Thermal Analysis of Native and Cultivated Soils After Long-Term Incubation.

Thermal analysis techniques have been typically used for the identification and quantification of soil mineral constituents, but their use has recently gained increasing importance in the study of soil organic matter (SOM). Due to the distinctive exothermic reactions associated with the thermal oxidation of organic matter, thermal analysis has been proposed as a relatively rapid and low-cost option for characterizing organic matter stability. Despite promising results already obtained, the number of tested soils and fractions are not yet large enough and additional experiments must be performed to demonstrate the potential link between thermal and biological SOM stability.

In a previous study, four different soil surface samples (Indian Head, SK; Mandan, ND; Akron, CO; and Waggoner Ranch, TX), each with a native and cultivated land use, were collected and incubated at 35°C for 588 days. CO₂ measurements were taken periodically over the course of the incubation. We report on the thermal analysis (i.e., thermogravimetry and different scanning calorimetry) of samples performed before and after incubation of these soils samples, as a means to assess changes in SOM quality during incubation.

Soil C concentrations and proportions of mineralizable C differed significantly among the four sites and land use treatments. Thermal analysis results were characteristic for each soil and were strongly correlated with the variations in C contents detected in the samples. Comparison of results before and after incubation by curve subtraction reflected the stability of the organic matter mineralized during the incubation and suggests that incubation depleted mainly thermally labile material (< 425° C) rather than more stable (> 425° C).

These results provide strong evidence for the use of thermal analysis to rapidly characterize SOM quantity and quality of many soils.