Black carbon enriches short-range-order ferrihydrite in Amazonian Dark Earth: Interplay mechanism and environmental implications

Our study underpins the mechanism of organo-mineral interaction between black carbon (BC, biochar) and associated minerals in the historical BC-rich Amazonian Dark Earth (ADE) by using synchrotron-based microscopic (TXM), microspectroscopic (μFTIR) and spectroscopic (XAS and μ-diffraction) approaches. The BC-rich ADE contained over 100% more poorly crystalline minerals than the adjacent tropical soil. Linear combination fitting of k-spacing in the X-ray Absorption Spectra (XAS) revealed that ferrihydrite contributed to 81.1% of the Fe-minerals in BC. A small but distinct peak was observed at 5.7 Å⁻¹ in the extended X-ray absorption fine structure k oscillation of BC, revealing the presence of Fe[sbnd]C (including Fe-O-C) covalent bonds. No Fe[sbnd]C path was yielded by the XAS fitting when an obvious peak downshift of the first (Fe[sbnd]Fe¹) shell was observed, suggesting that the availability of inner-sphere Fe[sbnd]C complexation was limited to the BC surface and interphase region. The main minerals for organo-mineral complexation were short-range-order (SRO) ferrihydrite on BC instead of corner-sharing FeO₆ octahedra. Compared to ADE, the coordination number of the first (Fe[sbnd]Fe¹) and second (Fe[sbnd]Fe²) shell was higher in BC, revealing a higher degree of order in coordination between the neighboring Fe mineral crystals. Black C limited the progressive aging of amorphous Fe phases and greatly enriched SRO ferrihydrite in the redox-fluctuating and high-leaching environment. The transformation of SRO ferrihydrite into the more crystalline Fe oxides was controlled by the local pH environment. A strong signal from the complexed phenolic group (aryl-OH, 1241 cm⁻¹) and a distinct band of inner-sphere complexation (Fe-aryl C, 1380–1384 cm⁻¹) were identified in the FTIR spectra. The enrichment of poorly crystalline minerals can have positive feedback on the long-term stabilization of BC. The scale-up application of biochar to agricultural and ecological systems may have a long-lasting impact on the enrichment and transformation of the SRO minerals in the soil.