Carbon isotopic composition,turnover and origins of soil CO2 in a monsoon evergreen broadleaf forest in the Dinghushan Biosphere Reservoir,South China

Carbon isotopic compositions of soil CO₂ in rainy season (July) from two natural soil profiles (DHLS &; DHS) in the monsoon evergreen broadleaf forest in the Dinghushan Biosphere Reservoir (DBR), South China, are presented. Turnover and origins of soil CO₂ are preliminarily discussed in this paper. Results show that the content of soil CO₂ varies between 6120 and 18718 ppmv, and increases with increasing depth until 75 cm, and then it declines. In DHLS, soil CO₂ δ ¹³C ranges from −24.71‰ to −24.03‰, showing a significant inverse correlation (R ₂=0.91) with the soil CO₂ content in the same layer. According to a model related to soil CO₂ δ ¹³C, the soil CO₂ is mainly derived from the root respiration (>80%) in DHLS. While in DHS, where soil CO₂ δ ¹³C ranges from −25.19‰ to −22.82‰, soil CO₂ is primarily originated from the decomposition of organic matter (51%–94%), excluding the surface layer (20 cm, 90%). Radiocarbon data suggest that the carbon in soil CO₂ is modern carbon in both DHLS and DHS. Differences in ¹⁴C ages between the “oldest” and “youngest” soil CO₂ in DHLS and DHS are 8 months and 14 months, respectively, indicating that soil CO₂ in DHLS has a faster turnover rate than that in DHS. The Δ¹⁴C values of soil CO₂, which range between 100.0‰ and 107.2‰ and between 102.5‰ and 112.1‰ in DHLS and DHS, respectively, are obviously higher than those of current atmospheric CO₂ and SOC in the same layer, suggesting that soil CO₂ is likely an important reservoir for Bomb-¹⁴C in the atmosphere.