Tree‐induced alterations to soil properties and rhizoplane‐associated bacteria following 23 years in a common garden

Key functional root traits, including mycorrhizal association and root diameter, can help project ecosystem processes like root turnover and soil carbon sequestration. It is less clear, however, how such traits relate to variations in soil biology and chemistry. Here, we examined the impact of tree species with varied root traits on soil properties and rhizoplane bacterial composition, focusing specifically on mycorrhizal association type and root diameter. Within a long-term common garden in central Pennsylvania, USA, we selected three arbuscular mycorrhizal (AM) and three ectomycorrhizal (EM) tree species and assessed changes in (1) soil and leaf chemistry and (2) bacterial composition along fine absorptive roots, through 16S rRNA gene sequencing. AM trees increased soil pH and soil available nitrogen relative to EM trees, and mycorrhizal association type was significantly associated with rhizoplane-associated bacterial composition. Absorptive root diameter did not clearly explain soil variability but was associated with changes in the composition of rhizoplane-associated bacteria. Tree-mediated shifts to soil properties were linked to mycorrhizal association type, whereas rhizoplane recruitment of bacteria was linked to both mycorrhizal type and root diameter. This has implications for predicting changes in biogeochemical processes following shifts in tree species composition.