SoilPhysical Fractions as Unique Habitats for Bacteria

Blackwood, C.B., A.J.M. Smucker, and E.A. Paul

Presented at the ASM at Snowbird (2000-08-02 to 2017-12-05 )

Soil samples collected at normal sampling scales may contain numerous microbial habitats defined by a unique assemblage of species.  This could mask the responses of microbial communities to changes in the environment as well as cause the high species richness and evenness detected in soil bacterial ribosomal clone libraries.  We hypothesized that physical soil fractions that have been shown to differ chemically from each other would contain different eubacterial communities.  In addition, different plant communities/soil management regimes will select for a different subset of species within each habitat, amounting to unique communities within each habitat-management regime combinations.  Terminal restriction fragment length polymorphism (T-RFLP) of the 16S ribosomal gene was used to characterize dominant community members in each soil fraction sample.  There was no community differentiation between  community profiles of 0-2, 2-4, 4-6.3 mm aggregates of their internal and external layers.  Light fraction, shoot residue, and rhizosphere communities were distinct from heavy fraction communities.  Alfalfa soil eubacterial communities were different from conventional, continuous corn soil bacterial communities in all soil fractions.  Organic rotation corn soil communities were more variable and were divided between the groups created by communities under monoculture.  We have shown that soil fractions defined by unique physical and chemical characteristics can harbor unique eubacterial communities, although not all soil fractions proved to be distinct habitats as hypothesized.  The samples from soil taken under continuous plant monocultures show that the resources and disturbance regimes presented by different plant species and soil management practices can also cause divergence in species assemblages.  Both habitat diversity and plant diversity have the potential to increase bacterial species richness and evenness found in soil, and this is reflected in the increased variability in the organic rotation corn soil communities.

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