Soil Organic Carbon and Aggregate Associated Changes in Three Subtropical Evergreen Forest Ecosystems of China

The stability of soil organic carbon (SOC) affects long-term carbon storage in forest ecosystems. However, the knowledge about SOC stability and carbon mineralization in subtropical ecosystems, particularly in the Dinghushan region, Guangdong, China, still needs to be explored. Therefore, the experiment was conducted in three distinct forest types (coniferous forest (CF), mixed coniferous broad-leaved forest (MCBF), and monsoon evergreen broad-leaved forest (MEF) at two soil depths (0–25 cm and 25–50 cm) to assess soil aggregate distribution, aggregate-associated SOC, SOC mineralization, and SOC fractions. The results indicated that MEF had the highest aggregate-associated SOC levels, surpassing MCBF and CF by 32.87% and 18.38% (within 2 mm soil aggregates); 52.48% and 18.39% (within 0.25 mm soil aggregates), 46.11% and 3.69% (within > 0.25 mm soil aggregates); and 57.12% and 11.73% (within > 0.053 mm soil aggregates), respectively. Moreover, MEF demonstrated the highest cumulative carbon mineralization rate, surpassing MCBF and CF by 28.35% and 12.71%, respectively. The SOC fractions, i.e., C1: very labile fraction, C2: labile fraction, and C3: less labile fraction were higher in MEF, surpassing mixed MCBF and CF by 18.92% and 4.51% (C1), 76.84% and 48.15% (C2), and 29.92% and 23.14% (C3), respectively. Conversely, the highest non-labile carbon (C4) was observed in the CF, with increments of 6.65% and 22.92% relative to MEF and MCBF. Overall, the study explored soil aggregate stability and SOC mineralization in the Dinghushan region, providing insights into SOC stability to guide the conservation and restoration of these forest ecosystems.