Mechanisms of soil organic carbon stabilization and its response to conversion of primary natural broadleaf forests to secondary forests and plantation forests

Soil organic carbon (SOC) is highly susceptible to land cover change. The assessment of SOC stabilization mechanisms is therefore crucial to understand the carbon (C) dynamic in terrestrial ecosystems. However, the mechanisms underlying the stabilization of SOC following forest conversion of primary natural broadleaf forests (BF) to secondary forests (SF) and plantation forests (PF) remain unclear. Here, we investigated the soil aggregate distribution and associated SOC concentration, and the dynamics of iron (Fe) and aluminum (Al) oxides in BF (24 sites), SF (25 sites), and PF (16 sites) soils in subtropical China. Results showed that SOC concentrations both in the bulk soil and within aggregates significantly decreased when BF were converted to SF and PF, and these reductions were more pronounced in the topsoil (0–10 cm) than those in the subsoil (10–30 cm). Soil macroaggregates (>250 μm) accounted for the largest proportions of aggregate fractions by mass (40.3 %, 38.9 %, and 39.4 %) and their associated SOC decreased with forest conversion and contributed the greatest proportions of C (11.1 g kg⁻¹, 10.2 g kg⁻¹, and 9.6 g kg⁻¹) to the bulk soil C in BF, SF, and PF, respectively, suggesting that the SOC within macroaggregate contributed to the largest decrease in SOC. The decreases in SOC within macroaggregate, especially with the conversion of BF to PF, were mainly ascribed to the reductions of tree biomass, C stocks of litter and root, and the decreases in the concentration of the Fe_d/Al_d and Fe_p/Al_p oxides but not the concentration of total Fe/Al oxides in gravel and soil. These findings recommend that mixtures of suitable native broadleaf species (e.g., BF species) with plantation species to enhance SOC stabilization in PF, thereby improving the stabilization and sequestration of SOC. Overall, these results help explain why soil C stabilization decreases following forest conversion, and propose an approach for the rehabilitation of plantation forests.