An interlaboratory comparison of mid-infrared spectra acquisition: Instruments and procedures matter

Diffuse reflectance spectroscopy has been extensively employed to deliver timely and cost-effective predictions of a number of soil properties. However, although several soil spectral laboratories have been established worldwide, the distinct characteristics of instruments and operations still hamper further integration and interoperability across mid-infrared (MIR) soil spectral libraries. In this study, we conducted a large-scale ring trial experiment to understand the lab-to-lab variability of multiple MIR instruments. By developing a systematic evaluation of different mathematical treatments with modeling algorithms, including regular preprocessing and spectral standardization, we quantified and evaluated instruments' dissimilarity and how this impacts internal and shared model performance. We found that all instruments delivered good predictions when calibrated internally using the same instruments' characteristics and standard operating procedures by solely relying on regular spectral preprocessing that accounts for light scattering and multiplicative/additive effects, e.g., using standard normal variate (SNV). When performing model transfer from a large public library (the USDA NSSC-KSSL MIR library) to secondary instruments, good performance was also achieved by regular preprocessing (e.g., SNV) if both instruments shared the same manufacturer. However, significant differences between the KSSL MIR library and contrasting ring trial instruments responses were evident and confirmed by a semi-unsupervised spectral clustering. For heavily contrasting setups, spectral standardization was necessary before transferring prediction models. Non-linear model types like Cubist and memory-based learning delivered more precise estimates because they seemed to be less sensitive to spectral variations than global partial least square regression. In summary, the results from this study can assist new laboratories in building spectroscopy capacity utilizing existing MIR spectral libraries and support the recent global efforts to make soil spectroscopy universally accessible with centralized or shared operating procedures

Combining a coupled FTIR-EGA system and in situ DRIFTS for studying soil organic matter in arable soils

An optimized spectroscopic method combining quantitative evolved gas analysis via Fourier transform infrared spectroscopy (FTIR-EGA) in combination with a qualitative in situ thermal reaction monitoring via diffuse reflectance Fourier transform infrared spectroscopy (in situ_<i>T</i> DRIFTS) is being proposed to rapidly characterize soil organic matter (SOM) to study its dynamics and stability. A thermal reaction chamber coupled with an infrared gas cell was used to study the pattern of thermal evolution of carbon dioxide (CO₂) in order to relate evolved gas (i.e., CO₂) to different qualities of SOM. Soil samples were taken from three different arable sites in Germany: (i) the Static Fertilization Experiment, Bad Lauchstädt (Chernozem), from treatments of farmyard manure (FYM), mineral fertilizer (NPK), their combination (FYM + NPK) and control without fertilizer inputs; (ii) Kraichgau; and (iii) Swabian Alb (Cambisols) areas, Southwest Germany. The two latter soils were further fractionated into particulate organic matter (POM), sand and stable aggregates (Sa + <i>A</i>), silt and clay (Si + C), and NaOCl oxidized Si + C (rSOC) to gain OM of different inferred stabilities; respiration was measured from fresh soil samples incubated at 20 °C and 50% water holding capacity for 490 days. A variable long path length gas cell was used to record the mid-infrared absorbance intensity of CO₂ (2400 to 2200 cm⁻¹) being evolved during soil heating from 25 to 700 °C with a heating rate of 68 °C min⁻¹ and holding time of 10 min at 700 °C. Separately, the heating chamber was placed in a diffuse reflectance chamber (DRIFTS) for measuring the mid-infrared absorbance of the soil sample during heating. Thermal stability of the bulk soils and fractions was measured via the temperature of maximum CO₂ evolution (CO₂_max). <br><br> Results indicated that the FYM + NPK and FYM treatments of the Chernozem soils had a lower CO₂_max as compared to both NPK and CON treatments. On average, CO₂_max of the Chernozem was much higher (447 °C) as compared to the Cambisol sites (Kraichgau 392 °C; Swabian Alb 384 °C). The POM fraction had the highest CO₂_max (477 °C), while rSOC had a first peak at 265 °C at both sites and a second peak at 392 °C for the Swabian Alb and 482 °C for the Kraichgau. The CO₂_max increased after 490 day incubation, while the C lost during incubation was derived from the whole temperature range but a relatively higher proportion from 200 to 350 °C. In situ_<i>T</i> DRIFTS measurements indicated decreases in vibrational intensities in the order of C-OH = unknown C vibration < C-H < −COO/C =C < C = C with increasing temperature, but interpretation of vibrational changes was complicated by changes in the spectra (i.e., overall vibrational intensity increased with temperature increase) of the sample during heating. The relative quality changes and corresponding temperatures shown by the in situ_<i>T</i> DRIFTS measurements enabled the fitting of four components or peaks to the evolved CO₂ thermogram from the FTIR-EGA measurements. This gave a semi-quantitative measure of the quality of evolved C during the heating experiment, lending more evidence that different qualities of SOM are being evolved at different temperatures from 200 to 700 °C. The CO₂_max was influenced by long-term FYM input and also after 490 days of laboratory incubation, indicating that this measurement is an indicator for the relative overall SOM stability. The combination of FTIR-EGA and in situ_<i>T</i> DRIFTS allows for a quantitative and qualitative monitoring of thermal reactions of SOM, revealing its relative stability, and provides a sound basis for a peak fitting procedure for assigning proportions of evolved CO₂ to different thermal stability components