Saline soil can both absorb and release atmospheric carbon dioxide (CO₂); however, the factors underlying the mechanisms that drive CO₂ transport and transformation in saline soils remain unclear. This study examined the absorption of atmospheric CO₂ by saline soil in an effort to better understand the factors that contribute to this exchange. We investigated the relationship between carbon sources and sinks using in-situ observations of saline soil in the Hetao Irrigation District, Inner Mongolia, China. Eight saline soil samples, each of which exhibited different electrical conductivities (EC), were collected from 2014 to 2018. CO₂ was sampled and analyzed by closed static-chamber-gas chromatography. We found that atmospheric CO₂ was absorbed by all eight soil samples, but EC levels influenced the carbon exchange between the soil and the atmosphere. Between 2014 and 2018, cumulative CO₂ absorbed by the two soil samples with the lowest EC values was 662.03 g·m⁻² and 1322.17 g·m⁻²; whereas for the same period, the cumulative CO₂ absorbed by the six soil samples with the highest EC values ranged from 55.05 g·m⁻² to 600.52 g·m⁻². Diurnal and nocturnal observations showed that saline soil absorbed atmospheric CO₂ at night and released it into the atmosphere during the day, with a maximum night-time absorption value of −41.4 mg·m⁻²·h⁻¹. Eighty-nine percent of the differences observed in dissolved inorganic carbon (ΔDIC) values across the soil samples were explained by differences in CO₂ accumulation (ΔCO₂) (r² = 0.8922, n = 7). CO₂ was absorbed by saline soil, where it was converted into DIC. As soil salinity increased, the cumulative amount of CO₂ in the soil-atmosphere system increased significantly. As a result, we conclude that saline soils with high EC exhibit an enhanced ability to absorb carbon.