Root diameter and depth significantly affect bacterial community composition and network dynamics. In black locust (_{Robinia pseudoacacia} L.), bacterial alpha diversity decreased with root diameter, with coarse roots (>2 mm) showing fewer orders but higher relative abundance. Fine roots (1 mm) had more complex and stable rhizosphere networks, while intermediate-diameter roots (1–2 mm) had a high proportion of keystone taxa, suggesting a keystone-reliant strategy.