Objective  This paper aims to elucidate the variation characteristics and main influencing factors of soil CO₂ concentration at different depths of the Tibetan Plateau forest ecosystem, so as to provide a scientific basis for variation mechanism of soil respiration CO₂ emission.

  Method  We selected Sejila Mountain (Lulang Section) in Tibet of southwestern China as a model area and a typical forest ecological area. Soil temperature (S_t) and water content (S_w) at different depths (5, 10, 20, 40 and 60 cm) were measured in situ for analyzing the distribution characteristics of soil CO₂ concentration at different levels and its response to S_t and S_w in the Abies georgei var. smithii forest.

  Result  (1) The daily mean value of soil CO₂ concentration at all levels was 40 cm > 60 cm > 20 cm > 10 cm > 5 cm, the mean values were 5094, 4965, 4613, 4119 and 3925 μmol/mol, respectively, and there was a significantly positive correlation among different depths. (2) On the diurnal variation scale, soil CO₂ concentrations at 5 cm and 10 cm layers had opposite variation characteristics, showing a “V”-shaped and a “single peak” curve, respectively. The peak value and valley value of soil CO₂ concentration at 5 cm layer appeared at 22:00−23:00 and 12:00−13:00, respectively, and the 10 cm appeared at 16:00−17:00 and 00:00−01:00, respectively, but no obviously diurnal variation in other layers. (3) The diurnal variations in the soil CO₂ concentration were not completely synchronized with the S_t, and showed a lagged behind changes, showing an approximate ellipse shaped curve in 5 cm and 10 cm layers. (4) The soil CO₂ concentration in the surface layer (5 and 10 cm) is mainly affected by water content, while the deeper layer (40 and 60 cm) soil is mainly affected by soil temperature.

  Conclusion  S_t and S_w have different effects on soil CO₂ concentration at different depths, and ultimately affect soil CO₂ transport and dynamics.