Vegetation restoration stimulates soil carbon sequestration and stabilization in a subtropical area of southern China

Vegetation restoration affects the stability of soil organic carbon (SOC) by changing the composition of soil carbon pools, including active carbon (C a), the labile pool of SOC; slow carbon (C s), the physically stabilized pool of SOC; and resistant carbon (C r), the chemically stabilized pool of SOC. The aims of this study were to determine how SOC pools changed during restoration of a subtropical forest and to what extent vegetation characteristics and soil properties affected the changes in SOC pools. Soil samples were collected to 40 cm in four plant communities along a restoration chronosequence: scrub-grassland (4–5 years), shrubs (10–12 years), coniferous and broadleaved mixed forest (45–46 years), and evergreen broadleaved forest (90–91 years). Laboratory incubations were used to measure CO2 production during SOC mineralization, and acid hydrolysis was used to measure C r. The CO2 production and C r data were fitted to a three-component first-order kinetic model to determine the C a and C s. Pearson's correlations, stepwise multiple line regressions, and variation partitioning analysis were used to determine the key factors that affected SOC pools. The results showed that vegetation restoration increased the contents of SOC from 1.67 to 47.6 g kg−1, C a from 0.03 to 0.35 g kg−1, C s from 1.32 to 24.5 g kg−1, and C r from 0.33 to 22.8 g kg−1. During vegetation restoration, the increase in SOC was primarily due to carbon (C) stored in stable pools (i.e., C s or C r), and the portion of C r in total SOC increased markedly from 18.5 to 56.3%. Fine root biomass was the primary driver that controlled SOC pools during vegetation restoration. The C/N ratio of litter had a greater effect on C a and C s than that of other factors, whereas the soil clay content contributed secondarily to C r. The results suggest that vegetation restoration increases not only the amounts of SOC, C a, C s, and C r but also the stability of the SOC pool in subtropical soil. The relatively rapid increases in C s and C r following vegetation restoration played a crucial role in C sequestration. Therefore, strong measures to preserve natural forests and facilitate vegetation restoration should be the primary approach to increase long-term soil C sequestration in this region.