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

Abstract

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 (R 2 > 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.

Temperature Thresholds of Pyrogenic Dissolved Organic Matter in Heating Experiments Simulating Forest Fires

“…In addition, based on the small heating temperature interval, we found that the fluorescence index (FI) increased, the biological index first increased and then decreased, and the humification index (HIX) first decreased and then increased with heating temperature (Figure S3). Such results complemented a recent study that found wildfires elevated the FI and HIX values of DOM …”

Effects of Environmental Aging on Wildfire Particulate and Dissolved Pyrogenic Organic Matter Characteristics

“…GDS-1 shows the lowest yield (1.1 g DOC kg –1 char C) followed by GDS-4 (2.1 g DOC kg –1 char C) and then GDS-30 (3.4 g DOC kg –1 char C). The low DOC yields are similar to those previously observed for laboratory-produced , and natural , chars and reflect the dominance of low solubility ConAC structures in the solid char samples.…”

Experimental Open Air Burning of Vegetation Enhances Organic Matter Chemical Heterogeneity Compared to Laboratory Burns

“…Dissolved organic carbon (DOC) and total dissolved nitrogen (TDN) were simultaneously measured on a Shimadzu TOC-L Total Organic Carbon Analyzer in precombusted amber vials within a week of leaching and filtering. Leached C and N in mg g C –1 were calculated as in eq of Fischer et al: Leachable .25em DOC .25em ( mg g C − 1 ) = DOC .25em ( mg L − 1 ) × leaching .25em volume .25em ( L ) / mass .25em of .25em dry .25em material .25em ( g ) × normalC .25em content .25em of .25em dry .25em material .25em ( mg C g − 1 ) …”

“…Dissolved organic carbon (DOC) and total dissolved nitrogen (TDN) were simultaneously measured on a Shimadzu TOC-L Total Organic Carbon Analyzer in precombusted amber vials within a week of leaching and filtering. Leached C and N in mg g C –1 were calculated as in eq of Fischer et al: …”