Fine roots vary dramatically in their functions, which range from resource absorption to within-plant resource transport. These differences should alter resource availability to root-associated microorganisms, yet most root microbiome studies involve fine root homogenization. We hypothesized that microbial filtering would be greatest in the most distal roots. To test this, we sampled roots of six temperate tree species from a 23-year-old common garden planting, separating by branching order. Rhizoplane bacterial composition was characterized with 16S rRNA gene sequencing, while bacterial abundance was determined on a subset of trees through flow cytometry. Root order strongly impacted composition across tree species, with absorptive lower order roots exerting the greatest selective pressure. Microbial carrying capacity was higher in absorptive roots in two of three tested tree species. This study indicates lower order roots as the main point of microbial interaction with fine roots, suggesting that root homogenization could mask microbial recruitment signatures.

Plant species identity influences soil microbial communities directly by host specificity and root exudates, and indirectly by changing soil properties. As a native pioneer species common in early successional communities, Masson pine (Pinus massoniana) forests are widely distributed in subtropical China, and play a key role in improving ecosystem productivity. However, how pine forest composition, especially the dominance of plant functional groups, affects soil microbial diversity remains unclear. Here, we investigated linkages among woody plant composition, soil physicochemical properties, and microbial diversity in forests along a dominance gradient of Masson pine. Soil bacterial and fungal communities were mainly explained by woody plant community composition rather than by woody species alpha diversity, with the dominance of tree (without including shrub) species and ectomycorrhizal woody plant species accounting for more of the variation among microbial communities than pine dominance alone. Structural equation modeling revealed that bacterial diversity was associated with woody plant compositional variation via altered soil physicochemical properties, whereas fungal diversity was directly driven by woody plant composition. Bacterial functional groups involved in carbohydrate and amino acid metabolism were negatively correlated with the availability of soil nitrogen and phosphorus, whereas saprotrophic and pathogenic fungal groups showed negative correlations with the dominance of tree species. These findings indicate strong linkages between woody plant composition than soil microbial diversity; meanwhile, the high proportion of unexplained variability indicates great necessity of further definitive demonstration for better understanding of forest–microbe interactions and associated ecosystem processes.

Pine wilt disease (PWD) caused by the pine wood nematode (Bursaphelenchus xylophilus) is a serious threat to coniferous forests worldwide. However, little is known about how soil microbial diversity responds to PWD and associated management practices. We investigated the community composition and diversity of bacteria and fungi in bulk and rhizosphere soil of Masson pine (Pinus massoniana Lamb.) forests following 0, 1, and 5 year PWD, with the dead pine in a certain plot being either managed (logged and removed from the plot) or unmanaged (maintained as standing dead wood). Both bacterial and fungal alpha diversity decrease after 5 year PWD and logging, with response degree being different between site locations. Alpha diversity of rhizosphere fungi, rather than bacteria, significantly decreases with the disease and logging. We observe an increase in the relative amount of bacterial functional groups involved in carbohydrate and amino acid metabolism after PWD infection and logging practice. With the disease infection, the relative abundance of ectomycorrhizal fungi decreases, while the relative abundance of saprotrophic fungi increases. Compared with logging treatment, unmanaged practice had a weaker effect on soil microbial communities. Our findings provide new insights into the short-term responses of soil microbial diversity to management practices after PWD infection.