3.1. FTIR-ATR spectra of soil samples
The average FTIR-ATR spectra of the soil samples are shown in Fig. 3. The peak at 3610 cm-1 was attributed to the stretching vibration of O-H in clay minerals, water, carboxyl groups, and hydroxyl groups (Madejová & Komadel, 2001; Nayak & Singh, 2007; Peltre et al., 2014; Xu et al., 2018). The peak at approximately 1660 cm−1 was attributed to the stretching vibrations of C=C/C=O in amides and aromatics (Madejová & Komadel, 2001). The stretching vibration of H–O–H from clay bound water has been highlighted for the peak at 1630 cm-1. The peak at 1434 cm-1 has been attributed to the stretching vibration of CO32- in carbonate (Soriano-Disla et al., 2013). The stretching vibration of C–H in methyls explains the peak spanning 1445–1350 cm-1(Bernier et al., 2013; Calderón et al., 2013; Movasaghi et al., 2008). The stretching vibration of Si–O in silicates (quartz from clay minerals) is present in the 1030–950 cm−1 region (Madejová, 2003). The peak at 915 cm-1 has been attributed to the bending vibration of Al–OH from kaolinite and smectite (Madejová & Komadel, 2001). The stretching vibration of C–O from carbonate is visible via the peak at 872 cm-1. The out-of-plane vibrations of amine (NH2) explain the peak at 761 cm−1 (Smidt & Meissl, 2007).