Quantification of charred materials in anthropogenic soils using the Rock-Eval® method: case studies from a recent biochar amended soil in Europe and a Bolivian archaeological site.

Biochar – the by-product of biomass pyrolysis incorporated into soils - has been proposed as a negative emission technology due its resistance to biological decomposition. However, the addition of pyrolyzed material in soil is very common in archaeological sites, such as Terra Preta. Although biochar is known to persist in soils for decades to millenia, determining their residence time is still a debated issue and would require the accurate quantification of biochar once incorporated into soils. Current methods to quantify pyrolyzed materials in soils are still limited due to their complexity and/or time-consuming aspect. This study proposes a new and reliable method to quantify biochar in soil, using the Rock-Eval® thermal (RE) method. Based on continuous measurement of hydrocarbons, CO and CO₂ released during successive pyrolysis and oxidation stages, the RE method has been increasingly used in the last 20 years to quantify carbon in soils and to distinguish carbon pools owing to their thermal properties. We hypothesized that biochar presents specific thermal properties allowing to track its content under field conditions.

The method of biochar quantification was developed using biochar-soil mixtures made from six biochars and four soils presenting contrasting characteristics. Analyses were made with the RE 6 device (Vinci Technology). This method was then tested under field conditions (1) in European soils where biochar was recently amended and (2) in tropical soils at Isla del Tesoro (Bolivia), where pyrolysed material was added several millennia ago due to past human activities. C-biochar quantification in the biochar-soil mixtures is highly correlated with the amount of CO₂ released between 410 and 610°C during the oxidation stage of the RE analysis. When applied under field conditions in European soil recently amended with biochar, the good estimation of C-biochar content confirmed the possibility to extend the method of biochar quantification to field conditions. In the soil from Bolivia, this method showed similar trend of the relative enrichment of carbon from pyrolysed material along the soil profile when compared to the benzene polycarboxylic acid (BPCA) method, thereby suggesting that pyrolyzed material from archaeological sediments can also be quantified using RE.

Altogether, these results highlight the ability of RE to provide an accurate determination of biochar concentration, the cornerstone to assess the fate of pyrolyzed material in a wide range of environments.