The relationship of soil organic carbon to 210Pbex and 137Cs during surface soil erosion in a hillslope forested environment

The relationships of erosion-induced displaced soil organic carbon (SOC) to discharged 210Pbex and 137Cs in a hillslope forested area were investigated using runoff plots in three different forest types. A significant correlation was found between the displaced SOC and the radionuclides in each examined forest type. Compared with 137Cs, however, the discharge of 210Pbex is more closely correlated with the displaced SOC and shows a far lower RMSE value by approximately a factor of two. This finding suggests that 210Pbex and SOC are more likely than 37Cs and SOC to move together in the course of soil erosion processes. Moreover, because 137Cs was introduced in the 1960s by thermonuclear bomb tests and is being depleted, its availability in the environment could be limited in the foreseeable future. 210Pbex, in contrast, is continuously replenished from its ubiquitous natural source (222Rn) in a manner similar to the replenishment of SOC from sources consisting of organic material (e.g., litter). In the light of this basic understanding, our findings provide practical evidence of the potential of 210Pbex application for assessing and documenting the water-induced distribution of SOC on forested hillslopes.