Turnover Rate of Soil Organic Matter and Origin of Soil ¹⁴Co₂ in Deep Soil from a Subtropical Forest in Dinghushan Biosphere Reserve, South China

This paper examines the carbon isotopes (13C, 14C) of soil organic carbon (SOC) and soil CO2 from an evergreen broadleaf forest in southern China during the rainy season. The distribution of SOC δ13C, and SOC content with depth, exhibits a regular decomposition of SOC compartments with different turnover rates. Labile carbon is the main component in the topsoil (0–12 cm) and has a turnover rate between 0.1 and 0.01 yr–1. In the middle section (12–35 cm), SOC was mainly comprised of mediate carbon with turnover rates ranging between 0.01 and 0.025. Below 35 cm depth (underlayer section), the SOC turnover rate is slower than 0.001 yr–1, indicating that passive carbon is the main component of SOC in this section. The total production of humus-derived CO2 is 123.84 g C m–2 yr–1, from which 88% originated in the topsoil. The middle and underlayer sections contribute only 10% and 2% to the total humus-derived CO2 production, respectively. Soil CO2 δ13C varies from –24.7‰ to –24.0‰, showing a slight isotopic depth gradient. Similar to soil CO2 δ13C, Δ14C values, which range from 100.0‰ to 107.2‰, are obviously higher than that of atmospheric CO2 (60–70‰) and SOC in the middle and underlayer section, suggesting that soil CO2 in the profile most likely originates mainly from SOC decomposition in the topsoil. A model of soil CO2 Δ14C indicates that the humus-derived CO2 from the topsoil contributes about 65–78% to soil CO2 in each soil gas sampling layer. In addition, the humus-derived CO2 contributes ∼81% on average to total soil CO2 in the profile, in good agreement with the field observation. The distribution and origin of soil 14CO2 imply that soil CO2 will be an important source of atmospheric 14CO2 well into the future.