The legacy of severe fire on carbon dynamics within montane watersheds: linking soils to streams

Abstract:

The warming climate will continue to increase the size and severity of wildfires, particularly in the Intermountain West. Predicting how fire alters the net ecosystem carbon balance requires an understanding of how carbon is stored, processed, and transferred in both terrestrial and aquatic ecosystems. In-situ sensors, weekly sampling, and incubations within five montane watersheds with varying fire history reveal that burned landscapes export a greater fraction of soil organic matter (SOM) to streams and the atmosphere. Laboratory and in-field incubations reveal differences in the processing of organic matter within the terrestrial and aquatic ecosystems, respectively. Both soil organic matter (SOM) and dissolved organic matter (DOM) exhibited a general increase in bioavailability within burned watersheds, even 17 years post burn. Analysis of the aquatic and soil microbiomes (16S rRNA) reveal greater richness and diversity within unburned systems, suggesting different functional potentials for carbon cycling and potentially more retention of soil C. In contrast, increased C bioavailability, combined with greater water yield in burned systems, results in greater lateral losses to aquatic ecosystems. Stable isotope analysis reveals a dominant terrestrial signal in stream inorganic C exports, specifically the respiration of SOM, within all watersheds. Isotopic evidence illustrates greater variability in sources within the reference watersheds over the summer; likely due to shifting flow paths associated with increased evapotranspiration within the forest, rather than grass dominated landscapes as the growing season progressed. Collectively our data suggests that fire can increase the relative export of C from terrestrial ecosystems for decades, essentially transferring a greater fraction of terrestrial net ecosystem production to aquatic ecosystems for further export downstream and to the atmosphere as CO₂.