Mangrove forests store substantial amounts of carbon, particularly in their soils, where plant roots play a crucial role in soil organic carbon accumulation. Among them, fine roots (<2 mm in diameter) contribute significantly to soil carbon storage. However, the depth-specific distribution, production, mortality, and decomposition of fine roots remain poorly understood, particularly in deeper soil layers. This study investigates fine root dynamics across a 1-m soil profile in a subtropical mangrove forest at the Gaburumata River estuary, Ishigaki Island, Japan, over a one-year period (July 2024–January 2025). Fine root dynamics were analyzed across five depth layers (0–15, 15–30, 30–50, 50–75, and 75–100 cm) using the sequential soil core method and litterbag experiments. The continuous inflow method was applied to quantify root production, mortality, and decomposition, while fine root standing mass was assessed to examine the vertical distribution of biomass and necromass. The results show that live fine roots were mainly concentrated in the upper soil layers (0–30 cm), accounting for approximately 50–70% of total fine-root biomass, whereas dead roots were more evenly distributed throughout the soil profile. Fine-root necromass dominated the fine-root pool, representing about 74–90% of total fine-root mass. More than 30% of fine root necromass decomposed within six months across all soil depths, indicating that fine-root decomposition occurs continuously throughout the soil profile, including deeper layers. Fine root production and mortality exhibited clear seasonal variation, with higher values during periods of increased rainfall and active plant growth, suggesting close links between fine-root dynamics, climatic conditions, and plant phenology. Annual fine-root net primary production (to 1 m) reached 38.2 Mg ha⁻¹ yr⁻¹, accompanied by a high turnover rate (6.5 yr⁻¹), reflecting rapid fine-root cycling in this mangrove ecosystem. Depth-related patterns showed different roles of soil layers in belowground carbon dynamics, with surface soils receiving active carbon inputs, while deeper layers contribute to carbon transformation and longer-term storage. Overall, this study highlights the importance of incorporating both fine-root decomposition and deep soil layers when estimating fine-root production, which is essential for accurately assessing root-derived carbon inputs and understanding depth-related soil carbon dynamics in mangrove ecosystems.