Objective  This paper aimed to clarify the infrared spectral characteristics and their change patterns of soil organic matter functional groups in buried ancient and modern paddy soils, and to reveal the effect of iron oxide on the spectral response of organic matter functional groups at different environmental conditions and different buried depths.

  Method  The spectral characteristics of original soil, the removal of organic matter soil, and the removal of iron oxide soil in buried ancient and modern paddy soils in Shanlonggang was analyzed by Fourier infrared spectroscopy, and the differences and the depth profile of soil organic matter functional groups were comparatively analyzed.

  Result  The infrared spectral characteristics of curve shape, absorption peak and the intensity of absorption peak were unanimous in the same soil profile under the three treatments, especially the several typical absorption peaks related to organic functional groups: 2925 cm⁻¹ (aliphatic peak), 2854 cm⁻¹ (aliphatic peak), 1634 cm⁻¹ (aromatic peak), 1401 cm⁻¹ (aliphatic peak). The differential spectra of organic matter removal were manifested in the same soil profile as: the absorption peak intensity of organic functional groups changes towards stronger, and the absorption peak intensity of organic functional groups in the differential spectra of iron oxide removal is greater.

  Conclusion  There is a close relationship between the functional groups of organic matter and iron-containing minerals. Removing iron oxide is more conducive to observing the changes in the infrared spectral characteristics for organic matter functional groups, and this change is particularly obvious in buried ancient rice soil. The structural stability of organic functional groups in the buried ancient paddy soil is followed as 1634 cm⁻¹ (aliphatic peak) > 1401 cm⁻¹ (aliphatic peak) > 2925 cm⁻¹ (aromatic peak) > 2854 cm⁻¹ (aliphatic peak). The profile variation shows that the deeper soil buried, the more aromatic carbon and the less aliphatic carbon.