3.4. Two-dimensional correlation spectroscopy (2DCOS) analysis
To clarify the association between the salt-related elements and SOM, FTIR–ATR was employed. For complicated materials, such as soils, the peaks of the functional groups and molecule bonds located in different submicron domains may be heterogeneous and overlap in 1 D FTIR–ATR spectra. However, the overlapping peaks could have different responses to external perturbations along spatial and micro scales. 2DCOS is built on the classical cross-correlation analysis of spectral intensity variations observed for a sample system of interest under the influence of external perturbations. The concentration of salt was set as the external perturbation.
The generalized two-dimensional synchronous and asynchronous salt content–dependent correlation spectra of the soil samples are plotted in Fig. 7. Auto-peaks appearing at diagonal positions represent the overall susceptibility of the corresponding spectral region to change in spectral intensity, which could be applied to the study system as an external perturbation. In Fig. 7a, the auto-peaks along the diagonal line of the synchronous spectrum, located at Ф (1698, 1698), Ф (1548, 1548), Ф (1264, 1264), and Ф (995, 995) cm-1, have been assigned to the C=C stretching in amides and carboxylic acids (1698 cm-1), N–H and C–N stretching in Amide II (1548 cm-1), C–OH and C–H of phenols and aromatics (1264 cm-1), and Si–O in clay minerals (995 cm-1). This indicates the components involved in the binding of salt compounds in soil. The main auto-peak, Ф (3700, 3700) cm-1 (Fig. 7c), showed that the O–H stretching in clay minerals was involved in the binding of salt-related compounds (Sun et al., 2019). The off-diagonal peaks in the synchronous map exhibited correlated signals (Yu et al., 2012). The negative peaks at Ф (995, 1548) and Ф (995, 1264) cm-1 indicate that the intensity of Si–O increased while that of N–H/C–N and C–OH/C–H decreased. The negative cross-peaks at Ф (2303, 3717) and Ф (2303, 3642) cm-1 suggest that the intensity of the CO–H stretching in carbohydrates changed directions, to that of O–H in clay minerals.
The asynchronous maps were anti-symmetric, which could provide information on the sequential order of functional groups when combined with analysis of the synchronous maps. In the range of 1500–650 cm-1 (Fig. 7b), there were four main negative peaks at Ψ (995, 1680), Ψ (995, 1502), Ψ (995, 1451), and Ψ (995, 1241). According to Noda and Ozaki’s principle (Noda and Ozaki, 2005; Noda, 2018), the sequence of binding affinities of bands with salt-related compounds followed the order: 995 → 1680 cm-1 and 995 → 1502 cm-1. This means that the binding of functional groups in soil with salt-related compounds followed the sequence: Si–O > C=O and Si–O > C=C. Similarly, the negative peaks at Ψ (893, 995) and Ψ (760, 995) indicated that the binding affinities of C–H (893 cm-1) and NH2(757 cm-1) with salt-related compounds were prior to that of Si–O. The main off-diagonal peaks distributed in the region of 4000–3000 cm-1 (Fig. 7d) were related to the O–H/N–H in clay minerals, water, alcohols, phenols, carboxyl groups, and hydroxyl groups (Ma et al., 2019).