New science says biochar carbon sink has duration over 1000 years
A paper published in December 2022 has a few very interesting things to say about the persistence of the carbon sink created when biochar is sequestered. A large share of the biochar carbon sink persists over 1000 years. We at Reverse Carbon think this may be a game changer as so how biochar is regarded as a CDR method. This may potentially boost demand for CRCs from biochar project activities on voluntary carbon markets.
What the paper[1] says is that biochar contains two distinct carbon pools, polycyclic aromatic carbon (PAC) and semi-persistent carbon (SPC), with different degrees of permanence and therefore has two different carbon sink values. The climate service obtained from the stable fraction of biochar (PAC) can be considered to have a similar risk of reversal as DACCS, BECCS, and and Enhanced Weathering.
Biochar produced at pyrolysis temperatures above 550°C and presenting a molar H:C ratio below 0.4 holds a 75 percent fraction PAC, which will persist after soil application for more than 1000 years independent of the soil type and climate. The remaining 25% of the biochar carbon may be considered as SPC presenting a mean residence time in soil of 50 to 100 years depending on soil type and climate.
This is the background: Studies so far 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, however that it is the degradation of the SPC that is observed during an experiment that spans over just a few years and that projecting the degradation behaviour of the SPC pool onto the degradation curve of the entire biochar is not adequate and biases our understanding of long-term carbon dynamics. (Over a timescale of several thousands of years, parts of molecules contained in the PAC pool may eventually also be degraded, but this fact is barely contained in degradation data measured only during the first decade after biochar sequestration).
[1] Schmidt HP, Abiven S, Hageman N, Meyer zu Drewer J: Permanence of soil applied biochar. An executive summary for Global Biochar Carbon Sink certification, the Biochar Journal 2022, www.biochar-journal.org/en/ct/109, pp 69-74