Forest type and stand age co-regulate iron-associated carbon and microbial life-history strategies in red soils

Microorganisms are important regulators of soil organic carbon (SOC) accumulation and stabilization, which is modulated by soil minerals, especially iron (Fe) oxides. Here, we analyzed soils from different forest types (coniferous forests, mixed forests, and broad-leaved forests) and stand ages (40-, 20-, and 10-year) in a subtropical acidic red soil region. SOC peaked at 20 years and declined by 40 years, while decreasing with soil depth. Free iron oxides (Fed) content was higher in mixed forests and broad-leaved forests than in coniferous forests, and iron-bound organic carbon (Fe-OC) showed the same pattern among forest types, with Fe-OC dominated by coprecipitation (C:Fe molar ratio >6). Bacterial communities shifted from r- to K-strategists with stand age, with K-strategists (e.g., Chloroflexi) dominating overall and r-strategists (e.g., Gemmatimonadota) enriched in mixed forests. Fungal richness was highest in 40-year broad-leaved forests. Functional predictions suggested potential shifts in microbial functional traits across forest types: coniferous forests stabilize carbon via high-quality litter in oligotrophic conditions; mixed forests showed acidophilic bacteria negatively correlated with Fed, inhibiting Fe-OC dissociation; and broad-leaved forests enhanced Fe-OC coprecipitation via unspecific monooxygenase. Structural equation modeling (SEM) analysis showed that Fed accumulation strongly promoted Fe-OC synthesis. Correlation analysis further indicated that SOC was significantly positively correlated with Fed, carbon fractions, and microbial activity (p < 0.05). In conclusion, plantations in the red soil region synergistically enhance soil carbon sequestration potential by regulating iron oxide dynamics and microbial functions, and deep soils, especially 40-year mixed forests, harbor substantial carbon sequestration capacity.