A Fourier‐Transform Infrared Study of Biochar Aging in Soils

Core Ideas Aged biochars in different soils were analyzed using DR‐FTIR in this study. Native soil organic C, biochar C, and minerals created distinct absorption bands. Ar/Al band ratio systematically changed in soils with high‐temperature biochar. Ar/Al ratios and biochar C mineralized in the soils were linearly correlated. DR‐FTIR is a simple procedure for long‐term monitoring of high‐temperature biochars. We used diffuse reflectance Fourier‐transform infrared (DR‐FTIR) spectroscopy, X‐ray diffraction (XRD), and chemical and isotopic analyses to characterize the light fraction of four contrasting soils (control and biochar‐amended soils) to determine changes in biochar properties after aging. Two Eucalyptus saligna Sm. wood biochars, produced at 450°C (B450) and 550°C (B550), were incubated separately in each of the four soils for up to 12 mo at 20, 40, and 60°C. Total C and isotopic (δ 13 C) methods were used to quantify the amounts of biochar C and native C mineralized during incubation. The DR‐FTIR spectra of the light fraction showed distinct absorption bands representing native soil organic C, biochar C, and mineral constituents present in the soils; the mineral bands were consistent with XRD data of the clay fraction of the four soils. Analysis of the DR‐FTIR spectra in the ν(C–H) bands showed that the ratio of the aromatic ν(C–H) bands systematically increased relative to the aliphatic ν(C–H) bands with increasing mineralization of biochar C in the B550 amended soils, and this relationship was unique for each soil type. In contrast, this relationship was not observed for the B450 amended soils that contained a relatively smaller proportion of aromatic C.