Linking soil redistribution to soil organic carbon using 210Pbex along different complex toposequences in a karst region, southwest China

Abstract Understanding the magnitudes and patterns of soil and associated soil organic carbon (SOC) redistribution is of great importance for assessing both soil degradation and rocky desertification in fragile karst ecosystems. However, to date reliable information on the application of excess lead-210 (210Pbex) for quantifying dynamics of SOC-soil redistribution relationship is limited on the hillslopes due to the highly heterogeneous landscape after land use change in karst systems. In this study, three typical toposequences (grassland, forests, and agroforestry) were selected in a typical karst graben basin of southwestern China. A total of 229 soil samples were collected along the toposequences to explore the soil redistribution process using a model coupled with measured 210Pbex, and then to ascertain the relationship of discharged 210Pbex to erosion-induced SOC. The results showed that most soil samples were significantly affected by soil erosion (reference value of 210Pbex: 5986 ± 756 Bq m−2). The output of the model demonstrated that the highest estimated net soil flux occurred in agroforestry (21.38 Mg ha−1 yr−1), followed by forests (8.39 Mg ha−1 yr−1) and grassland (−3.54 Mg ha−1 yr−1). Meanwhile, the SOC concentration showed a decreasing order of agroforestry > forest > grassland on the hillslopes, implying that while agroforestry help increase SOC sequestration potential, intensive human intervention also carries a high risk of soil erosion in karst regions. The fitting correlation of 210Pbex and SOC in surface soils displayed the decreasing order of grassland (R2 = 0.80) > forests (R2 = 0.69) > agroforestry (R2 = 0.42), indicating that fallout 210Pbex could be used directly as a tracer to quantify the dynamics of the SOC-soil redistribution relationship in grassland toposequence. Our results provide practical evidence of the potential of 210Pbex for quantifying SOC-soil redistribution along hillslopes with complex toposequences in a karst region.