Soil organic carbon fractions and sequestration across a 150-yr secondary forest chronosequence on the Loess Plateau, China

•SOC concentrations increased rapidly at early successional stages (< 50-yr).•Topsoil (0–20 cm) stored nearly half of SOC stock across successional stages•The majority of total SOC stock existed in the non-labile fraction.•Long-term vegetation succession increased SOC stocks and sequestration.•Deeper soil layers appeared to play a positive role in SOC sequestration.

Restoration of natural vegetation is recommended as an effective approach to restore soil function and rebuild degraded ecosystems. Information is lacking about the long-term results of secondary forest succession on the Loess Plateau with respect to soil organic carbon (SOC) fractions and sequestration in the root-zone soil profile. We investigated the differences in SOC fractions down to 100 cm depths along a 150-yr chronosequence, including cropland (control) and five successional stages (pioneer weeds, herbage, shrub, early forest, and climax forest). Total, labile, and non-labile SOC concentrations increased rapidly at early successional stages (before shrub, < 50-yr) and then gradually leveled off. Total SOC stock was highest at the climax forest stage (64.3 Mg ha− 1) and lowest in cropland (39.9 Mg ha− 1). Nearly half (~ 44.8%) of total SOC stock was stored in surface soils (0–20 cm) and the majority (76.4%) existed in the non-labile fraction. The ratio of labile to non-labile fraction decreased with depth but remained stable across successional stages. The mean SOC sequestration potential and rate relative to cropland were 20.5 Mg ha− 1 and 0.73 Mg ha− 1 yr− 1, respectively. Although the SOC sequestration potential decreased with depth (64.7% for 0–20 cm vs. 35.3% for 20–100 cm), it decreased by 3.1 Mg ha− 1 in the surface and increased by 3.4 Mg ha− 1 in the subsurface from herbage to climax forest stage. This study indicated that long-term secondary forest succession played a positive role in SOC sequestration on the Loess Plateau, especially in the subsurface soil layers.