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.