Carbon stabilization pathways in soil aggregates during long-term forest succession: Implications from 813C signatures

Forest restoration increases organic carbon (OC) sequestration mainly via the additional litter input and im-provements in soil structure that result in biochemical and physical C stabilization over the short term. However, the pathways of long-term C stabilization in soil aggregates during forest succession are unclear. To characterize the long-term C fluxes, the aggregate-associated OC content and C flow pathways were examined over 160 years of secondary successional chronosequence from Lespedeza bicolor to Quercus liaotungensis. The contribution of plant-derived C (litters and fine roots) to aggregate-associated OC was assessed using 13C natural abundance. The proportion of macroaggregates increased but microaggregates decreased along forest succession, and the macroaggregate-associated OC content increased from 4.6 to 28 g kg-1 during succession. The 13C enrichment trend was common in all aggregate size classes: macroaggregates to silt and clay size classes. The maximum 813C was -23 parts per thousand in the silt and clay size classes in the pioneer weed stage at 20-30 cm soil depth. The C pathways followed the trend from macroaggregates to silt and clay size classes. The intensity of the C flows decreased in the topsoil (<10 cm), but plant-derived C stocks within the aggregates increased during forest succession over the 160 years. Notably, fine roots made a greater contribution to the OC accumulation within aggregates than aboveground litter did. The microbial biomass was an important factor affecting fine root-derived C stocks in the aggregates. Concluding, this analysis of natural 813C signature provides detailed insights into long-term C sta-bilization pathways associated with soil aggregates during forest succession.