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

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 betweenC 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 < 0.05) higher than that in the PF, in addition to the SOC stock, although there were nonsignificant differences between the BF and MF. The β value was significantly(p < 0.05) positively correlated with the soil recalcitrance index, total nitrogen, and available nitrogen contents but was significantly(p < 0.01) negatively correlated with soil pH.Conclusions: Our results demonstrate that SOC has lower turnover rates in old-growth forests, accompanied by higher soil chemical recalcitrance, nitrogen status, and lower soil pH. This finding helps to elucidate the mechanism underlying C sequestration in old-growth forest soils, and emphasizes the important value of old-growth forests among global C sinks.