Optical properties and photobleaching of wildfire ashes aqueous extracts†

Abstract

Wildfires can severely degrade soils and watersheds. Post-fire rain events can leach ashes and altered dissolved organic matter (DOM) into streams, impacting water quality and carbon biogeochemistry. The photochemical properties and persistence of DOM from wildfire ash leachates are not well understood. To establish a range of properties, wildfire DOM leachates were generated from (i) surficial [grey and black] wildfire ashes, (ii) mineral soils below ash, and (iii) unimpacted soils from two Colorado wildfire scars. Subsequently, the leachates were studied under simulated sunlight. Photochemical properties of absorbance, fluorescence and ¹O₂ quantum yield (Φ_F and Φ_¹O2) were determined for thirteen wildfire leachates. Φ_¹O2 of ash leachates was greatest (7.6 ± 3.4%), followed by underlying mineralized soil leachates (4.6 ± 0.7%), and control soil leachates (Φ_¹O2 = 3.9 ± 1%). Correlations between increasing E₂ : E₃, Φ_F, Φ_¹O2 suggest that surface ash leachates with elevated molar absorptivity may play an important role in ¹O₂ production that is not well documented. Interestingly, photobleaching experiments comparing ash DOM to unimpacted soil DOM revealed ash leachates lost fluorescence, absorbance, while producing CO₂ at rates ∼3 fold greater than soils. This suggests that aromatic features of ashes may cause degradation of wildfire DOM faster than unimpacted DOM in the environment.