Legacy effects of fire continue to shape the soil microbiome in an eastern deciduous forest

Gregory Martin, Plant and Soil Sciences, West Virginia University, Morgantown, WV, Walter P. Carson, Biological Sciences, University of Pittsburgh, Pittsburgh, PA, Ember Morrissey, Division of Plant and Soil Sciences, West Virginia University, Morgantown, WV and Zachary Freedman, Department of Soil Science, University of Wisconsin, Madison, WI

Background/Question/Methods
Soils offer some of the most complex and diverse microbial habits on earth and are sensitive to disturbances, many of which are expected to increase as a result of global climate change. Within forest ecosystems, soil microbial communities perform many critical functions, including nutrient cycling and carbon storage. Furthermore, these communities can be affected by land disturbances, some of which may produce long lasting legacy impacts. We investigated how soil microbial communities are impacted by the legacy effects of three disturbances experienced by temperate forests across the eastern U.S., namely, canopy gaps, grazing by large herbivores (i.e., deer), and surface fire. To address this question, we collected soil samples from a long-term disturbance experiment implementing canopy gaps, herbivore exclusion and surface fire in the Monongahela National Forest, WV. Bacterial and fungal communities were analyzed with amplicon sequencing along with key soil parameters (e.g., soil carbon, nitrogen, ammonium, nitrate, pH).
Results/Conclusions
The legacy effects of fire emerged as a dominant force shaping soil bacterial communities as well as soil nitrogen content while the impacts of canopy gaps and herbivore exclusion were weaker and more variable. In the bulk soil, fire increased soil pH (p < 0.01), but did not have an effect on total carbon, SOM or labile carbon. Ammonium was higher in the presence of fire (p < 0.01), while nitrate/nitrite and total nitrogen were unchanged. Fire also decreased total bacterial abundance (p < 0.01) and increased both bacterial evenness (p < 0.01) and diversity (p = 0.03). While there were no clear legacy effects of fire on fungal abundance, evenness or diversity, the combination of herbivore exclusion and canopy gap creation decreased fungal evenness (p = 0.03) and diversity (p = 0.03). None of the treatments had a discernable effect on bacterial or fungal community composition. These findings suggest the long-term effects of fire outweigh those of canopy gap creation and herbivore exclusion in belowground dynamics. Additionally, carbon cycling and storage may be more resilient than nitrogen cycling to the legacy of fire. Lastly, bacterial communities may be more sensitive than fungal communities to the long-term effects of fire.