Erosion-driven changes in soil cation exchange capacity quantified using barium isotopes

Luke Bridgestock, Bridgestock , Emily Stevenson, J. Jotautas Baronas, Alasdair Knight, Harold Bradbury, Alexandra Turchyn, Edward Tipper

Human activities have perturbed the balance between rates of soil erosion and formation, driving declines in soil quality. However, quantifying these soil imbalances remains challenging, especially at large scales. Here we present a novel isotope mass balance approach that can be used to quantify river catchment wide rates of change in cation exchange capacity (CEC), a key soil quality metric, in response to erosional perturbations. The approach uses differences in Ba isotope composition between river export fluxes and chemical weathering inputs to solve for rates of change of Ba storage within upstream soil exchange pools. Accumulation of Ba within soil exchange pools, due increases in CEC, results in the preferential export of heavier Ba isotopes in river dissolved and erosional outputs. Data for river water and suspended particulate material (SPM) samples from the Irrawaddy and Salween river demonstrate the utility of the approach. The Ba isotopic data are used to quantify associated changes in catchment wide CEC. During peak discharge conditions the Salween River basin is balanced within uncertainty of the data for its catchment wide CEC budget. By contrast, the Irrawaddy River basin features net CEC accumulation of at least 35 ± 5 meq/m2/year during peak discharge conditions. This net CEC accumulation in the Irrawaddy River basin is interpreted to reflect SPM deposition in floodplains, while the CEC balance observed in the Salween River basin likely reflects the lack of a significant floodplain area in this catchment. These results demonstrate the utility of this approach to place quantitative constraints on river catchment wide soil quality (CEC) changes driven by human perturbations to terrestrial sediment transport, including the reduction of SPM deposition in floodplains due to dam construction and accelerated soil erosion due to land-use and climate change.