Interactions between biochar and clay minerals in changing biochar carbon stability

The science-informed design of stable carbonaceous materials as ‘green’ soil amendment will be indispensable for improving the soil fertility and carbon sequestration ability. In this study, a series of biochars were prepared from mineral-rich cellulosic corn straw (C), lignocellulosic pine wood (P), and lignin-rich walnut shell (W) at 500 or 700 °C. Their change of carbon stability after 90-day interaction with two typical soil clay minerals (i.e., kaolinite or montmorillonite) under a field-relevant condition (mass ratio of biochar to soil clay at 1:5) was evaluated as carbon loss (%) determined by the K₂CrO₇-H₂SO₄ oxidation method. The spectroscopic analyses demonstrated that the highly graphitized and microporous W-biochars exhibited a high carbon stability (35.6–40.2% C loss) that could be further enhanced in the presence of kaolinite or montmorillonite. This promotion was probably ascribed to the transformation from the aromatic C–C/C[dbnd]C functionality to the ester C–O and methyl C–H configurations on the biochar surface forming stable organo-mineral complexes (i.e., C–O–Al) with the clay minerals. In contrast, a substantial level of labile C fraction was observed in the C- and P-biochars (e.g., 94.8% C loss of P700-M) after incubation with the clay minerals, especially for montmorillonite with high CEC, swelling capacity, and week interlayer bonding. This adverse impact was possibly attributed to the aliphatic C–C/C[dbnd]C bonding with low oxidation resistance after co-precipitation with the clay minerals. The results of this study can provide deeper insight into the evolution of physicochemical properties, porous structure, and carbon interactions during long-term biochar application for carbon sequestration and sustainable development.