Spectral Signals of Organic Matter Change As a Function of Moisture Content - Implications for Soil Water Repellency.

Characterization of the molecular structure of soil organic matter (OM) using Fourier transformed infrared spectroscopy (FTIR) is typically reported for dry materials. However, in soil systems, microenvironments frequently cycle between wet and dry. The correlation of soil water repellency and moisture content necessitates the consideration of changes in the molecular structure of OM under various moisture contents in order to explore the chemical mechanisms by which soil water repellency forms and resolves, and the external drivers that trigger those mechanisms. In the work presented here, we focused on the interpretation of attenuated total reflectance FTIR spectroscopy to generate meaningful information about the molecular structural changes of individual components of soil OM during wetting and drying cycles. We show that for molecules such as stearic acid and methionine the spectral signal does not change regardless of the quantity of water added. For other molecules in aqueous environments such as cellulose and dead microbial biomass, a straightforward separation of the spectral response into a water signal and a compound related signal can be demonstrated. Finally, there are organic compounds such as citric acid that interact with water molecules in a way that generates a novel and unique spectral signature with features that are not observed for either of the pure phases. We discuss the implications of moisture dependent variations in spectral information for the interpretation of FTIR spectra from soils. From this we hope to add to the understanding regarding the mechanisms responsible for soil water repellency which what will allow agronomists to develop and implement management practices that will be more effective at reducing soil erosion, increasing crop productivity, increasing water use efficiency, and increasing the resiliency of the ecosystem services of these fragile soils.