Ash trees (Fraxinus spp.) have suffered catastrophic losses (>99% mortality) due to the infestation of the emerald ash borer (EAB), an invasive wood-boring beetle. Ash trees are typically dominant in riparian forests and have been suggested to be a foundational species filling in after the loss of American elm to Dutch elm disease. Effects of ash mortality and the belowground microbial associations of ash are understudied despite the importance of soil microbial communities to soil ecological function and integrity.
To characterize potential associations between ash trees and soil bacterial communities we sampled soils from 11.2m radius plots with (n=23) and without (n=9) ash trees across 7 forests in central Ohio. Tree species were censused and ash tree condition was gathered at each plot where 0-10cm soil cores were randomly sampled. DNA was extracted from soils and paired-end sequencing of 16S rRNA amplicons was conducted using Illumina MiSeq. Bacterial relative abundances, community structure, and functional potential was revealed using QIIME and PICRUSt metagenomic analysis software.
Ash trees associate with a unique belowground community. Ash and non-ash plots differed significantly in overall bacterial community structure (PERMANOVA; p=0.002). The most abundant bacterial phylum, Acidobacteria, had significantly higher relative abundance in non-ash plots (Welch’s T-test; p<0.001), and of the remaining 6 most abundant phyla, all had significantly lower abundances in non-ash plots (p<0.05). Soil pH was significantly lower in non-ash plots (p<0.001) and was revealed through redundancy analysis (RDA) to be a major driver of bacterial abundance, explaining the greater abundance of the acidophilic Acidobacteria. The presence of ash may directly affect soil pH through cation accumulation, and thus indirectly influence bacterial community structure. Functionally, these community differences in non-ash plots significantly increased the genetic potential for carbohydrate (p<0.001) and nitrogen (p<0.01) metabolic pathways. Increases in the relative abundance of these genes could indicate higher rates of decomposition and nitrogen transformations in future non-ash forests. Establishing a benchmark for ash tree associations with soil bacterial communities will help to better understand changes in ecosystem dynamics following the anticipated loss of ash tree species.