Stand age and soil drivers mediate the transition from particulate organic carbon to mineral-associated organic carbon and regulate carbon pool management index in Haloxylon ammodendron sand-fixing forests

Understanding soil carbon pool variation with stand age in planted Haloxylon ammodendron sand-fixing forests is essential for accurately evaluating the carbon sink function of arid sandy ecosystems. This study investigated a 5–40-year H. ammodendron plantation chronosequence along the southern margin of the Badain Jaran Desert, with adjacent mobile dunes as controls. Soil carbon fraction and the carbon pool management index (CPMI) were analyzed across the 0–100 cm depth. The results indicate that, with stand age, the mean soil inorganic carbon pool (0–100 cm), soil organic carbon pool, dissolved organic carbon (DOC), easily oxidizable carbon (EOC), mineral-associated organic carbon (MAOC)and CPMI all exhibit positive trends. In the 0–100 cm soil depth, SOC and particulate organic carbon (POC) reached their minimum levels in the 5-year-old H ammodendron stands. In contrast, at the 0-5 and 5-20 cm soil depths, the lowest soil organic carbon stocks (SOCS), SOC, and POC were observed in 10-year-old stands. SOC composition shifted from POC-dominated to MAOC-enriched, indicating enhanced stabilization. Labile organic carbon dominated the surface layers, while stable carbon forms were relatively more abundant in deeper horizons. Soil nutrients and vegetation characteristics are key regulators of soil carbon pools; CPMI is mainly co-influenced by these factors plus labile organic carbon fractions. Additionally, reduced bulk density and pH, and increased clay, silt, and porosity, collectively promoted both carbon sequestration and stability. Overall, planted H. ammodendron forests simultaneously strengthen both the capacity and stability of soil carbon pools in arid sandy lands.