Tracking soil carbon processes in two temperate forests at different successional stages using stable and radioactive carbon isotopes

The profile characteristics of soil carbon reflect the combined effects of vegetation and erosion. However, the quantitative assessment of soil carbon in profile and its dynamics affected by vegetation restoration and erosion is still extremely inadequate. In this study, we synthesized soil and vegetation properties, stable carbon isotopic compositions, and radiocarbon ages to investigate the soil carbon processes in two temperate forests of different successional stages (Populus forest versus Quercus forest) on the Chinese Loess Plateau. The objectives of this study are to (1) examine the dynamics of soil organic carbon (SOC) and soil inorganic carbon (SIC) in the topsoil and subsoil, and (2) evaluate the effect of vegetation restoration and historical erosion on the changes of SOC and SIC in the temperate forests. We found that from Populus to Quercus forests, the SOC stock increased in the topsoil but declined in the subsoil, whereas the SIC stock exhibited a completely opposite variation trend. The significant increase in litter biomass enhanced the SOC in the topsoil, while a stronger historical erosion and priming effect would contribute to the decrease of SOC in the subsoil along succession. Changes in δ¹³C and Δ¹⁴C in the soil profile provided evidence that the turnover rate of SOC increase in the topsoil but decrease in the subsoil along succession. Over the past 10,000 years, due to soil erosion process, the estimated loss of the SOC and SIC was 26.2 and 64.1 Mg ha^-1 in Populus forest and 20.2 and 42.6 Mg ha^-1 in the Quercus forest. Our results indicated that the litter input and erosion processes make great effect on the carbon dynamics in the topsoil and subsoil of the study region.