Altered Composition and Microbial versus UV-Mediated Degradation of Dissolved Organic Matter in Boreal Soils Following Wildfire

Abstract

Production, transport, and degradation of terrestrial dissolved organic matter (DOM) influence carbon (C) and nutrient cycling in both soils and downstream aquatic ecosystems. Here, we assessed the impacts of wildfire on DOM production, composition, and reactivity (biodegradation versus UV degradation) from soils of upland forest and peatland ecosystems. Soil C solubility was lowest for upland char samples, highest from surface soils in unburned spruce stands and decreased with a higher degree of peat humification regardless of fire history. Soil nitrogen (N) became relatively more soluble in both upland and peat soils post-fire, as leachate C/N decreased. Biodegradability was lower for DOM leachates from burned than unburned soils, both in upland and peatland sites. Several DOM composition indices were related to biodegradability; with the strongest relationship for specific UV absorbance at 254 nm (indicator of aromaticity). Parallel factor analysis revealed distinctive characteristics of leachates from burned soils and char that were related to low biodegradability and high UV-mediated losses. Relative to dark incubations, incubation under UV conditions led to greater C losses for highly aromatic leachates, but reduced losses for leachates with low aromaticity. This suggests that UV-mediated degradation could provide a pathway for highly stable terrestrial organic matter, including char, to become rapidly mineralized and released to the atmosphere once it reaches aquatic ecosystems in dissolved form. Together our results demonstrate that wildfire can potentially alter both turnover of DOM in terrestrial soils and linkages between terrestrial and aquatic C cycling through its influence on terrestrial DOM production and composition.