Pyrolysis of Soil Organic Matter

Organic matter extracted from O2 and Bh horizons of a Podzol soil was pyrolyzed between room temperature and 540°C. Samples were withdrawn at regular temperature intervals and analyzed by chemical and spectroscopic methods. The carbon content of the chars increased with temperature accompanied by a simultaneous decrease in oxygen. The chars of both organic matter preparations heated to 540°C. contained identical percentages of carbon and hydrogen but no oxygen. Some of the nitrogen and sulfur in the original organic matter was very stable and was recovered in the chars heated to the highest temperature. Phenolic OH groups were more stable than COOH groups but both were eliminated between 250° and 400°C. Both types of functional groups were more stable in the Bh horizon organic matter than in O2 horizon organic matter, apparently because the former was more aromatic. The infrared spectra of the chars were in general agreement with the chemical data. On the basis of the analytical data the low temperature peaks in the DTG curves of both organic matter preparations were due mainly to the elimination of functional groups, whereas the high‐temperature maxima resulted from decomposition of the nuclei. Using the graphical‐statistical method of Van Krevelen, the main reactions governing the pyrolysis of O2 horizon organic matter were found to be: (a) dehydrogenation (up to 200°C.); (b) a combination of decarboxylation and dehydration (between 200 and 250°C.); and (c) continuous dehydration. The main reaction determining the pyrolysis of Bh horizon organic matter was dehydration.