Several previous studies have examined the influence of tree species on the chemical and physical properties of soils, either in natural stands or in experimental “common gardens.” However, no study to date has assessed the impact of the largest trees on earth, giant sequoia (Sequoiadendron giganteum), on the underlying soil microbial communities. We expected giant sequoia trees to have large effects on the soil microbial communities based on our previous research showing substantial differences in the soil chemical and physical properties under giant sequoia compared to co-occurring sugar pine trees (Pinus lambertiana). To evaluate the relative impact of these trees on soils, we sampled upper mineral soils (0-5 cm) under the crown of individual mature giant sequoia trees and co-dominant, old-growth sugar pine trees in two groves within Yosemite National Park. These two groves (Mariposa and Merced) occur on similar soils (both coarse-loamy, isotic, frigid Ultic Haploxeralfs) that are derived from contrasting parent materials (metavolcanic and metasedimentary, respectively). To determine tree species effects on soil microbial community structure, microbial DNA was extracted and the 16S rRNA gene (bacteria/archaea) and the internal transcribed spacer (fungi) were sequenced on the Illumina MiSeq platform.
Although, the species richness of 16S communities was similar for each tree species within each grove (about ~6,000 OTUs identified), the 16S community structure differed significantly by tree species and grove, and there was no tree species by grove interaction. Mantel tests across all samples demonstrated that the 16S community composition correlated significantly with several soil chemical properties, including pH, carbon to nitrogen ratio, extractable “base” cations, and extractable aluminum and iron. As with 16S communities, ITS community species richness was similar for each tree species within each grove (about ~200 OTUs identified), and the ITS community structure differed significantly by tree species and grove. However, there was a significant interaction between tree species and grove for ITS communities. Mantel tests conducted across all samples showed that the ITS community composition correlated significantly with many of the same soil chemical variables that the 16S communities were correlated to, with the carbon to phosphorus ratio as an additional soil correlate. Our results suggest that individual giant sequoia trees can strongly influence the soil microbial communities underneath their crowns, and these communities are likely shaped by the contrasting chemistry of the soils that develop underneath them.