Temporal Variation of Chemical Characteristics of Soil and Dissolved Black Carbon in Surface Soils after Wildfire.

The 2013 Rim Fire was the third largest wildfire in California history locating around the watersheds of drinking water source for 2.6 million residents near San Francisco. To understand how the wildfire affects the temporal variation of terrestrial organic matter and its potential impacts on source water, we collected 0-5 cm surface soils in wildfire affected forests in October 2013, December 2013, and August 2014, representing immediately, one-month, and ten-month after fire.  The chemical composition of soil and dissolved organic matter were characterized using Pyrolysis–gas chromatography–mass spectrometry (Py-GCMS), fluorescence excitation-emission matrix, and disinfection byproduct (DBP) formation potential. Py-GCMS data suggested that the fraction of aromatic composition, a main character of black carbon, in soil increased with time. The consistence of temporal trends in specific conductivity and dissolved organic carbon (DOC) of both field samples and laboratory sequential extractions suggested dissolution was the major mechanism on DOC and salt exports. However, the temporal variation of dissolved nitrogen and its reactivity in forming nitrogenous DBPs could not be explained by dissolution only and was likely to be affected by other biogeochemical processes such as microbial alternation. Our results indicate a potential asynchronous terrestrial input of fire-affected carbonaceous and nitrogenous DBP precursors from forest floor to the waterbody.