FTIR spectral properties affected by OM-cation interactions

Soil organic matter (OM) interacts with cations like Ca by using C=O and OH functional groups. Such interactions are known to protect soil OM against decomposition. This process affects the bonding strength of functional groups. Changes in bonding strength are assumed to shift the wavenumber region of OH and C=O absorption band maxima in Fourier transform infrared (FTIR) spectra. The aim is to analyze the extent of such shifts to determine presence and strength of OM–cation interaction. Solutions of PGA and Chia seed mucilage were mixed at different ratios with CaCl₂ solution. The mixtures were freeze dried. FTIR spectra of PGA–Ca, and mucilage-Ca mixtures indicate that the OH band is affected by the presence of Ca. However, the C=O band maximum and the CH/C=O ratio were not affected. For the PGA–Ca and mucilage-Ca mixtures the shift in OH band maxima relative to PGA and mucilage, respectively, increases with Ca content. Such shifts in OH band maxima are in a similar range as the ones observed for the outer compared to inner regions of an intact chia seed mucilage droplet. The results suggest that it is necessary to know the relation between OM-cation interactions and band shifts for the correct interpretation of the FTIR spectra of soil and rhizosphere samples.