Can Bomb 14 C Indicate Dynamic Replacement of Soil Carbon in Eroded Agricultural Landscapes?.

Subsoil organic matter situated below 30 cm can represent over half of the total organic C in many areas of the world. In agricultural landscapes, water and tillage processes redistribute soil organic matter whereby upper slopes can lose a majority of their inherent C, and toeslope and footslope landscape positions can gain large quantities of soil C. This process can be an important mechanism in global C dynamics, but studies assessing the effect have not been conclusive. In this study we utilize radiocarbon measurements on eroded and redistributed soils, in order to better understand C dynamics in these landscapes. In general, Δ¹⁴C values decrease linearly or exponentially under normal or undisturbed conditions. Four of the five depositional profiles studied here contained inversions of Δ¹⁴C which deviated from the typical depth profile under natural conditions. These perturbations were a result of erosion of recently-assimilated bomb-sourced ¹⁴C due to thermonuclear testing in the 1950’s and 1960’s, and this was verified by ¹³⁷Cs measurements on the same samples. We investigated if recent assimilation of ¹⁴C from bomb-testing could be indicatative of dynamic replacement of C in eroded landscape positions. Three of the five sites clearly showed a large contribution of young bomb-sourced C, suggesting dynamic replacement of eroded soil C.