Two-Dimensional Correlation IR Spectroscopy of Humic Substances of Chernozem Size Fractions of Different Land Use

Diffuse reflectance FTIR measurements with two-dimensional correlation spectroscopy (2D-COS) are used for accurate band identification of chernozem comprising soil organic matter (SOM), including humic substances and mineral silicate matrix. Samples of different land use (native steppe, shelterbelt, bare fallow, and arable land) of a long-term field experiment were compared. Homospectral 2D-COS maps for size fractions obtained by wet fractionation were built, and the fraction size was used as a correlation-building variable (external perturbation) of 2D-COS. Synchronous 2D-COS maps are characterized by main correlation regions at 4000-3600 (hydrogen bonds), 1800-1150 (SOM), and 1100-200 cm(-1) (quartz matrix). SOM range can be used as a signature of the samples distinguishing two pairs, native steppe-bare fallow and arable land-shelterbelt, by correlations at 1340-1320 cm(-1) (CH2) and 1670 cm(-1) (aromatic -C=C-). Asynchronous 2D-COS maps show bands at 3690-3620, 2930-2830, in the range of 1640-1250 (8 bands), 1160, 1070, 797, 697, 505, and 400 cm(-1), the latter 5 indicate the increasing proportion of silicate to quartz in small fractions. The manifestation of asynchronous correlation bands at 1650, 1580-1560, 1444, 1340, and 1250 cm(-1), which have no major contribution from inorganic soil components, are due to carbonyl, carboxylate, and aromatic C-C; their appearance order (accumulation of corresponding substances in larger factions) is different for each land use. The proposed approach provides the identifying SOM components with enough reliability for SOM IR bands that are weaker compared to mineral matrix bands in original IR spectra.

Automated summary

Diffuse reflectance FTIR measurements with 2D-COS were used to accurately identify the bands of chernozem soil organic matter (SOM) and mineral silicate matrix. The study compared samples from different land use types of a long-term field experiment. Homospectral and synchronous 2D-COS maps revealed correlation regions related to hydrogen bonds, SOM, and quartz matrix. The SOM range had distinct signatures, distinguishing different land use pairs. Asynchronous 2D-COS maps showed bands related to the proportion of silicate to quartz in small fractions, as well as carbonyl, carboxylate, and aromatic C-C in larger factions. The proposed approach provides reliable identification of SOM components with weaker IR bands compared to the mineral matrix.