Soil organic carbon accumulation along a chronosequence of vegetation colonization on debris flow fans in Southwest China

•Soil properties improve with vegetation colonization.•SOC concentration increases with vegetation colonization.•Biospheric OC fraction increase with vegetation colonization.•Alkyl and O-alkyl C fractions increase with vegetation colonization. Soil is one of the largest terrestrial carbon (C) storages, which offers great potential to mitigate climate change through C sequestration. In this study, we focused on soil organic carbon (SOC) accumulation along a chronosequence of vegetation colonization on inactive debris flow fans by investigating the temporal variation of SOC concentrations and properties such as C:N ratios, δ13C values and chemical compositions of soil fractions in both topsoils (0–10 cm) and subsoils (40–50 cm) with vegetation colonization. We found that SOC concentration increased with soil property improvement (such as increase of soil aggregation and cation exchange capacity). Petrogenic OC from parent materials and biospheric OC from vegetation are the two main sources of SOC, and vegetation colonization increased the fraction of biospheric OC in bulk SOC. Fine soil fractions were found to have higher C concentrations, i.e. higher C adsorption efficiency per unit soil mass. As a result, fine soil particles were more enriched in alkyl and O-alkyl C that are the main composition of biospheric OC added to soils after vegetation colonization. Our results show that vegetation colonization on debris flow fans not only increases SOC concentrations, but also makes the bulk SOC become more labile.