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).