Abstract: Soil pore CO₂ can be several to dozens of times higher than the soil-atmosphere interface respiration. Consequently, even minor fluctuations in this subsurface gaseous carbon pool could lead to significant changes in atmospheric CO₂ concentrations. However, current understanding of CO₂ dynamics and its isotopic composition in the vadose zone of alpine meadows remains limited. In this study, we conducted systematic measurements of CO₂ concentrations, fluxes, and its C isotopic compositions (¹³C/¹²C, δ¹³C-CO₂) at the soil-atmosphere interface and within the vadose zone (0, 10, 20, 40, 60, and 100 cm) throughout the growing season (May–September) in an alpine meadow. Combined with structural equation modeling, we elucidated the contribution of the subsurface CO₂ pool to soil respiration and its underlying regulatory mechanisms. Results showed that CO₂ concentrations in the vadose zone accumulated with increasing soil depth, ranging from 407.04 ppm-22998.03 ppm across the 0–100 cm profile. During the growing season, the cumulative CO₂ flux at the soil-atmosphere interface was 8847.3 kg CO₂-C·hm^-2; the fluxes decreased with soil depth in the vadose zone, with 11568.7, 4357.2, 5155.1, 2726.2, and -1608.6 kg CO₂-C·hm^-2 for the 0–10, 10–20, 20–40, 40–60, and 60–100 cm layers, respectively. Soil-atmosphere interface respiration originated primarily from the 0–10 cm surface layer, where CO₂ production exceeded surface emissions by approximately 30%, indicating that soil respiration in the alpine meadow mainly derives from the upper 10 cm. The δ¹³C-CO₂ values at the soil-atmosphere interface were significantly higher than those in the soil vadose zone, with ranges of -16.61 to -13.40‰, -22.61 to -20.18‰, -22.81 to -19.54‰, -23.11 to -21.00‰, -23.09 to -18.63‰, and -23.07 to -19.99‰ for 0, 0–10, 10–20, 20–40, 40–60, and 60–100 cm depths, respectively. Structural equation modeling revealed that depth exerted a positive indirect effect on CO₂ concentration (+0.308) but a negative indirect effect on CO₂ flux (-0.718). The key dominated factors underlying this decoupling were total nitrogen (TN) (regulating concentration), pH (regulating flux), and soil temperature (regulating both). The "concentration accumulation-emission suppression" pattern in the vadose zone resulted from multiple interacting factors, including attenuated root-microbe activity, substrate quality deterioration, nutrient limitation, and restricted gas diffusion due to soil compaction. TN exerted a positive regulatory effect on δ¹³C-CO₂ values, whereas soil water content showed the opposite trend; its negative effect reflected the regulatory role of moisture on diffusion fractionation. Based on the calculated δ¹³C-CO₂ signature (Keeling curve approach) confirmed that the accumulated CO₂ in the vadose zone was partially derived from the decomposition of deep old carbon characterized by more ¹³C-enriched signatures. The δ¹³C-CO₂ data, together with concentration and flux models, collectively revealed a complete picture of the carbon cycle encompassing "carbon stock–carbon flux–carbon source characteristics." The "surface-dominated respiration with widespread retention in the vadose zone" pattern observed in this alpine meadow suggests that the 0–10 cm soil layer governing surface respiration may be of universal significance. The negative effect of pH on CO₂ emissions may partially offset the promotional effect of warming; this balancing mechanism will profoundly regulate the response intensity of alpine meadows to climate change, providing a critical scientific basis for regional carbon budget assessments.