Changes in the chemical composition of soil organic matter including water-soluble component during incubation: A case study of coniferous and broadleaf forest soils

The objective of this study was to clarify what changes occurred in the amount and chemical properties of the soil organic matter (SOM) and dissolved organic matter (DOM) in the soil following isothermal incubation for 170 days. In this study, the soil samples were collected from coniferous and broadleaf forests in Toyama, Japan. The changes in the amount of carbon dioxide generation, the composition of the organic components of the SOM and its chemical properties during the incubation period were investigated. Likewise, DOM was extracted and examined for changes in chemical properties. For that purpose, pyrolysis-gas chromatography-mass spectrometry using tetramethylammonium hydroxide (TMAH-Py-GC/MS), absorption spectrophotometry, and fluorometry were used. From the amount of CO2 generated during incubation, it was found that 10%-15% of the SOM was mineralized. More CO2 was found to have been generated by the soil of the coniferous forests than by the soil of the broadleaf forests, probably because the former was more strongly affected by temperature changes. The DOM increased during the incubation period by day 55 and then sharply decreased. The absorption and fluorescence spectra showed that the DOM in the soil contained large amounts of humic substances after 97 days of incubation, which comprised organic compounds that did not readily decompose.