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) and qualitative in situ thermal reaction monitoring via diffuse reflectance Fourier transform infrared spectroscopy (in situT 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 to different qualities of soil organic matter (SOM). Soil samples were from three different sites, i.e. (i) the Static Fertilization Experiment, Bad Lauchstädt (Chernozem) from treatments of farmyard manure (FYM), mineral fertilizer (NPK), combination (FYM + NPK) and control without fertilizer inputs, and cropped soils from the (ii) Kraichgau and (iii) Swabian Alb (Cambisols) areas, Southwest Germany. Soils from Kraichgau and Swabian Alb were further fractionated into particulate organic matter (POM), sand and stable aggregates (Sa + A), silt and clay (Si + C), and NaOCl oxidized Si + C (rSOC) to gain OM of different inferred stabilities. Fresh soil samples from the Kraichgau and Swabian Alb were incubated at 20 °C and 50% water holding capacity for 490 days in order to measure soil respiration under controlled conditions. A variable long path length gas cell was used to record the mid-infrared absorbance intensity of carbon dioxide (2400 to 2200 cm−1) being evolved during soil heating from 25 to 700 °C with a heating rate of 68 °C min−1 during an initial ramping time of 10 min and holding time of 10 min. Separately the heating chamber was placed in a diffuse reflectance chamber (DRIFTS) for measuring the mid-infrared absorption of the soil sample during heating. Thermal stability of the bulk soils and fractions was measured via the temperature of maximum CO2 (2400 to 2200 cm−1 evolution (CO2). Results indicated that the FYM + NPK and FYM treatments of the Chernozem soils of Bad Lauchstädt had a lower CO2max as compared to both NPK and CON treatments. On average CO2max in Bad Lauchstädt was much higher (447 °C) as compared to the Kraichgau (392 °C) and Swabian Alb (384 °C) sites. The POM fraction had the highest CO2 (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 was found to increase 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 situT 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 situT DRIFTS measurements enabled the fitting of four components or peaks to the evolved CO2 thermogram from the FTIR-EGA measurements to have 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 was influenced by long-term farmyard manure input and also by 490 days of laboratory incubation, indicating that this measurement can be an indicator for the relative overall SOM stability. The combination of FTIR-EGA and in situT DRIFTS was shown to be useful for monitoring the rate of thermal decomposition of different soils and SOM fractions which were related to their relative stability. This knowledge was used for a peak fitting procedure for assigning proportions of evolved CO2 to different thermal stability components.