LAUSR

Purpose The objectives of the study were as follows: (a) to determine the response of soil organic carbon (SOC) fractions to vegetation restoration; (b) to examine the contributions of aggregate-associated OC to total soil OC accumulation along vegetation restoration, (c) to identify the factors that affect SOC accumulation along natural vegetation restoration in a karst region in Southwest China. Materials and methods Four vegetation restoration stages, namely, grassland, shrubland, shrub-arbor mixed forestland, and arbor forestland, were compared with cropland (CL). Soil samples were collected at depths of 0–10 cm and separated into five aggregate size fractions. SOC, light fraction OC (LFOC), easily oxidizable OC (EOC), and aggregate-associated OCs were determined for different aggregate sizes and total soil. Results and discussion Natural vegetation restoration increased macroaggregate amount but decreased the fractions of meso- and microaggregates. Vegetation restoration significantly increased total SOC, EOC, and LFOC concentrations and stocks and soil aggregate-associated OC concentrations. The responses of EOC and LFOC in total soil and soil aggregates were more sensitive than those of SOC along with vegetation restoration. Aggregate-associated OC concentrations generally increased with a decrease in aggregate size. Macro- and microaggregate-associated OC stocks increased, but mesoaggregate-associated OC stocks decreased following the conversion of CL to a natural vegetation ecosystem. The increase in SOC stocks was primarily attributed to the macroaggregate-associated OC stocks and their changes. OC concentrations and stocks in total soil and the soil aggregates were significantly positively related to exchangeable calcium content. Conclusions Vegetation restoration considerably affects the amount of soil aggregates, OC concentrations, and stocks in total soil and soil aggregates. Changes in OC concentrations and stocks can be more pronounced in the liable carbon fraction than in total SOC. The increase in SOC was mostly attributed to OC accumulation in macroaggregates. Exchangeable calcium also affected soil OC accumulation in total soil and soil aggregates.