Uncovering the Biochemical Significance of Soil Organic Matter Thermal Stability in Soils of Contrasting Genesis.

Thermal analysis (i.e., TG-DSC) has emerged as a promising technique for understanding and quantifying the quality/stability of bio-molecular and organo-metallic materials that comprise soil organic matter (SOM). Despite recent analytical advances (data generation and post-processing), clear interpretation between measured energetic properties (stability or yield) and ecological or management implications remain uncertain. Previous work has established the relevance of three thermal stability regions, where general classes of biomolecular materials are believed to experience exothermic oxidization. These thermal stability regions are nominally classified as Exo-1 (185°-340° C: carbohydrates, proteins, lipids), Exo-2 (340°-450° C: lipids, aromatic material), and Exo-3 (450°-550° C: condensed and covalently bound C). The goal of this study was to quantify the biochemical changes in SOM that correspond to these three soil thermal regions through the coupling of thermogravimetry (TG)/ differential scanning calorimetry (DSC) with FTIR spectroscopy.

Five surface soils (0-10 cm) that represent a wide range in organic carbon concentration (13 to 243 g C kg^-1), soil mineralogy (kaolinitic, smectitic and amorphous), and land-use (agricultural, rangeland, and forests) were selected from across the continental United States. Samples were analyzed using TG-DSC, heating from ~25° C to six end-point temperatures: 185°, 340°, 450°, 550°, and 700° C. After samples were combusted to each end-point temperature, transmission FTIR spectroscopy was used to analyze the molecular composition.  In general, our results show a loss of polysaccharides and lipids in the Exo-1 region, further loss of lipids and aromatic compounds in the Exo-2 region, followed by further loss of aromatic compounds in the Exo-3 region. These results confirm previous assertions as to the specific biomolecules thermally decomposed within each thermal regions and support the utility of SOM thermal stability as a proxy for SOM stability.