Towards understanding the long-term fate of biochar in Terra Preta

Terra Preta soils, or anthropogenic dark earths (ADE), were formed beneath pre-Columbian settlements in Central Amazonia. Unlike surrounding weathered soils poor in soil organic matter, ADE soils are rich in nutrients and C, largely because of the biochar that has remained in the soil for over 2000 years. The reasons underlying this stability of particulate biochar remain unclear. This study aimed to elucidate stabilization processes by (i) characterizing the chemistry of biochar particles in ADE and (ii) comparing surface and interior chemical composition. Analyses included bulk chemical composition, benzene polycarboxylic acids as biochar markers, microstructure and surface elemental composition, functional groups, and the distribution of organic and inorganic compounds and their interactions within the particles. Bulk chemical analysis revealed relatively low total organic carbon and black carbon contents, suggesting mineral-organic interactions. Surfaces showed high oxygen content, indicating oxidation, and microscopy revealed mineral-organic coatings attached to particle surfaces. These coatings likely protect the aromatic biochar core from further degradation. Elemental depth profiles further indicated increasing C, K, and Ca contents with particle depth, suggesting mineral–biochar interactions extend into the interior. Thus, both surface coatings and internal interactions contribute to the long-term stability of ADE biochar.