Bacterial composition of soils in ponderosa pine and mixed conifer forests exposed to different wildfire burn severity Carolyn F. Weber * , J. Scott Lockhart, Emily Charaska, Ken Aho, Kathleen A. Lohse Idaho State University, Department of Biological Sciences, Pocatello, ID 83209, USA article info Article history: Received 9 March 2013 Received in revised form 21 September 2013 Accepted 12 November 2013 Available online 26 November 2013 Keywords: Las Conchas fire Wildfire 16S rRNA gene sequencing Soil microbiology Disturbance ecology Ponderosa pine abstract Soil microbial communities influence the rate and trajectory of ecosystem recovery after wildfire, but how their composition varies with burn severity in different vegetation types is largely unknown. This study utilized high throughput amplicon sequencing of a bacterial 16S rRNA gene fragment to determine the bacterial community composition in soils that were unburned, moderately burned (“low burn”) and severely burned (“high burn”) in ponderosa pine (‘P’) and mixed conifer (‘M’) forests, three months after the Las Conchas fire (New Mexico, USA; July 2011). Community composition was distinct in unburned M and P soils, but it was similar in high burn soils, despite differences in initial and post-burn M and P soil parameters (i.e. pH, moisture, organic matter, carbon and nitrogen content), which are known to correlate with shifts in bacterial community composition. Richness tended to be lower in the high burn M soils relative to unburned M soils, while it was similar across all P soils. Collectively, our findings indicate that high burn severity may result in bacterial communities shifting to similar compositions within a few months post-fire, even if the initial communities, as well as initial and post-burn soil physical and chemical properties are distinct. Published by Elsevier Ltd. 1. Introduction The frequency and severity of wildfires in the Western U.S. have significantly increased over the last 25 years (Conard et al., 2001; Dombeck, 2001; Grissino-Mayer and Swetnam, 2000; Westerling et al., 2006). These changes have been attributed to both land management practices and climate change. Long-term fire sup- pression has led to biomass and organic matter accumulation that fuel wildfires of increased size, duration, and frequency that release large pulses of nutrients into the environment (Neary et al., 1999). The impacts of wildfire on overall ecosystem recovery and func- tioning, particularly nutrient cycling, are intimately tied to the composition and structure of soil microbial communities. As a result, there is an increased need for a thorough understanding of the resistance and resilience of soil microbial communities to climate change and associated disturbances, such as wildfire, and how this may influence the rate and characteristics of ecosystem recovery (Allison and Martiny, 2008; Bardgett et al., 2008; Fierer et al., 2010). Previous studies have documented the impacts of wildfire on soil physical and chemical properties including increased soil pH, increased soil hydrophobicity and decreased soil structural stability (Certini, 2005; MacDonald and Huffman, 2004) as well as a reduction in the quantity and quality of soil organic matter (Certini, 2005; Neary et al., 1999). Additionally, organic nitrogen (N) may be volatilized or converted to inorganic forms through mineralization. Mineralization can increase biologically available N, but if it is not consumed, large fluxes from ecosystems can occur. Immediately after a fire, the predominant form of inorganic N is ammonium, a direct product of burning, which is subsequently converted to ni- trate through the process of nitrification (Certini, 2005; Johnson et al., 2007; Neary et al., 1999; Prieto-Fernandez et al., 1998; Smithwick et al., 2005). Soil microbial communities play a critical role in carbon (C) and N cycling. Thus, identifying associations between wildfire severity and community characteristics provides a foundation for gaining insights into the possible impacts of fire on ecosystem functioning. Surface soil temperatures in wildfires typically exceed 200  C causing a substantial decrease in total microbial biomass and changes in soil physical and chemical characteristics that influence microbial community composition (Dumontet et al., 1996; Certini, 2005; Mabuhay et al., 2006; Hamman et al., 2007). To date, studies have documented that post-fire microbial communities are often less diverse and their composition may be influenced by * Corresponding author. Idaho State University, Department of Biological Sci- ences, 921 S. 8th Avenue, Mail Stop 8007, Pocatello,ID 83209, USA. Tel.: þ1 208 282 2149; fax: þ1 208 282 4570. E-mail address: webecaro@isu.edu (C.F. Weber). Contents lists available at ScienceDirect Soil Biology & Biochemistry journal homepage: www.elsevier.com/locate/soilbio 0038-0717/$ e see front matter Published by Elsevier Ltd. http://dx.doi.org/10.1016/j.soilbio.2013.11.010 Soil Biology & Biochemistry 69 (2014) 242e250