Fluorescence and Absorbance Indices for Dissolved Organic Matter from Wildfire Ash and Burned Watersheds

Wildfires generate significant amounts of ash and burned soils that can leach altered dissolved organic matter (DOM) to watersheds. In this work, we analyzed the absorbance and fluorescence spectrum of DOM leached from 40 ash and soil samples collected from two conifer forest burn scars. DOM fluorescence quantum yield at 350 nm (ϕf350) was elevated in all ash and burned soil leachates. Wildfire DOM fluorescence was also consistently shifted toward the ultraviolet (UV) region relative to unburned materials, which limited the utility of several pre-defined optical indices, such as the humification index (HIX) and SUVA254, for wildfire DOM differentiation. In contrast, E2:E3 and ϕf350 resolved differences between ash, burned mineral soil, and unburned soil DOM across an assortment of physical settings. Mixtures of a freshwater DOM reference material and wildfire ash leachate confirmed that specific fluorescence peaks A and C, ϕf350, and E2:E3 were well correlated (R2 > 0.92) to mass increases of ash-derived DOM. These findings were compared to 31 surface water samples collected from burned tributaries within the Cameron Peak Fire burnscar. Surface waters draining from the burn scar were also elevated in ϕf350, E2:E3, SUVA254, FI, and UV-shifted fluorescence peaks A and C at intensities comparable to ash mixing experiments.