Effects of Forest Enclosure Duration on Soil Microbial Carbon Metabolic Functions in a Subtropical Deciduous Broadleaved Forest

Forest enclosure is widely used for degraded forest restoration; however, microbial functional recovery across enclosure durations in subtropical deciduous secondary broadleaved forests remains poorly understood. This study evaluated microbial carbon metabolic recovery and associated environmental factors across a chronosequence of forests enclosed for 0–1, 5–6, 11–12, 20–24, and 28–34 years, with an old-growth forest as the reference. Forest enclosure significantly enhanced microbial carbon metabolic capacity and functional diversity. Average Well Color Development (AWCD) and Shannon, Simpson, and McIntosh indices increased with enclosure duration, with the lowest values in 0–1 year stands and the highest values in old-growth forests. Enclosure duration significantly altered carbon source utilization patterns, particularly amino acids and carboxylic acids, with carboxylic acids becoming the dominant utilized carbon source category after 5–6 years of enclosure. Furthermore, Bray–Curtis distance analysis revealed progressive convergence of microbial carbon-use profiles toward the old-growth reference state with increasing enclosure duration (Spearman’s ρ = −0.61, p = 0.0155). Soil carbon, nitrogen, moisture, bulk density, pH, and microbial diversity were significantly associated with these functional changes. These findings highlight microbial carbon-use profiles as effective indicators of soil functional recovery and provide insights for incorporating microbial traits into future forest restoration and management strategies.