Soil sample drying temperature affects specific organic mid-DRIFTS peaks and quality indices

Abstract

Peak area (PA) integration of diffuse reflectance mid infrared Fourier transform spectroscopy (DRIFTS) spectra is useful to study soil organic matter (SOM) quality and as a potential modelling proxy. Residual water of soil samples after drying affects DRIFTS spectra, mainly in the region >2500 cm−1. Therefore, SOM quality related PA at different wavenumbers should be influenced to varying degrees. We studied how absolute, normalized and relative PA related to SOM functional groups (at 2930, 1620, 1530 and 1159 cm−1), as well as their ratios were influenced by oven drying of soil samples at increasing temperatures (from 32 °C to 105 °C). All organic and even mineral associated PA significantly increased with drying temperature. The PA of aliphatic CH stretching (2930 cm−1), located on the shoulder of the broad OH stretching PA, was influenced most strongly. Our results indicate that using 105 °C as drying temperature and storing samples in a desiccator is the best way to minimize water interference. These findings apply to relative PA and ratios of organic compounds, while no effect of drying temperatures on PA correlations to C or N were found.