4.3. Effects of crop rotation and fertilization on the organic and mineral functional groups
The soil FTIR-PAS and FTIR-ATR spectra could discover the changes in the molecular structure of soil organic and mineral as the adsorption bands of FTIR had a high correlation with SOC (R2=0.617, 0.650), TN (R2=0.585, 0.606), and POXC (R2=0.606, 0.584) in the PLSR model (Fig. 6b). According to the variable in projections of POXC in the PLSR model, the POXC content showed a unique correlation with the bands of νC─H from aliphatic and methyl compounds which were considered as intrinsically easily degradable (Smidt & Meissl, 2007). Peltre, Bruun, Du, Thomsen, and Jensen (2014) also reported a chiefly positive correlation between the labile fraction of SOC and the similar band at 1520−1400 cm−1. Moreover, the POXC content was also correlated with the bands of νC─O (1160 cm−1) and δC─O (1050 cm−1) which attributed to polysaccharides, nucleic acids, proteins, and carbohydrates. The polysaccharides and carbohydrates are considered intrinsically labile (Amelung, Brodowski, Sandhage-Hofmann, & Bol, 2008). The soil νC─H band in some treatments significantly increased under RG and RR rotations while it had no significant change under RW rotation (Fig. 6), which was consistent with the change in soil POXC content. These results suggested that fertilization under RG and RR rotations increased the labile fraction of SOC. The bands of νO─H/νN─H at 3600−3400 cm−1attributed to water, alcohols, phenols, carboxyl, hydroxyl groups, and amides also significantly increased under the RG rotation. An increase of amides groups under RG rotation confirmed a greater increase of TN under RG rotation which potentially attributed to the biological N fixation of rhizobia on Chinese milk vetch. The νC═C/νC═O and νC═C bands at 1720−1600 cm−1 and 1515 cm−1 were associated with carboxylic acids, amides, and aromatics, respectively. These compounds are considered to be degradation-resistant due to the recalcitrant nature of aromatic structures and due to the organo-mineral associations (Lützow et al., 2006). The organo-mineral associations are possibly formed through polyvalent cation bridges with clay surfaces and enhancing hydrophobicity to resist the microorganisms and their enzymes (Peltre et al., 2014). In this study, the intensities of νC═C and νC═O bands were significantly reduced after RG rotation rather than RR and RW rotations, suggesting RG rotation reduced the relative abundance of the stabilized SOC. In addition, the decrease of νCO32− and δAl─OH demonstrated that the relative abundances of soil carbonates, kaolinite, and smectite decreased after crop rotation and fertilization, especially for the fertilization treatments. The decrease of these soil minerals possibly was caused by the dilution effect by increased organic matter in the soil.