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

Abstract

Human activities have perturbed the balance between soil erosion and formation rates, driving declines in soil quality. Quantifying these soil imbalances remains challenging, especially at large scales. Here we present a novel isotope mass balance approach 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. Data for river water and suspended particulate material (SPM) samples from the Irrawaddy and Salween river demonstrate the utility of the approach. River dissolved loads and exchange pools of co-existing SPM loads feature a Ba isotope fractionation of Δ^138/134Ba _diss-sp = 0.23 ± 0.14 ‰. Variations in the Ba isotope composition of the sum of these two Ba pools in river outputs (δ^138/134Ba_out) are predominantly controlled by SPM concentration. At low SPM concentrations δ^138/134Ba_out are offset to higher values relative to average rocks. This is interpreted to reflect the accumulation of isotopically light Ba in upstream soil exchange pools due to CEC increases. Using a mass balance framework, catchment wide changes in CEC are quantified from the Ba isotopic data. During peak discharge conditions the Salween River basin is balanced within uncertainty of the data for its catchment wide CEC budget, while the Irrawaddy River basin features net CEC accumulation of at least 35 ± 5 meq/m²/year. 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 limited floodplain area of this catchment. These results demonstrate the utility of this approach to quantify river catchment wide soil quality (CEC) changes driven by human perturbations to terrestrial sediment transport.