Microbial communities in a boreal forest podzol profile and responses to the presence of Pinus sylvestris seedling roots

Abstract: The boreal biome consists of coniferous forests that cover much of the northern hemisphere, and constitute an important natural resource for the forest industry, but has also gained attention in the light of global warming in acting as potential sinks of atmospheric carbon dioxide. Both forest production and carbon sequestration are closely connected to the activities of soil microorganisms, which play major roles in nutrient cycling and decomposition of plant biomass. Boreal forest soils are commonly classified as podzols, which are characterized by their clear horizon development with depth. Ecological surveys of soil biota commonly focus on the topsoil. The aim of this experiment was to examine if the organic, eluvial and illuvial horizons of a Swedish boreal forest podzol harboured distinct communities of fungi, bacteria and archaea, and further if roots of Scots pine seedlings, through input of rhizodeposition, would induce shifts in microbial communities in the different soil categories. The study was conducted as a microcosm experiment in combination with denaturing gradient gel electrophoresis (DGGE) fingerprinting of phylogenetic marker sequences, ITS rDNA for fungi and 16S rDNA for archaea and bacteria. Soil sampled at the time of experimental set-up served as control for community shifts caused by incubation time alone. All soil horizons harboured distinct communities of fungi, bacteria and archaea. Fungi and bacteria displayed complex community profiles for all soil samples, but richness for both groups tended to be highest in the organic soil. The archaeal community profiles were relatively simple for all soil samples, though their richness appeared to be highest in the eluvial soil. Fungi was the microbial group that displayed the strongest response to the presence of Scots pine seedling roots, and ectomycorrhizas were observed in all microcosms with seedling, although the organic, eluvial and illuvial horizons harboured distinct morphotypes. In contrast, neither bacteria nor archaea responded much to the presence of Scots pine seedlings, but the bacterial communities underwent a clear shift in composition during the incubation period, especially in the organic soil. These results highlight the importance of including all layers in a soil profile to obtain a holistic view on the ecology of soil microorganisms and their roles in ecosystem functioning.