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

• Published in: Biogeosciences Vol. 10; no. 5; pp. 2897 - 2913
• Main Authors: Demyan, M. S, Rasche, F, Schütt, M, Smirnova, N, Schulz, E, Cadisch, G
• Format: Journal Article
• Language: English
• Published: Katlenburg-Lindau Copernicus GmbH 02.05.2013; Copernicus Publications
• Subjects: Analysis; Infrared spectroscopy; Soils
• ISSN: 1726-4189; 1726-4170; 1726-4189
• DOI: 10.5194/bg-10-2897-2013

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 sub(T) 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 (CO sub(2)) in order to relate evolved gas (i.e., CO sub(2)) to different qualities of SOM. Soil samples were taken from three different arable sites in Germany: (i) the Static Fertilization Experiment, Bad Lauchstaedt (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 + A), 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 degree 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 CO sub(2) (2400 to 2200 cm super(-1)) being evolved during soil heating from 25 to 700 degree C with a heating rate of 68 degree C min super(-1) and holding time of 10 min at 700 degree 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 CO sub(2) evolution (CO sub(2) sub(max)). Results indicated that the FYM + NPK and FYM treatments of the Chernozem soils had a lower CO sub(2) sub(max) as compared to both NPK and CON treatments. On average, CO sub(2) sub(max) of the Chernozem was much higher (447 degree C) as compared to the Cambisol sites (Kraichgau 392 degree C; Swabian Alb 384 degree C). The POM fraction had the highest CO sub(2) sub(max) (477 degree C), while rSOC had a first peak at 265 degree C at both sites and a second peak at 392 degree C for the Swabian Alb and 482 degree C for the Kraichgau. The CO sub(2) sub(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 degree C. In situ sub(T) 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 sub(T) DRIFTS measurements enabled the fitting of four components or peaks to the evolved CO sub(2) 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 degree C. The CO sub(2) sub(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 sub(T) 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 CO sub(2) to different thermal stability components.