Soil microbial community response to ectomycorrhizal dominance in diverse neotropical montane forests

Abstract

Abstract Ectomycorrhizal (EM) associations can promote the dominance of tree species in otherwise diverse tropical forests. These EM associations between trees and their fungal mutualists have important consequences for soil organic matter cycling, yet the influence of these EM-associated effects on surrounding microbial communities is less well known. We examined fungal and bacterial/ archaeal community composition in surface soil samples from mixed arbuscular mycorrhizal (AM) and ectomycorrhizal (EM) stands as well as stands dominated by EM-associated Oreomunnea mexicana in four adjacent watersheds varying in soil fertility in the Fortuna Forest Reserve, Panama. We hypothesized that EM-dominated stands would support distinct microbial community assemblages relative to the mixed AM-EM stands due to differences in carbon and nitrogen cycling associated with the dominance of EM trees. We expected this microbiome selection in EM-dominated stands would lead to lower overall microbial community diversity and heterogeneity, with tighter correspondence between fungal and bacterial/ archaeal communities. We measured fungal and bacterial/ archaeal community composition via high-throughput Illumina sequencing of the ITS2 (fungi) and 16S rRNA (bacteria/archaeal) gene regions. We analyzed differences in alpha and beta diversity between stand mycorrhizal types, as well as the relative abundance of fungal functional groups and various microbial taxa. We found that fungal and bacterial/ archaeal community composition differed based on stand mycorrhizal type. There was lower bacterial/ archaeal diversity and lower relative abundance of fungal saprotrophs and pathogens in EM-dominant than AM-EM mixed stands. However, contrary to our prediction, there was lower homogeneity for fungal communities in EM-dominated stands compared to mixed EM-AM stands. Overall, we demonstrate that EM-dominated tropical forest stands have distinct soil microbiomes relative to surrounding diverse forests, suggesting that EM fungi may filter microbial functional groups in ways that could potentially influence plant health or ecosystem function.