Impact of Soil Microstructure Geometry on DRIFT Spectra: Comparisons with Beam Trace Modeling

Abstract

Fourier‐transformed infrared spectroscopy in diffuse reflectance mode (DRIFT) has been proposed as a tool for the characterization of organic matter (OM) composition at intact soil surfaces; however, the local properties and the geometry of intact structural surfaces (e.g., biopores and cracks) affect the reflection in as yet unknown ways. Our goal was to develop an approach to correct for surface geometry effects. The objectives were to analyze the effects of (i) particle size, (ii) porosity, and (iii) specific surface shapes on the infrared signal intensity by comparing measured with simulated reflectance data. Mid‐infrared DRIFT spectra were obtained from differently textured quartz samples and from gypsum blocks with defined surface shapes as models for soil porous systems. A beam tracing model (BTM) was used for the numerical description of the infrared beam propagation in such model soils. The measured DRIFT signal intensity and the simulated bihemispherical reflectance both decreased with increasing quartz particle size; the resolution of the DRIFT spectra decreased with particle size. The geometric effects of the microtopography can be explained by the local variations in the distance between the collection mirror of the DRIFT device and the sample surface. The DRIFT‐measured effects of particle size and surface topography on the signal intensity agreed with the BTM simulation results. The results suggest that a radiative transfer model is useful for interpretations of DRIFT data obtained at intact structural surfaces. The OM properties can be analyzed with DRIFT for surfaces that consist of particles <70 μm and have relatively small, i.e., <1‐mm, relief differences.