Abstract: Aims Individual size reflects the radial growth of trees, while fine roots are highly sensitive to changes in the rhizosphere soil environment. Investigating the relationship between functional traits of fine root and the microenvironment of rhizosphere soil in trees at different sizes can contribute to elucidating interaction mechanisms within the forest subsurface ecosystem of trees at the individual level. Methods In this study, fine roots and rhizosphere soil sampling were conducted based on Machilus pauhoi individuals of different sizes in an 11-year-old plantation. The aim is to analyze the relationship between fine root functional traits, rhizosphere soil nutrients contents, microbial community structure, and enzyme activities of M. pauhoi individuals of different sizes. Important findings The results indicate that: 1) There are differences in fine root functional traits and rhizosphere microenvironment among trees of different sizes. Significant differences were observed in traits such as fine root biomass, specific root length, root length density, root volume density, root tissue density, root nitrogen content and root phosphorus content. Except for the specific root area and root tissue density, other fine roots functional traits were highest in the medium-sized individuals, and rhizosphere soil carbon, nitrogen, and phosphorus content were also highest in medium-sized M. pauhoi. 2) Variability coefficients of fine root traits such as specific root length, root volume density, fine root biomass, and rhizosphere soil fungal and actinomycete contents were relatively large among M. pauhoi individuals of different sizes, particularly pronounced in medium-sized individuals and relatively low in small-sized individuals. All individuals tend to adapt to environmental changes by adjusting traits such as root volume density, fine root biomass, rhizosphere soil fungal contents and nitrate nitrogen content. 3) The fine roots at different tree sizes adopted different resource utilization strategies. Medium-sized M. pauhoi exhibited larger specific root length, root nitrogen content, and root phosphorus content, indicating a resource acquisition strategy to optimize nutrient acquisition capacity. Small-sized individuals with larger root tissue density adopted a resource conservation strategy to enhance their ability to cope with environmental stress, while large individuals exhibited a balanced strategy between above-ground and below-ground growth. 4) Soil total carbon content, microbial biomass carbon content, actinomycete contents, ammonium nitrogen content, acid phosphatase activity, and microbial biomass nitrogen content in the rhizosphere soil microenvironment were identified as the main factors influencing fine root functional traits of M. pauhoi. The relationship between fine root functional traits and rhizosphere microenvironment varied among individuals of different sizes. Fine roots of small-sized M. pauhoi individuals were mainly affected by rhizosphere soil nutrients, while those of medium-sized M. pauhoi individuals were primarily influenced by the content of actinomycetes and acid phosphatase activity in the rhizosphere soil microbial community structure. Fine root traits of large-sized individuals were affected by both rhizosphere soil nutrients and soil microbial community structure, including bacteria and actinomycetes content. The finding provide theoretical insights for implementing afforestation of M. pauhoi on a microtopographic scale, precisely developing artificial forest tending and thinning measures, and cultivating large-diameter timber plantations. Moreover, they offer important reference values for exploring the relationship between plants and microtopographic environments based on intra-specific or individual variation.

Key words: fine root function trait, rhizosphere soil, soil microbial community structure, soil enzyme activity, tree size, Machilus pauhoi plantation