Increases in global wildfires and fire severity are expected to result from global warming. Severe wildfires not only burn surface vegetation but also affect forest soil. Humic substances play key roles in the transport of nutrients and the carbon cycle in terrestrial ecosystems. In this study, we evaluated the effects of forest fires on the chemical properties of fulvic acid (FA) and humic acid (HA) extracted from non-burned and burned forest soils in Gunma, Japan. The differential thermal analysis of FA indicated that the intensity of exothermic reaction peak at 400 °C was 2-fold higher than that from non-burned soil. Based on pyrolysis-gas chromatography-mass spectrometry analysis with tetramethyl ammonium hydroxide, the amount of pyrolysate compounds in FA from burnt soil was significantly lower than that in FA from non-burnt soil. Therefore, we can conclude that the forest fire caused the significant change in the properties of FA such as increasing the aromaticity and refractory. In addition, the concentration of dissolved organic carbon with low molecular weight in surface soil increased after forest fire. This study suggests that the denaturation of soil organic matter by wildfire can affect the carbon cycle in terrestrial ecosystems.
Keywords: Fulvic acid; Humic acid; Soil organic matter; TMAH-pyrolysis-GC/MS; Three-dimensional excitation-emission matrix; Wildfires.