Quantity vs. quality: divergent carbon stabilization pathways under eucalyptus and natural succession

While the conversion of abandoned pastures to forest systems is a known soil carbon (C) sequestration strategy, the functional trade-offs between physical C accumulation and biogeochemical recovery under active versus passive restoration pathways remain debated. We evaluated soil C and nitrogen (N) dynamics, total stocks, physical fractions (POM and MAOM), and isotopic signatures (δ 13 C and δ 15 N), down to a 1-m depth, across a 22-year chronosequence in the Brazilian Atlantic Forest comparing active silviculture (Eucalyptus cultivation, EU), passive restoration (Natural Succession, SN), and a degrading baseline (Abandoned Pasture, PA). At 22 years, EU accumulated the highest total C stocks (151.4 Mg ha −1 ), maximizing long-term storage via physically protected MAOM and rapid C turnover (>90% C 3 -derived), while maintaining a highly efficient and conservative N cycle. Conversely, driven by diverse, low-C/N litter inputs, SN prioritized biogeochemical functionality (“Quality”); it maintained high levels of labile C, restored N-POM stocks, and exhibited progressive δ 15 N enrichment to signal a dynamic nutrient cycle. Meanwhile, the PA acted as a continuous C source over time, likely because insufficient root inputs failed to offset ongoing microbial decomposition. These divergent trajectories dictate the trade-off between storage and functionality: Eucalyptus is highly effective for long-term carbon sequestration (Quantity), whereas Natural Succession restores soil functional quality and nutrient cycling (Quality). Crucially, unassisted recovery in abandoned pastures (PA) failed to halt C losses, demonstrating that overcoming basal decomposition limits requires substantial biomass inputs, achieved either through active management or successful successional establishment.