Topsoil Carbon Spatial Patterns and Successional Shifts in Dominant Controls in Tropical Forests on Hainan Island

Soil organic carbon (SOC) is a key component of terrestrial carbon stocks, yet its spatial variability, recovery potential, and dominant controls across tropical forest succession remain insufficiently quantified. We combined field measurements from 40 natural forest plots on Hainan Island, China, with laboratory analyses and multi-source environmental data to assess topsoil SOC (0–20 cm) distribution, recovery potential, and regulatory drivers. In natural forests on Hainan Island, topsoil SOC stocks ranged from 33.06 to 62.60 Mg · C · ha−1. Using the median (Q0.5) SOC of old-growth forests as the reference level, recovery potential ranged from 12.60 to 42.14 Mg · C · ha−1. Topsoil SOC exhibited clear spatial heterogeneity across the island, with higher values in more continuous forest areas. Secondary forests generally exhibited lower current SOC but greater recovery potential, whereas old-growth forests showed higher and more stable stocks. Multivariate analyses revealed a clear successional shift in dominant controls: total phosphorus (TP) was the primary predictor in secondary forests, while total nitrogen (TN) dominated across the full gradient, particularly in old-growth forests. These findings highlight stage-dependent SOC regulation and the critical role of soil nutrient status in shaping tropical forest carbon recovery.