Shrub expansion drives coupled declines in wetland particulate and mineral-associated organic carbon stability

Increasing shrub encroachment into wetlands has been shown to reduce the stability of mineral-associated organic carbon (MAOC), yet its influence on particulate organic carbon (POC) stability remains unclear. To determine whether shrub expansion drives coupled declines in POC and MAOC stability, this study established a shrub expansion sequence consisting of wet meadow (control), dwarf-shrubland (early stage), and tall-shrubland (late stage). The stability of POC was characterized across the 0–15, 15–30, and 30–45 cm soil layers (C-rich and ecologically sensitive) using the temperature at 50% mass loss (TG-T 50 ), the temperature at 50% energy release (DSC-T 50 ), and contact angle measurements. Along the sequence, TG-T 50 , DSC-T 50 , and contact angle decreased by 8.7–21.2 °C, 5.9–17.1 °C, and 16.0–26.3 o , respectively, whereas the thermally labile fraction increased by 3.7–5.9%. Structural equation modeling revealed that shifts in litter quality along the sequence modified the chemical structure (represented by characteristic functional groups) and stoichiometry of POC, ultimately driving the decline in its stability. Notably, POC and MAOC exhibited consistent changes in both thermal stability and stoichiometric ratios. Collectively, our findings demonstrate that shrub expansion reduce POC stability via increased high-quality litter inputs, which alter its chemical properties. Moreover, the coupled behavior of POC and MAOC provide a novel framework for predicting wetland soil carbon pool responses to vegetation shifts. Effective management of wetland C sinks requires attention to the coupled behavior of POC and MAOC.