Application of Differential Scanning Calorimetry to Determine Enthaplic Character of Composts, Dissolved Organic Carbon (DOC), and Soils.

Using differential scanning calorimetry (DSC) on biogeochemical samples is a novel quantitative approach to analyzing these materials.  Along with standard thermal analysis metrics (endothermic versus exothermic transformations, ash content, and mass loss) we are able to determine the enthalpic value associated with each reaction.  Currently two key exothermic reactions due to organic matter have been repeatedly measured in soil materials: a low temperature region (~350°C) associated with nominally ‘labile’ aliphatic compounds and lower molecular weight fragments and a higher temperature region (~550°C) associated with nominally ‘recalcitrant’ aromatic compounds and mineral stabilized C.  Working with municipal yard waste composts, whole soils, and flocs of metal coagulants with wetland-derived DOC, we are working to characterize enthalpic values of major reactions and develop relationships with other chemical properties (organic C content, C/N values, δ¹³C values, etc.).

Second derivatives of thermal traces identify fine differences between sample treatments such as chemical or physical separation, composition of organic fractions, stages of ecosystem soil development and changes in soils as influenced by different vegetation types on common soil such as revegetation of disturbed soils.  Preliminary analysis of DOC and coniferous forest soils under varying understory control management treatments show patterns of varying soil organic carbon stability. 

Future efforts will include coupling the DSC with an isotope ratio mass spectrometer for evolved gas analysis in real time during the thermal cycle to correlate the enthalpic value of each reaction with its unique δ¹³C signature.  Along with C mass data, this combination of information will facilitate greater understanding of the C cycle based on energetic inputs (chemical energy from photosynthesis) and C stabilization and associated chemical bond energy based on units that easily cross ecosystem boundaries (Joules).  Isotope analysis of other elements will also provide insight into mineral changes and their energies (¹⁸O) and N pool dynamics (¹⁵N).