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Abstract: Forest floor chemistry and microbial communities can be influenced by forest land management, such as harvesting and prescribed burning. Here, we used phospholipid fatty acid (PLFA) and multiple carbon-source substrate-induced respiration (MSIR) analyses to characterize microbial communities of deciduous, mixedwood and coniferous boreal forest floors with different silvicultural treatments. The sites were stem-only harvested with 10% retention, and silvicultural treatments consisting of slash being evenly distributed on the site and then burned, or not burned. The burned sites exhibited lower microbial biomass and greater NO3 − concentrations than the unburned sites. However, burning appeared to have no effect on forest floor microbial community structure or function. On the other hand, during drier months (August sampling), the composition of forest floor microbial communities appeared to be strongly influenced by topographic position rather than stand related differences. Harvested sites located at higher elevations had similar microbial communities, regardless of the overstory composition, while coniferous and mixedwood sites located at lower elevations had similarly structured microbial communities that were distinct from deciduous sites. Differences in microclimatic conditions of the forest floor between higher elevation sites and lower elevations sites may select for some microbial groups over others. Indicator analysis found a strong association of a fungal PLFA biomarker (20:1ω9c), with sites at higher elevation, while a biomarker for actinomycetes (10Me19:0) was strongly associated with deciduous sites at lower elevation. Structural differences in microbial communities observed between sites at higher and lower elevations appear to be linked to seasonal patterns in moisture, as previous studies in this region found no apparent effect of elevation during times of higher monthly precipitation. [Copyright &y& Elsevier]

Abstract: With the growing interest in silvicultural techniques that more closely emulate natural disturbance regimes, there is a need to better understand how partial harvesting affects the soil microbial community in stands with varying ecological characteristics, e.g., tree species composition. Four and a half and 5.5 years post-harvest, we used phospholipid fatty acid (PLFA) and substrate-induced respiration (SIR) analyses to compare the microbial biomass and microbial community structure of forest floors from stands dominated by white spruce (Picea glauca; SPRUCE) or by trembling aspen (Populus tremuloides; ASPEN) and from mixed-species (MIXED) stands in northern Alberta, Canada, that had been clearcut, partial-cut with 20% retention, partial-cut with 50% retention or left uncut (controls). PLFA and SIR analyses revealed that ASPEN forest floors supported a larger microbial biomass with a very different community structure than MIXED or SPRUCE forest floors. The microbial community structure of these soils appeared to be strongly affected by the presence of white spruce and the composition of the understory vegetation. There were no effects of timber harvesting detected within or across stand types on any of the variables measured, with the exception of the PLFA 16:1ω5, which was relatively more abundant in the clearcuts and 50% retention treatments than in the uncut controls, perhaps in response to an increased forest floor pH and grass cover in the disturbed areas. The resilience to timber harvesting of the forest floors from these stands may be the result of efforts to minimize soil disturbance during harvesting and to allow vegetation to regenerate naturally. From the perspective of the forest floor microbial community, partial harvesting does not appear to have any benefit over clearcut harvesting at these boreal forest sites. [Copyright &y& Elsevier]

The boreal forest is the single largest terrestrial store of carbon on Earth, and approximately 25% of these carbon stocks are in the forest floor. Climate change is expected to alter boreal vegetation, and aspen-dominated stands will replace conifers in Western Canada. We investigated how these vegetation shifts could affect the composition and function of soil microbial communities, using forest floor samples from the Ecosystem Management Emulating Natural Disturbance (EMEND) project in northwestern Alberta, Canada. Soil microbial communities were surveyed in rhizosphere and bulk forest floor of 17-year-old spruce clear-cuts where aspen was naturally regenerating, mature stands of aspen and spruce, and 17-year-old clear-cuts of aspen. Phospholipid fatty acid (PLFA) analysis was used to characterize microbial community composition and multiple substrate induced respiration (MSIR) to quantify microbial community functional capacity. Carbon source utilization by microorganisms was investigated through natural abundance isotope analysis of individual PLFAs. Rhizosphere samples had a significantly higher proportion of fungi and a higher gram-negative to gram-positive bacteria ratio compared to bulk soil. Fungi and gram-negative bacteria biomarkers in the rhizosphere showed 13C depletion compared to bulk forest floor, indicating that they had assimilated more newly-photosynthesized carbon than bulk forest floor microbes. Aspen trees exerted a greater influence over their rhizospheres than spruce trees in terms of microbial community composition and functional capacity, and aspen rhizospheres showed the highest basal respiration. In less than two decades, aspen regeneration in former spruce stands shifted the composition of microbial communities towards that of native aspen stands, with the rhizosphere microbiome responding more quickly than bulk forest floor. This study suggests that predicted vegetation shifts in the boreal have the potential to cause more immediate and profound changes in the rhizosphere, and emphasizes the need to include the rhizosphere in future studies. • Clear differences existed between rhizosphere and bulk soil microbial communities. • Natural abundance 13C PLFA patterns differed between rhizosphere and bulk soil. • Vegetation shift caused changes in microbial communities in less than two decades. • Rhizosphere microbes responded more readily to the vegetation shift. [ABSTRACT FROM AUTHOR]