Abstract

Introduction Grassland desertification substantially reduces soil organic carbon (SOC) stocks. However, the responses of two distinct SOC fractions—mineral-associated organic carbon (MAOC) and particulate organic carbon (POC)—to desertification, as well as the underlying mechanisms governing these responses, remain poorly understood. Methods To address this knowledge gap, we conducted a field study across the typical steppe (TS) and desert steppe (DS) ecosystems of Inner Mongolia, China, to investigate the dynamics of MAOC and POC and their underlying drivers, including iron/aluminum (Fe/Al) oxides, exchangeable calcium (Ca ex ) and microbial community composition. Results Our results reveal a significant decline in MAOC concentrations under desertification, with mean values of 15.66 g/kg (0–10 cm) and 14.99 g/kg (10–20 cm) in the TS, and 12.10 g/kg (0–10 cm) and 11.64 g/kg (10–20 cm) in the DS. Unlike MAOC, POC concentrations significantly varied in only the surface layers of TS and DS, with mean values of 6.22 g/kg in the TS and 4.73 g/kg in the DS. This indicates divergent vertical responses of MAOC and POC to grassland desertification. In the DS, reduced fungal abundance—coupled with lower concentrations of amorphous Fe/Al oxides (Fe o /Al o ) in the fine soil fraction—collectively constrained MAOC accumulation. However, because aboveground biomass contributes the majority of litter input preferentially to the surface soil layer, sparse vegetation in the DS primarily limits POC accumulation in the surface horizon. Discussion This study provides the first comprehensive assessment of the differential responses of MAOC and POC to desertification in Inner Mongolian grasslands and elucidates the associated biogeochemical mechanisms.