Abstract: Aims As a key component of terrestrial ecosystem carbon pools, soil carbon storage in grasslands, encompassing both soil organic carbon (SOC) and inorganic carbon (SIC) pools, plays a crucial role in terrestrial carbon cycling and climate feedback. However, there is relatively limited systematic understanding on driving mechanisms inte-grating both SIC and SOC fractions, compared to the extensive focus on SOC. The comprehensive understanding of soil carbon storage between different grassland types remains unclear. Methods Here, we conducted a field survey in temperate grasslands of Inner Mongolia, selecting two grassland types: typical steppe and meadow steppe. We measured soil physicochemical properties, plant biomass and chemical traits, as well as microbial biomass carbon and community composition. In addition, we acquired a suite of explan-atory factors including climatic, edaphic, vegetational, and microbial variables. Boosted regression trees analyses and structural equation modeling were then used to investigate the relative importance of the four factors and poten-tial mechanisms in affecting total soil carbon storage and its organic and inorganic components. Important findings The results indicated that the SIC stock in the 0 - 60 cm soil layer of typical steppe (2.75 ± 0.15 kg C m-2) was significantly higher than that in meadow (0.45 ± 0.03 kg C m-2), whereas SOC stocks showed no significant difference (8.61 ± 0.19 kg C m-2 for typical steppe and 8.32 ± 0.17 kg C m-2 for meadow). Conse-quently, the total soil carbon stock in typical steppe being significantly higher than that in meadow. The SOC con-tent of both grassland types decreased with soil depth. The SIC content in typical steppe exhibited pronounced ac-cumulation in deeper soil layers, a pattern that was absent in meadow. Biotic and abiotic factors, including plant, climate, and soil, jointly influenced total soil carbon and its SOC and SIC components, with distinct regulatory mechanisms between grassland types. In typical steppe with strong water limitations, soil carbon storage was pri-marily regulated by climate factors, whereas in meadow steppe with relatively higher moisture availability, soil fac-tors played a more prominent role. These findings provide a scientific basis for accurately assessing the soil carbon storage in temperate grasslands and enhance our understanding of the distribution mechanisms of soil carbon under multi-factor interactions.

Key words: Temperate grasslands, soil carbon stock, soil carbon fractions, organic-inorganic carbon coupling, climate-plant-soil interactions