PNNL

Abstract

• Traditionally, fine roots were grouped using arbitrary size categories, rarely capturing the heterogeneity in root physiology, morphology and functionality among different fine root orders. Fine roots with different functional roles are rarely separated in microbiome-focused studies and may result in confounding microbial signals and host-filtering across different root microbiome compartments. • Using a 26-year common garden experiment, we sampled fine roots from four temperate tree species that varied widely in root morphology and mycorrhizal type and sorted them into absorptive and transportive fine roots using root branching order. The rhizoplane and rhizosphere root compartments were characterized using 16S rRNA gene and ITS region amplicon sequencing, and shotgun metagenomics for the rhizoplane to identify potential microbial functions. Fine roots were subject to metabolomic analyses to spatially characterize resource availability. • We observed differences between the bacterial rhizoplane and rhizosphere compartments for absorptive but not transportive fine roots. Rhizoplane bacteria, as well as the root metabolome and potential microbial functions, differed between absorptive and transportive fine roots, but not the rhizosphere bacteria. Functional differences were driven by sugar transport, peptidases and urea transport. • Our data highlights the importance of root function when examining root-microbial relationships, emphasizing different host selective pressures imparted on different root microbiome compartments.