Thermal-gradient analysis of soil organic matter using an elemental analyser – A tool for qualitative characterization?

Abstract

Analysing the thermal stability of soil organic matter (SOM) by heating soil material up to 900 °C and recording the time/temperature-dependent evolution of CO2, using an elemental analyser, may be an easily available tool to obtain information on the composition and stabilization of SOM. We studied four sample sets, including size fractions of mineral horizons, particulate organic matter (POM), soil aggregates, and thermally treated materials. All analyses were conducted with a thermal gradient of 70 K min−1, with i) stages of constant temperature at 400, 600 and 900 °C (‘DIN 19359′), ii) stages at 450, 600 and 900 °C (modified DIN method), or iii) continual heating. With both stage methods, the large majority of organic carbon (OC) was present in the least thermally stable fraction, thermally decomposing at T ≤400 or 450 °C, respectively. However, neither did these methods result in the same relative proportions of fractions of similar thermal stability, nor did they result in time/temperature-dependent CO2-evolution patterns characteristically reflecting the different sample sets. Thus, results of the stage-heating methods were operationally defined, as different SOM fractions in terms of composition or stabilization mechanism were combusted, but showing similar patterns. Nonetheless, most POM samples revealed less thermal stability than the other fractions with any method. Evaluation of CO2 development during continual heating provided more options, including T of maximum CO2 evolution, T50, that is, T at which 50% of CO2 has evolved, and peak deconvolution using Gauss functions. The mineral samples showed at least two maxima and variable T50 values up to 550 °C. Most POM fractions had lower T50, but not in general, possibly caused by the presence of black carbon or strongly complexing cations. The aggregate samples were mostly in between the mineral and POM samples. Irrespective of the method applied, direct unambiguous identification of samples in terms of composition or stabilization mechanisms was not possible. Nonetheless, we consider an elemental analyser a valuable tool to evaluate the thermal stability of large samples sets at relatively low cost and time effort. Particularly peak deconvolution from continual heating offers the option of quantifying OC fractions, differing in thermal stability.