Quantity and quality decoupling in soil carbon sequestration: A case study focusing on the stability of soil organic matter in reclaimed coal gangue soils

Assessing soil organic matter (SOM) stability is crucial for evaluating the quality of reclaimed carbon pools. Herein, we integrated Fourier transform infrared (FT-IR) spectroscopy, high-throughput sequencing, and physicochemical assays to investigate SOM stability and its driving factors in reclaimed coal gangue soils. Reclamation treatments significantly increased soil organic carbon (SOC) contents by 40 %-71 % (8.62-8.78 g/kg) compared to the control (5.12 g/kg). However, this carbon accumulation coincided with a substantial reduction in soil aggregate stability, indicated by decreases of 16 %-28 % in mean weight diameter (MWD), and 18 %-43 % in geometric mean diameter (GMD). Compared to the control, reclaimed treatments exhibited a slight but non-significant decrease in SOM decomposition degree (Degree), defined by the FT-IR derived aromatic-C/aliphatic-C ratio; this ratio reflects SOM stability due to the high recalcitrance of aromatic-C and the lability of aliphatic-C toward microbial breakdown. Correlation analysis further revealed Degree was positively correlated with MWD, GMD, soil pH, but negatively correlated with SOC content and fungal richness. These relationships highlight a critical dynamic in reclamation: the rapid input of exogenous organic matter dominates the soil carbon pool, exceeding the rate of its microbial conversion to stable forms. Collectively, our findings demonstrated a decoupling process between the quantity (carbon accumulation) and the quality (carbon pool stability) of sequestered carbon. This study provides a mechanistic framework for assessing carbon sequestration and underscores the necessity for management practices that not only enhance carbon input but also promote its chemical stabilization and physical protection in engineered post-mining landscapes.