POM-Mediated Synergistic Enhancement of Organic and Inorganic Carbon in Saline–Alkali Soil Amelioration

Saline–alkali soils in coastal deltas impose major constraints on carbon stabilization, yet the mechanistic links between soil organic carbon (SOC) and inorganic carbon (SIC) under amelioration remain insufficiently understood. We conducted an in situ incubation experiment in the Yellow River Delta using low- and high-salinity soils to assess how straw inlay at 30 cm, with or without biochar incorporation in the 0–30 cm layer, regulates carbon fractions. We quantified dissolved organic carbon (DOC), particulate organic matter (POM), mineral-associated organic matter (MAOM), and both organic and inorganic carbon within each fraction.  Results show that straw, biochar, and their combination consistently promoted POM accumulation and reduced DOC:SOC ratios, particularly in high salinity soils, reflecting enhanced SOM stabilization. The bulk soil and POM fraction exhibited strong positive relationships between SOC and SIC, indicating synergistic enhancement of organic and inorganic carbon pools during amelioration. In addition, DOC:SOC ratio was negatively associated with SIC, suggesting that greater inorganic carbon accumulation corresponds to lower SOM lability and supporting a role for SIC in promoting SOM stabilization. These findings provide mechanistic evidence that POC-enriching amendments can simultaneously enhance SOC and SIC pools, offering an effective pathway to improve carbon sequestration and structural resilience in coastal saline–alkali ecosystems.