Thermal transformation of organic matter and impacts on water quality in fire-affected Jarrah Forest

Abstract

Fire in forested catchments significantly alters organic matter fluxes by generating dissolved organic matter (DOM) different from that generated under non-fire conditions. Elucidating the composition of DOM is key to understanding its persistence in the post-fire environment. This laboratory study aimed to establish a relationship between DOM quantity and quality with aspects of fire regime. Soil and litter samples were collected from areas with different burn histories (described as Time Since Last Fire – TSLF). Each sample was subjected to burn temperature treatments simulating different burn severity regimes in a muffle furnace (at 250°C – low severity, 350°C – moderate severity, and 500°C – high severity), and compared with reference unburned samples. Leachates were extracted from the control and treated samples. Analysis of DOC concentrations and characterization of DOM using fluorescence spectroscopy in the extracted leachates support the hypothesis that moderate burn severity generated the highest concentrations of DOM. For forest litter, increasing burn temperature reduced mean DOC from 7.93, 4.48, 0.64, and 0.53 mg/L per g of litter across unburned, 250°C, 350°C, and 500°C treatments respectively. Similarly, for soil, the mean DOC concentration ranged from 0.56, 1.30, 1.09, and 0.24 mg/L per g of soil. Simple organic compounds are lost during thermal degradation, enriching condensed aromatic compounds in both litter and soil samples indicated by increasing relative concentration of C1 (humic-like DOM constituents) at all burn temperatures. However, such phenomena could not be identified using commonly applied bulk normalised absorbance measurements such as SUVA254. In addition, in recently burned areas, soil at moderate burn severity generated high DOC concentrations dominated by labile DOM, while such an effect was not observed with litter. The results of this work show for the first time that management burns, which are widely used in southwestern Australia, will have a greater influence on DOM than previously realized.