Carbon Stabilization Pathways in Soil Aggregates During Long-Term Forest Succession: Implications from Δ13c Signatures

Forest restoration increases organic carbon (OC) sequestration mainly via the additional litter input and improvements 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 (litter and fine root) to aggregate-associated OC was assessed using 13 C natural abundance. The proportion of macroaggregates was found to increase but microaggregates decreased along forest succession, and the macroaggregate-associated OC content increased from 4.6 to 28 g kg-1 soil during succession. The 13 C enrichment trend appeared in aggregate size classes: macroaggregates to silt and clay size classes. The maximum δ13 C was -23‰ in the silt and clay size classes in the pioneer weed stage in 20−30 cm soil depth. The C pathways followed the trend of macroaggregates to silt and clay size classes. The intensity of the C flow decreased in the topsoil (< 10 cm), but plant-derived C stocks within the aggregates increased during forest succession over the 160 years. Notably, compared to the contribution from aboveground litter, there was a greater contribution from fine root to the accumulation of OC within aggregates. The fine root and microbial biomasses were the important factors affecting fine root-derived C stocks in the aggregates. This analysis of natural δ13 C abundance provides detailed insights into long-term C stabilization pathways associated with soil aggregates during forest succession.