon.
The paper also claims that studies of biochar have so far underestimated biochar’s durability. They have generally used controlled laboratory experiments to monitor degradation over a limited number of years, typically 1 to 8 years, and then mathematically extrapolated the results into the far future.
The authors of the paper argue that so far the degradation of the SPC has been observed during an experiment that spans over just a few years. The approach doesn’t adequately measure the degradation curve of the entire biochar and biases the understanding of its long-term carbon dynamics.