Bacterial Abundance in Dry Montane Forest Soils

All forest fi re events generate some quantity of charcoal, which may persist in soils for hundreds to thousands of years. However, few studies have eff ectively evaluated the potential for charcoal to infl uence specifi c microbial communities or processes. To our knowledge, no studies have specifi cally addressed the eff ect of charcoal on ammonia-oxidizing bacteria (AOB) in forest soils. Controlled experiments have shown that charcoal amendment of fi re-excluded temperate and boreal coniferous forest soil increases net nitrifi cation, suggesting that charcoal plays a major role in maintaining nitrifi cation for extended periods postfi re. In this study, we examined the infl uence of fi re history on gross nitrifi cation, nitrifi cation potential, and the nature and abundance of AOB. Soil cores were collected from sites in the Selway-Bitterroot wilderness area in northern Idaho that had been exposed twice (in 1910, 1934) or three times (1910, 1934, and 1992) in the last 94 yr, allowing us to contrast soils recently exposed to fi re to those that experienced no recent fi re (control). Charcoal content was determined in the O horizon by hand-separation and in the mineral soil by a chemical digestion procedure. Gross and net nitrifi cation, and potential rates of nitrifi cation were measured in mineral soil. Analysis of the AOB community was conducted using primer sets specifi c for the ammonia mono-oxygenase gene (amoA) or the 16S rRNA gene of AOB. Denaturing gradient gel electrophoresis was used to analyze the AOB community structure, while AOB abundance was determined by quantitative polymerase chain reaction. Recent (12-yr-old) wildfi re resulted in greater charcoal contents and nitrifi cation rates compared with sites without fi re for 75 yr, and the more recent fi re appeared to have directly infl uenced AOB abundance and community structure. We predicted and observed greater abundance of AOB in soils recently exposed to fi re compared with control soils. Interestingly, sequence data revealed that Clusters 3 and 4, and not Cluster 2, of genus Nitrosospira dominated these forest soils, with a shift toward Cluster 3 in recently burned sites. Wildfi re and Charcoal Enhance Nitrifi cation and Ammonium-Oxidizing Bacterial Abundance in Dry Montane Forest Soils