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

[摘要] An optimized spectroscopic method combining quantitative evolved gasanalysis via Fourier transform infrared spectroscopy (FTIR-EGA) incombination with a qualitative in situ thermal reaction monitoringvia diffuse reflectance Fourier transform infrared spectroscopy (insitu_T DRIFTS) is being proposed to rapidly characterize soil organicmatter (SOM) to study its dynamics and stability. A thermal reaction chambercoupled with an infrared gas cell was used to study the pattern of thermalevolution of carbon dioxide (CO₂) in order to relate evolved gas (i.e.,CO₂) to different qualities of SOM. Soil samples were taken from threedifferent arable sites in Germany: (i) the Static FertilizationExperiment, Bad Lauchstädt (Chernozem), from treatments of farmyardmanure (FYM), mineral fertilizer (NPK), their combination (FYM + NPK) andcontrol without fertilizer inputs; (ii) Kraichgau; and (iii) SwabianAlb (Cambisols) areas, Southwest Germany. The two latter soils were furtherfractionated into particulate organic matter (POM), sand and stableaggregates (Sa + A), silt and clay (Si + C), and NaOCl oxidized Si + C(rSOC) to gain OM of different inferred stabilities; respiration wasmeasured from fresh soil samples incubated at 20 °C and 50%water holding capacity for 490 days. A variable long path length gas cellwas used to record the mid-infrared absorbance intensity of CO₂ (2400to 2200 cm⁻¹) being evolved during soil heating from 25 to700 °C with a heating rate of 68 °C min⁻¹ andholding time of 10 min at 700 °C. Separately, the heatingchamber was placed in a diffuse reflectance chamber (DRIFTS) for measuringthe mid-infrared absorbance of the soil sample during heating. Thermalstability of the bulk soils and fractions was measured via the temperatureof maximum CO₂ evolution (CO_2max).

Results indicated that theFYM + NPK and FYM treatments of the Chernozem soils had a lowerCO_2max as compared to both NPK and CON treatments. On average,CO_2max of the Chernozem was much higher (447 °C) ascompared to the Cambisol sites (Kraichgau 392 °C; Swabian Alb384 °C). The POM fraction had the highest CO_2max(477 °C), while rSOC had a first peak at 265 °C at bothsites and a second peak at 392 °C for the Swabian Alb and482 °C for the Kraichgau. The CO_2max increased after 490 day incubation, while the C lost during incubation was derived from thewhole temperature range but a relatively higher proportion from 200 to350 °C. In situ_TDRIFTS measurements indicated decreases invibrational intensities in the order of C-OH = unknown C vibration< C-H < −COO/C =C < C = C with increasingtemperature, but interpretation of vibrational changes was complicated bychanges in the spectra (i.e., overall vibrational intensity increased withtemperature increase) of the sample during heating. The relative qualitychanges and corresponding temperatures shown by the in situ_TDRIFTSmeasurements enabled the fitting of four components or peaks to the evolvedCO₂ thermogram from the FTIR-EGA measurements. This gave asemi-quantitative measure of the quality of evolved C during the heatingexperiment, lending more evidence that different qualities of SOM are beingevolved at different temperatures from 200 to 700 °C. TheCO_2max was influenced by long-term FYM input and also after 490days of laboratory incubation, indicating that this measurement is anindicator for the relative overall SOM stability. The combination ofFTIR-EGA and in situ_T DRIFTS allows for a quantitative and qualitativemonitoring of thermal reactions of SOM, revealing its relative stability, andprovides a sound basis for a peak fitting procedure for assigningproportions of evolved CO₂ to different thermal stability components.

[学科分类] 地球化学与岩石