2.3.3. Fourier Transform Infrared Spectrum (FTIR) Analysis Test

The attenuated total reflection Fourier transform infrared spectroscopy method was adopted in this study. Six kinds of soil were dried at 100 ◦C for 24 h and tested by a Fourier infrared spectrometer after passing through a 200-mesh sieve. The mid-infrared region was used for detection in this study. The following procedures were used to analyze the spectrum [51].

The measured mid-infrared was tentatively divided into a characteristic functional group area (4000–1333 cm−1) and a fingerprint area (1333–400 cm−1), then the characteristic functional group was further divided into three wavebands (4000–2400 cm<sup>−</sup>1, 2400–2000 cm<sup>−</sup>1, and 900–400 cm<sup>−</sup>1), and the fingerprint area was finely divided into two wavebands (1333–900 cm−<sup>1</sup> and 900–400 cm−1). Infrared spectroscopies were taken as a preliminary analysis (compound type, functional group, the structural unit, etc.) based on the corresponding characteristic absorption bands to determine the possible functional groups and possible structural units.

Based on possible functional groups, possible structural units, the characteristic frequency table of the characteristic absorption bands of various compounds, and various factors affecting the movement of characteristic frequencies, the structural details were revealed. The standard spectra of related compounds determined according to the details were compared with the obtained spectra of soils under six environments to obtain specific functional group composition.
