Abstract: Changes in soil organic carbon in tropical secondary forests are critical to global carbon balance, with soil fungi playing a key role in soil carbon decomposition. However, the effect of soil fungal communities on the stability of soil organic carbon during the restoration of tropical secondary forests remains unclear. We investigated the secondary forests at the early (<30 years), middle (>40 years), and late (>70 years) stages of restoration and the old-growth forests (>300 years) in the Bawangling Branch of National Park of Hainan Tropical Rainforest, Hainan Island, China. Soil samples were collected from the humus (Ah) and eluvial (E) horizons. The chemical composition of soil organic carbon was analyzed using Fourier transform infrared spectroscopy. We further examined the relationships of fungal community diversity, composition, and network stability with organic carbon chemical stability. The results showed that total organic carbon content remained stable during forest restoration. However, polysaccharide carbon and aromatic carbon contents increased initially and then decreased, whereas aliphatic carbon showed an opposite trend. The chemical stability of soil organic carbon was highest in the middle-stage secondary forests. The Ah horizon exhibited significantly higher total organic carbon and its chemical fractions than the E horizon. While secondary forest restoration did not affect soil fungal alpha diversity, fungal richness declined markedly with increasing soil depth. Molecular ecological network analysis showed that the stability of the fungal community network increased with forest restoration and soil depth. Keystone fungal species were primarily ectomycorrhizal and saprophytic fungi, including Russula, Inocybe, Sebacina, Entoloma, Penicillium, Aspergillus, and Mortierella. Structural equation modeling demonstrated that the restoration of tropical secondary forests mainly regulated the stability of organic carbon by influencing the keystone fungal species. These findings provide insights into the microbial mechanisms regulating soil organic carbon dynamics during tropical secondary forest restoration and provide theoretical support for the ecological management of tropical secondary forests.

Key words: tropical lowland secondary forest, soil fungal community, molecular ecological network, soil organic carbon chemical fraction, soil organic carbon stability