Aggregate-associated soil organic carbon fractions in subtropical soil undergoing vegetative restoration

Precise assessment of soil organic carbon (SOC) storage requires understanding how vegetation and soil physico-chemical properties differ in SOC fractions. Therefore, we aimed to analyze the dynamics of aggregate-associated, labile organic carbon (LOC) fractions corresponding to depth to clarify the effect of vegetation and soil properties on water stable aggregate (WSA) mineral adsorption in subtropical, red soil with five vegetation restoration regimes. The results showed that the large macro-aggregate fraction dominated the degraded red soil, which had the highest content of dissolved organic carbon (DOC). WSA-associated, easily oxidized organic carbon (EOC) varied from 6.26 to 20.02 g/kg and was not affected by vegetation types. Schima superba pure forest significantly increased DOC (0.38 g/kg on average) and particulate organic carbon (POC, 7.92 g/kg on average), which had the highest biomass. Along with soil depth, WSA-associated POC declined, while exhibiting a growth trend with decreasing particle size, for example, the highest POC was found in <0.053 mm aggregates. The redundancy analysis ordination indicated that soil porosity and total nitrogen (TN) were the main soil parameters that explained the most variance. Meanwhile, the vegetation biomass, except for litter, were all significantly positively correlated with <0.053 mm aggregates. Leaf biomass played the most important role on DOC in macro-aggregates with a 53.42% contribution. For aggregate-related POC, the largest contribution was from the interactions between branch biomass and pH (47.78%) followed by TN (35.1%) of micro-aggregate-related POC. Leaf biomass, <0.053 mm aggregates, and TN can be used as indicators to evaluate the impact of vegetation restoration on WSA-associated SOC fractions. Broad-leaved forest or in combination with indigenous coniferous species was a better choice for SOC sequestration improvement in the study area for managing C supply, process, and flux in subtropical terrestrial ecosystems.