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 six months (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 fine root biomass was distributed throughout the 1-m soil profile, with necromass comprising approximately 60–95% of the total root mass, particularly in deeper layers. Fine root biomass was concentrated in the upper 50 cm (~50 g/m²) and gradually decreased with depth, reaching 15 g/m² in the 50–100 cm layer. Meanwhile, necromass was more pronounced in deeper soil layers, peaking at 50–75 cm at around 74 g before decreasing. 1-m fine root production (118.27 g/m² over 6 months) was lower than mortality (157.95 g/m² over 6 months), resulting in a net loss of root biomass. Production peaked at 15–30 cm (28.37 g/m² over 6 months) and was lowest at 50–75 cm (18.75 g/m² over 6 months). Mortality was highest at 0–15 cm and lowest at 75–100 cm, at 36.85 and 23.65 g/m² over 6 months, respectively. 1-m decomposition (126.41 g/m² over 6 months) was moderate but did not fully compensate for mortality, potentially leading to dead root accumulation in certain soil layers. Additionally, decomposition tended to increase in deeper layers, further influencing belowground carbon storage. These findings enhance depth-specific understanding of belowground processes, refining fine root production estimates for carbon models. Future research should investigate seasonal variations, environmental drivers, and long-term climate change effects on fine root dynamics and belowground carbon sequestration in mangrove ecosystems.