Alteration of Biochar Carbon Chemistry during Soil Incubations: SR‐FTIR and NEXAFS Investigation

Abstract

Near‐edge X‐ray absorption fine structure (NEXAFS) spectroscopy and synchrotron radiation based Fourier‐transform infrared (SR‐FTIR) microspectroscopy were utilized to systematically study the aging of three biochars under similar controlled conditions by tracking changes in the C chemistry of biochar in the presence and absence of a Typic Xerorthent soil. By utilizing both NEXAFS and SR‐FTIR, differences in the initial biochar C functional group composition due to feedstock (aromatic C was greater in walnut shell biochar than softwood feedstock) and pyrolysis temperature (no long‐range‐ordered C in wood feedstock made at 410°C compared with 510°C) were confirmed. The data provided spectroscopic evidence corroborating both the conceptual biphasic model for biochar degradation and the power model of organic matter continuum mass loss as biochar ages due to a more labile aliphatic biochar portion and an aromatic portion that is oxidized more slowly. Incubations in the presence and absence of soil revealed a decrease in the ratio of the 287.6 eV peak (aliphatic C) relative to the 285.5 eV peak (aromatic C) during the incubations. Binding through functional groups present on the aged biochar surfaces (e.g., quinones, phenols, carbonyls) as well as the physical protection of the biochar by the soil appears to retard biochar surface decomposition. This study provides high‐resolution spectroscopic data on discrete points on the biochar in addition to interactions between the soil and the biochar under conditions of minimal sample disturbance and destruction that corroborates current biochar stability and turnover models.