Reduced turnover rate of topsoil organic carbon in old-growth forests: a case study in subtropical China

Abstract Background Old-growth forests are irreplaceable with respect to climate change mitigation and have considerable carbon (C) sink potential in soils. However, the relationship between the soil organic carbon (SOC) turnover rate and forest development is poorly understood, which hinders our ability to assess the C sequestration capacity of soil in old-growth forests. Methods In this study, we evaluated the SOC turnover rate by calculating the isotopic enrichment factor β (defined as the slope of the regression between 13C natural abundance and log-transformed C concentrations) along 0–30 cm soil profiles in three successional forests in subtropical China. A lower β (steeper slope) is associated with a higher turnover rate. The three forests were a 60-year-old P. massoniana forest (PF), a 100-year-old coniferous and broadleaved mixed forest (MF), and a 400-year-old monsoon evergreen broadleaved forest (BF). We also analyzed the soil physicochemical properties in these forests to examine the dynamics of SOC turnover during forest succession and the main regulators. Results The β value for the upper 30-cm soils in the BF was significantly (p