3.4. Soil FTIR spectra
The FTIR-PAS spectra featured a broad peak at 3900−3700 cm−1 attributed to the stretching vibration (ν) of O─H from clay minerals (Table 1, Fig. 5a). A sharp peak of νO─H from clay minerals at 3620 cm−1 in FTIR-ATR spectra was also observed (Fig. 5b). The broadband ranged from 3600 to 3400 cm−1 attributed to νO─H from water, alcohols, phenols, carboxyl, and hydroxyl groups and νN─H from amides was both found in FTIR-PAS and FTIR-ATR spectra. The broadband at 3000−2800 cm−1 in FTIR-PAS spectra was dominated by νN─H from aliphatic methyl and methylene groups. The overtone of νCOH ranged from 2200 to 2000 cm−1 was derived from carbohydrates. The broadband ranged from 1720−1600 cm−1 was associated with νC═O from carboxylic acids and amides and νC═C from aromatics. The band at 1570−1540 cm−1 in FTIR-ATR spectra was dominated by νN─H and νC─N in plane from amide II. An obvious sharp peak at 1515 cm−1 in FTIR-PAS spectra attributed to νC═C from aromatics. The absorption band of νCO32− from carbonates also was confirmed at 1500−1300 cm−1 in FTIR-ATR spectra. The shoulder at 1445−1350 cm−1 in FTIR-PAS spectra was assigned to νC─H from methyls. The νC─O and bending vibration (δ) of C─O in FTIR-ATR were derived from polysaccharides, nucleic acids, proteins, and carbohydrates. The FTIR-ATR showed an intensive peak at 990 cm−1 which was associated with νSi─O from clay minerals. A shoulder at 915 cm−1 attributed to δAl─OH from kaolinite and smectite.
The scatterplots of the first two PCA scores (PC1 and PC2) from FTIR-PAS and FTIR-ATR spectra indicated an obvious separation between soil samples before and after treatments (Fig. S5a, b). The PC1 and PC2 scores of soil FTIR-PAS spectra were mostly positive after treatments and both the PC1 and PC2 had positive loadings at the wavenumbers of νO─H, νO─H/νN─H, νC═C/νC═O, νC═C, νC─H, and νSi─O, (Fig. S5c), indicating that these functional groups in soil increased after crop rotation and fertilization. Soils after treatments mostly had negative PC2 scores for FTIR-ATR spectra and had positive PC2 loading at the wavenumbers of νCO32−and νAl─OH (Fig. S5d), which suggested that soil CO32− and Al─OH decreased after crop rotation and fertilization. We further applied PCA on the differential spectra of FTIR-PAS and FTIR-ATR. Crop rotation, fertilization, and their interaction had significant effects on PCA scores for FTIR-PAS differential spectra, while only their interaction had a significant effect on PCA scores of FTIR-ATR differential spectra (Fig. 5c, d). An obvious separation between soils after RG and RW rotations was observed according to PC1 for FTIR-PAS differential spectra. PC1 showed high positive scores for soils after RG rotation but high negative scores for soils after RW rotation (Fig. 6a). In addition, PC1 had high positive loadings at the wavenumbers of νC═C/νC═O and νC═C, and high negative loadings at the wavenumbers of νO─H/νN─H (Fig. S5). This suggested that RG rotation increased the soil functional group of O─H and N─H and decreased the soil functional groups of C═C, C═O, and C═C (Fig. 6).
SOC, TN, and POXC contents had a commonly significant relationship with the stretching vibrations of O─H (3900−3700 cm−1) in the PLSR model of FTIR-PAS. However, SOC content showed a unique relationship with the stretching vibrations of C═C (1515 cm─1) and POXC content presented a unique relationship with the stretching vibrations of C─H (1445−1350 cm─1) in the PLSR model of FTIR-PAS (Fig. 5a). For FTIR-ATR, SOC, TN, and POXC contents were all significantly related to the stretching vibrations of CO32− (1500−1300 cm─1), C─O (1160 cm−1), Si─O (1030−950 cm−1), and the bending vibrations of Al─OH (915 cm─1) in the PLSR model (Fig. 5b).