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).