Long-term Impacts of Slash Pile Burning on Soil Biogeochemistry Provide Insights into Carbon Cycling Changes with Increased Wildfire Severity.

Forest vegetation and soils act as major global carbon (C) sinks. Increasing fire frequency and severity threatens the efficiency of forest C storage, especially where this altered disturbance regime results in a vegetation shift. Here we use slash pile burns, a common forest management practice, as a surrogate of severe wildfire to examine the ecological feedbacks between forest vegetation, soil, and microbial communities. We examined soils from a 60-year chronosequence of burn pile scars and paired surrounding regenerating lodgepole pine forest in the Medicine Bow-Routt National Forest, Colorado. Soils in slash burn pile scars had approximately 20% less C and generally lower microbial activity, with 40% lower respiration rates, compared to soils from their clear-cut harvest counterpart. In regenerating forest soils, respiration rates increased with greater amounts of soil C and higher C:N ratio of soil organic matter (SOM); these relationships were not found in burn pile soils. Tree recovery lags behind adjacent clear-cut areas, with woody vegetation often absent even 60 years after burning. Non-linear trends in forest revegetation are reflected in bulk soil characteristics. However, while soil characteristics and chemistry are correlated with respiration rates and SOM bioavailability within revegetating soils, they are not predictive of these rates in burn pile soils. This suggests that severe fire alters the fate of newly fixed C in the ecosystem, by not only shifting vegetation and soil recovery but also by altering the interactions between the microbiome and SOM, thus changing soil C sequestration. Used as a proxy for high burn severity, results from this chronosequence indicate that lodgepole pine forests of the Rocky Mountain West are vulnerable to permanent stand structure shifts and reduced C sequestration given wildfire trends.