Alder influences microbial community composition within its rhizosphere

Background/Question/Methods: Alder trees belong to a group of angiosperms that form symbiotic relationships with the nitrogen-fixing bacteria Frankia. Although Frankia strains are able to infect all species of alder trees, species-specific associations have formed at the discretion of the tree. Additionally, alder trees have been shown to alter soil chemical properties such as soil N and C concentrations and pH, and influence the growth of neighboring plants. It is therefore apparent that alder trees have a profound influence on their Frankia symbionts, soil properties and surrounding plant communities. However, the extent to which these trees are capable of influencing general soil microbial communities remains unclear. This study uses automated ribosomal intergenic spacer analysis to characterize the bacterial and fungal community composition of soil taken from within the alder rhizosphere, underneath the alder leaf canopy, and bulk soil not in direct contact with alder, in order to investigate the ability of alder trees to influence the local soil microbial community. Statistically significant differences in community composition were determined using permutational multivariate analysis of variance. 

Results/Conclusions: Statistically significant differences were found between bacterial and fungal communities from rhizosphere, canopy, and bulk soil. The distinction between communities in direct contact with alder from those outside of contact indicates that alder trees are influencing the local soil microbial community. Non-metric multidimensional scaling revealed that canopy soil community composition was intermediate between bulk and rhizosphere soil communities, possibly indicating a decrease in alder influence with increased distance from the tree. Overall, results indicate that alder trees do influence the general soil microbial community at sites in which they establish. As microbes are responsible for nutrient cycling and decomposition processes, a better understanding of the effect of alder on soil communities will lead to a better understanding of the impacts that alder trees have on these processes.