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.

北方森林是地球上最大的陆地碳储藏库，其中约有25％的碳储量位于森林地面。预计气候变化将改变北方的植被，而以白杨为主的林分将取代加拿大西部的针叶树。我们使用加拿大艾伯塔省西北部的生态系统管理模拟自然扰动（EMEND）项目的林底样品，研究了这些植被转移如何影响土壤微生物群落的组成和功能。调查了17岁的云杉阔叶林的根际和大块森林地板上的土壤微生物群落，其中白杨是自然再生的，白杨和云杉成熟的林分，以及17岁的白杨阔叶林。磷脂脂肪酸（PLFA）分析用于表征微生物群落组成，多底物诱导呼吸（MSIR）定量微生物群落功能。通过对单个PLFA的自然丰度同位素分析，研究了微生物对碳源的利用。与块状土壤相比，根际样品的真菌比例明显更高，革兰氏阴性菌与革兰氏阳性菌的比例更高。根际中的真菌和革兰氏阴性细菌生物标记物显示 与块状土壤相比，根际样品的真菌比例明显更高，革兰氏阴性菌与革兰氏阳性菌的比例更高。根际中的真菌和革兰氏阴性细菌生物标记物显示 与块状土壤相比，根际样品的真菌比例明显更高，革兰氏阴性菌与革兰氏阳性菌的比例更高。根际中的真菌和革兰氏阴性细菌生物标记物显示与大块森林地板相比，¹³ C耗竭，表明它们比大块森林地板微生物吸收了更多的新光合作用碳。就微生物群落组成和功能能力而言，白杨树对根际的影响大于云杉树，而白杨根际显示出最高的基础呼吸。在不到二十年的时间里，前云杉林分中的白杨再生使微生物群落的组成向天然白杨林分转变，根际微生物组的响应速度快于大块森林。这项研究表明，预测的北方植被变化可能会导致根际发生更直接和更深刻的变化，并强调在未来的研究中必须将根际包括在内。