Agricultural soil characterization by FTIR spectroscopy at micrometer scales: Depth profiling by photoacoustic spectroscopy

Abstract

In a Fourier transform mid-infrared photoacoustic spectroscopy (FTIR-PAS) mode, the scanning depths vary with the moving mirror velocity at specific wavenumbers, allowing depth profiling of materials. In this paper, we use FTIR-PAS spectroscopy to carry out a depth profiling of three types of agricultural soils (Fluvo-aquic soil, red soil and black soil) at micrometer scales by modulating the moving mirror velocity. The compositional and structural differences in soil organic matters were explored from the surface to a depth of about 4 μm. The results showed that the distribution of potential hydrophobic CH varied among different types of soils. The contribution of the soil organic matter (SOM) to the cation exchange capacity (CEC) also differed for each soil type. The results of the principal component analysis (PCA) indicated that most differences among the selected soil types occurred in the inner layer, and the discriminatory power of the soil spectra decreased with the reduction of scanning depths. Correlations between soil properties and spectral absorbance were carried out to determine the source of water extractable organic carbon (WEOC) of different soil types. For the black soil, WEOC were significantly and positively correlated to the absorbance at ~2920 cm−1, and this correlation increased with the probing depth at micrometer scales. However, the significant and negative correlation between the WEOC of the red soil, and the bond intensity of ~1650 cm−1 decreased with an increasing probing depth. This result demonstrates a selective retention of biotic components in the WEOC of the red soil. The WEOC of the black soil, with comparison to the red soil, has more contributions from the native organic matter. These results can be reasonably ascribed to different pedogenesis and cultivation systems that were practiced in the past.