Forest Type Regulates Soil Aggregate Stability and Soil Organic Carbon Stabilization in Subtropical Plantations

The influence of forest type on soil aggregates distribution, stability, and the contribution of aggregate-associated carbon (C) to bulk soil organic carbon (SOC) remains poorly understood. This may be crucial for the accumulation and persistence of SOC in subtropical ecosystems. In this study, we examined soil aggregate distribution and stability at two depths (0–15 and 15–30 cm) in 10-, 20-, and 30-year-old Cryptomeria japonica (Japanese cedar) and Chimonobambusa quadrangularis (square bamboo) plantations. We further assessed the contribution of carbon (C) associated with distinct aggregate fractions to bulk SOC. Across all stand ages and soil depths, macroaggregates accounted for 19–56% of total soil aggregates in Japanese cedar plantations, whereas their proportion was 30–337% higher in square bamboo plantations. In contrast, fine aggregates constituted 3–67% of total aggregates in Japanese cedar plantations, but were 29–94% lower in square bamboo plantations than in Japanese cedar plantations. Compared with Japanese cedar plantations, aggregate mean weight diameter (MWD) and geometric mean diameter (GMD) increased by 17–88% and 35–152%, respectively, in square bamboo plantations. In Japanese cedar soils, C and nitrogen (N) were primarily concentrated in coarse macroaggregates and fine macroaggregates, whereas in square bamboo plantations, C and N were mainly associated with coarse macroaggregates only. Both aggregate-associated soil C and N varied significantly with aggregate size and forest type, and Japanese cedar soils exhibited higher aggregate C/N ratios, particularly in older stands. Bulk SOC was positively correlated with macroaggregate-associated C in both forest types and with the silt and clay fractions in Japanese cedar plantations. MWD increased with higher macroaggregate C content and declined as the proportion of C in smaller aggregate fractions increased. These findings indicate that forest type plays a critical role in regulating soil aggregation and SOC stabilization pathways, with square bamboo plantations enhancing C sequestration by promoting macroaggregate formation and stability.