Effect of soil drying on FT-IR spectroscopic analysis: a methodological study in Luxembourg
Résumé
Optimum soil quality is a goal to be achieved in agriculture and forestry and in order to maintain a sustainable equilibrium of ecosystems. The evaluation of various parameters influencing the soil’s structure, fertility or even its chemical, physical or biological properties, like for instance soil organic matter, often relies on time-consuming and cost-intensive labwork. Fourier Transform Infrared (FT-IR) spectroscopy, however, has highly contributed to rapidly and cost-effectively assess these different parameters. Especially the mid-infrared (MIR) range (4000-400cm-1) has proven to be an effective tool to predict soil properties. Additionally, MIR spectroscopy represents a qualitative analysis and renders spectral interpretation less complex by easily attributing peaks to organic or mineral compounds on the spectrum. Our research focuses on characterizing various soil compounds (water-extractable organic matter and lipids) through MIR spectral analysis. The present study specifically investigates the effect of various drying conditions on soils by Fourier Transform infrared spectroscopy in attenuated total reflectance (ATR) mode. The OL and A horizons of five representative soils of Luxembourg were collected: a Stagnic cambisol (Humic, Dystric), a Haplic Acrisol (Arenic), a Stagnic Albeluvisol (Dystric, Siltic), a Cambic Leptosol (Calcaric) and a Haplic Cambisol (Eutric). Field-moist soil samples were directly sieved in a 2mm mesh and conditioned in accordance with the different drying procedures realized during this study, namely freeze-drying, oven-drying (25, 40, 80 and 105°C) and air-drying (20°C). The spectral analyses were performed after incorporation of KBr pellets on a Bruker Hyperion 2000 (Bruker Optik Gmbh, Ettlingen, Germany) Fourier Transform-Infrared Spectrometer (FT-IR) in Attenuated Total Reflectance mode (ATR). For each pellet, three spectra were recorded at different locations, in order to evaluate the reproducibility of the method. Additional conventional soil analyses were realized: pH (H2O, KCl, CaCl2), exchange capacity (Cohex) and related exchangeable cations, particle size analysis and CHNS elemental analysis, including the distinction between total and organic carbon contents, by means of acidic pretreatment of samples (HCl 2N). The outcome of this study shall define an optimal drying procedure for soil organic matter analysis coupled with infrared spectrometry. Indeed, traces of water in soil samples are known to produce broad bands around 3500cm- 1, which may mask relevant peaks related to soil. The expected results would show a significant attenuation or even an absence of the water band, depending on the pretreatment applied. In addition, this work will also be focused on the impact of the drying procedure on the spectral bands attributable to soil organic matter, possibly due to SOM degradation. Furthermore, the results from the preferred method should be reproducible independently from the type of soil chosen.