Experimentally Determined Traits Shape Bacterial Community Composition One and Five Years Following Wildfire


Dana Johnson1*, Benjamin Sulman2, Kara Yedinak3, Jamie Woolet4, Thea Whitman1 (twhitman@wisc.edu)


1University of Wisconsin–Madison, WI; 2Oak Ridge National Laboratory, Oak Ridge, TN; 3Forest Products Laboratory, USDA Forest Service, Madison, WI; 4Colorado State University, Fort Collins, CO



Wildfires represent major ecological disturbances, burning 2 to 3% of the Earth’s terrestrial area each year with sometimes drastic effects on above- and belowground communities. Soil bacteria offer an ideal, yet understudied system within which to explore fundamental principles of fire ecology due to their vast diversity and critical ecological functions, such as organic matter mineralization. To understand how wildfires restructure soil bacterial communities and alter their functioning, the team sought to translate aboveground fire ecology to belowground systems by determining: (1) which microbial traits are important post-fire; and (2) whether changes in bacterial communities post-fire affect carbon (C) cycling. Researchers employed an uncommon approach to assigning bacterial traits by first running three separate laboratory experiments in which they directly determined which microbes survive fires, grow quickly post-fire, or thrive in the post-fire environment, while also tracking carbon dioxide emissions. Then, to evaluate the importance of each trait in structuring soil bacterial communities, the team quantified the abundance of taxa assigned to each trait in a large field dataset of soils 1 and 5 years after wildfires of varying burn severities in a globally important ecosystem—the boreal forest of northern Canada. First, analogously to fire-adapted plants, fast-growing bacteria were found to rapidly dominate post-fire soils, but, in contrast to their larger counterparts, they return to preburn relative abundances between 1 and 5 years post-fire. Although both fire survival and an affinity for the post-fire environment were statistically significant predictors of post-fire community composition, neither was particularly influential. These results suggest that uncharacterized factors, such as vegetation recovery or bacterial dispersal, may be more important for determining community composition over decadal timescales. Second, results indicate that soil C fluxes post-wildfire are not likely limited by the microbial community, suggesting strong functional resilience. From these findings, the team offers a traits-based framework of bacterial responses to wildfire, which could be of broad use to researchers seeking to understand interactions between C cycling, bacterial communities, and disturbance, while the integrated experimental-field observational approach could be translated and applied to many different systems. For subsequent experiments, researchers have returned to the region and are investigating how wildfires may change microbial C use efficiency, using both glucose and pine wood as substrates. In addition, the team is determining whether incorporating laboratory-identified traits into a biogeochemical model improves predictions of C mineralization post-fire.