2.4. Fourier-transform infrared spectroscopy analysis
The molecular composition of SOC was characterized by Fourier transform
infrared (FTIR) spectroscopy (Thermo Fisher Scientific Nicolet iS10
spectrometer, USA) according to
Demyan
et al.
(2012).
Briefly, 2 mg air-dried ground soil samples (<0.147 mm) were
pressed into 100 mg potassium bromide (KBr) pellets (64 scans within the
spectral range 4000-400 cm−1 at a resolution of 4
cm−1 were averaged). Band maxima at wavenumbers around
2930, 2850, 1635, 1430 and 1040 cm−1 were selected and
were assigned to a range of organic functional groups (with limited
mineral interference). The peak areas of these five bands were
integrated using a tangential baseline from the onset to the offset of
each peak, and their relative peak areas of each band were calculated to
evaluate the assignments of soil C functional groups (Table S1). The
ratio of the peak areas at 1635 and 2930 cm−1(rA1635/rA2930 ratio) was defined as the resistance index and often
taken as an indicator of SOC stability (Demyan et al., 2012; Ernakovich
et al., 2015; Hou et al., 2019).