Phytolith-occluded organic carbon (PhytOC) is an important long-term (up to several thousand years) terrestrial carbon (C) fraction in forest ecosystems. The objectives of this study were to (i) investigate the spatial distribution of PhytOC in Lei bamboo (Phyllostachys praecox C.D. Chu & C.S. Chao.) forests under intensive management (mulching and fertilization) and (ii) assess the role of PhytOC in C sequestration in a Lei bamboo stand and across subtropical China. Phytolith concentrations in Lei bamboo plant components were (P < 0.05) in the following order: rhizome ≈ stump > leaf ≈ branch > culm. The distribution of PhytOC in bamboo leaves, branches, culms, rhizomes, and stumps was 22.2%, 12.1%, 16.1%, 15.9%, and 33.7%, respectively. The PhytOC stock was in the following order (P < 0.05): soil (9361 kg C·ha⁻¹) > mulching materials (197.5 kg C·ha⁻¹) > belowground plant parts (13.0 kg C·ha⁻¹) ≈ aboveground plant parts (12.8 kg C·ha⁻¹) ≈ litterfall (11.3 kg C·ha⁻¹). The PhytOC accretion rate in the vegetation in the Lei bamboo stand was 19.4 kg C·ha⁻¹·year⁻¹, equivalent to 71 kg CO₂-eq·ha⁻¹·year⁻¹. The soil PhytOC stock decreased markedly with depth and had an accretion rate of 325 kg C·ha⁻¹·year⁻¹ for the 0–60 cm soil layer. Based on a PhytOC accretion rate of 0.795 Mg CO₂-eq·ha⁻¹·year⁻¹, PhytOC accretion rate in the 2.62 × 10⁶ ha of Lei bamboo stands in southern China is estimated to be 2.08 × 10⁶ Mg CO₂-eq·year⁻¹. In conclusion, intensively managed Lei bamboo stands have a large potential in long-term C sequestration in the form of PhytOC, and the PhytOC stock belowground should not be ignored due to its contribution to the ecosystem level PhytOC stock.