Objective The grassland carbon reservoir plays a crucial role in carbon cycle studies and the realization of the "dual-carbon" goals. However, there is still a lack of studies on the characteristics of the grassland carbon reservoir in Inner Mongolia, and the research on the grassland carbon reservoir in Ulanqab city remains limited. Investigating the carbon storage and carbon density of the natural grasslands in Ulanqab (temperate steppe desert, temperate desert steppe, temperate typical steppe, temperate lowland meadow, temperate meadow steppe, and temperate mountain meadow) is important for evaluating the grassland carbon reservoir in the central and eastern regions of Inner Mongolia.

Methods A total of 16 representative sample plots were selected based on field investigations. Within each sample plot, three sub-plots (1 m × 1 m) were established. Through field investigations, aboveground living plants, litter, and roots within the sub-plots were collected, brought back to the laboratory, washed, dried at 85 °C, and weighed to calculate biomass. The carbon storage per unit area for grasses was obtained by multiplying the biomass by the carbon content rate. The soil profile was excavated in the center of each standard plot, and four soil layers were sampled: 0–20 cm, 20–40 cm, 40–60 cm, and 60–80 cm, to calculate soil carbon density. Basic data analysis was performed using Microsoft Excel, and one-way ANOVA was conducted using SPSS.

Results The species number, community coverage, and richness of the temperate mountain meadow were significantly higher than those of other grassland types. The vegetation biomass of the temperate mountain meadow was significantly higher, and root biomass accounted for 83.69%–91.72% of the total biomass. The vegetation carbon densities of the temperate steppe desert, temperate desert steppe, temperate typical steppe, temperate lowland meadow, temperate meadow steppe, and temperate mountain meadow were 128.21, 270.48, 364.03, 484.11, 759.00, and 981.80 g/m², respectively. Ecosystem carbon densities were 5.90, 6.31, 10.30, 20.71, 18.15, and 32.91 kg/m², respectively. Soil organic carbon density accounted for 95.72%–97.80% of the ecosystem carbon density and decreased with increasing soil depth. The total carbon density was positively correlated with precipitation, altitude, plant species richness, and community coverage, and negatively correlated with temperature.

Conclusions The total carbon storage of natural grassland in Ulanqab city is 324.99 Tg, and the carbon storage of the temperate typical steppe (146.62 Tg) is significantly higher than that of the other grassland types. The order of carbon storage from low to high is temperate mountain meadow < temperate meadow steppe < temperate steppe desert < temperate lowland meadow < temperate desert steppe < temperate typical steppe.