Carbon K-edge NEXAFS and FTIR-ATR spectroscopic investigation of organic carbon speciation in soils

Soil organic matter (SOM) is a fundamental component of soil and the global C cycle. We used C (1s) near-edge x-ray absorption fine structure (NEXAFS) and synchrotron-based Fourier transform infrared-attenuated total reflectance (FTIR-ATR) spectroscopy to speciate C and investigate the influence of land use on the composition of SOM in the humic substances extracted from clay and silt fractions. Soil samples were collected from natural forest, tea and Cupressus plantations and cultivated fields in Ethiopia. Carbon K-edge spectra revealed multiple C (1s) electron transitions in the fine structure of C NEXAFS region (284-290 eV) indicating the presence of aromatic-C, phenolic-C, aliphatic-C, carboxylic-C, and O-alkyl-C in the humic substances. It also exhibited good selectivity, where specific energy regions correspond to C in discrete functional groups. However, regions of slight overlap between 1s-3p/σ* transition of aliphatic-C and 1s-π* transition of carboxylic-C may not be excluded. Fourier transform infrared-attenuated total reflectance spectroscopy showed larger proportions of aromatic-C (25.5%, 21.9%) and asymmetric and symmetric aliphatic-C (19.7%, 15.2%) groups in the silt than in clay, respectively. However, smaller proportion of polysaccharides (19.3%, 11.5%) was obtained from the silt compared with clay. The proportions of phenols (20.7%, 20.4%), aliphatic deformation of CH₂ or CH₃ (13.1%, 14.5%), and carboxylic (9.8%, 8.3%) groups were of similar magnitude in both fractions. The proportion of polysaccharides decreased in the order: natural forests > plantations > cultivated fields, while recalcitrant aromatic-C increased in the order: natural forest < plantation < cultivation. Therefore, C (1s) NEXAFS and synchrotron-based FTIR-ATR spectroscopy are powerful, nondestructive techniques that can potentially be used not only to identify and fingerprint complex structural characteristics of organic C macromolecules but also to investigate the impact of long-term anthropogenic management on the composition and biogeochemical cycling of organic C in terrestrial ecosystems.