Dynamics of SOC density and driving factors during the restoration of artificial grassland and abandoned farmland in Mu Us Desert, China

•The SOCD of artificial grassland increased in 1–7 years and decreased in 9 years.•There was no significant change in SOCD in the early stage of vegetation restoration.•Microbial biomass dominated the SOCD of the topsoil.•Microbial biomass and soil texture jointly increased deep layer SOCD. Establishment of artificial grassland and abandoned farmland are effective measures to increase soil carbon sequestration and mitigate global warming; however, the difference between the two methods in terms of carbon sequestration efficiency and mechanisms at various times and at different soil depths remain unclear. This study investigates the soil organic carbon density (SOCD), soil physicochemical properties, microbial biomass, as well as the above- and belowground biomass of artificial grassland and abandoned farmland restored for one, three, four, seven, and nine years in Mu Us Desert, China. The results show that establishment of both land types could significantly increase soil carbon storage. The SOCD changed following a single-peak curve in the artificial grassland and steadily increased with restoration age in the abandoned farmland. The SOCD of the artificial grassland reached 1.7–2.4 times that of the abandoned farmland, but there was no significant difference (p > 0.05) between the two by the ninth year. The SOCD did not increase significantly in the early stages of restoration (1–3 years and 1–4 years for artificial grassland and abandoned farmland, respectively), which was delayed as the soil layer deepened. Compared to the abandoned farmland, the influence of plants on SOCD through microbial pathways was greater in the artificial grassland, however, the role of soil texture in abandoned farmland was more pronounced. Along with soil depth, the influence of plant factors on the SOCD decreased (40.37–18.19%), but the combined effects of the soil texture and microbial biomass determined the SOCD dynamics. Our results suggest that the combination of appropriate artificial interventions in abandoned farmland can not only quickly restore vegetation communities but also enhance the plant-soil interactions, improve soil texture and microbial functions, increase carbon capture and storage capabilities, and form more sustainable grassland ecosystems.