Use of ¹³⁷Cs and ²¹⁰Pb_ex fallout radionuclides for spatial soil erosion and redistribution assessment on steeply sloping agricultural highlands

The steeply sloping agricultural highlands in Korea have severe soil erosion. Estimation of both soil erosion and sedimentation in these highlands is necessary to make plans for soil-conservation measures, but it is not feasible using existing soil-erosion models. This study measured the site-specific concentrations of ¹³⁷Cs and ²¹⁰Pb_ex on both a highland slope (33% slope) and a reference site (undisturbed flat area) to estimate soil erosion and redistribution. The use of the fallout radionuclide (FRN) method was evaluated to see if it is a suitable method for characterizing soil erosion. Results were compared with those obtained with the Universal Soil Loss Equation (USLE), which is an empirical model that estimates annual soil erosion. The average concentrations of ¹³⁷Cs and ²¹⁰Pb_ex at the reference site were 11.57±0.24 Bq kg^-1 and 59.74±4.2 Bq kg^-1, respectively. Concentrations of ¹³⁷Cs and ²¹⁰Pb_ex in the experimental slope were 16.4% and 10.8%, respectively, of those at the reference site. Radionuclide inventories were lower at the upper point of the slope than those at the basal point of the slope. Concentrations of ¹³⁷Cs and ²¹⁰Pb_ex were significantly correlated with available phosphorus, organic matter, CEC, and exchangeable cations. Estimation of soil redistribution rate using ¹³⁷Cs and ²¹⁰Pb_ex showed site-specific variations at different points along the slope, and respective ranges were -17.46~-207.51 and 1.55~-581.38 Mg ha^-1 yr^-1, which indicated that more erosion was assessed by 210Pbex than by ¹³⁷Cs. Redistribution analysis showed that soil erosion occurred along the entire slope, except for the bottom point of the slope where 1.55 Mg ha^-1 yr^-1 of sediment accumulated. The USLE provided a single value of the average annual soil loss in the entire slope, which was either 166 or 398 Mg ha^-1 yr^-1, depending on the soil erodibility factor (soil series factor and calculated factor from soil sample analysis) used in the model. We conclude that the FRN method using ¹³⁷Cs and ²¹⁰Pbex radionuclides can be used to assess soil erosion and redistribution in steeply sloping agricultural highlands. Verification of soil-erosion values using the FRN method and soil-erosion models has been controversial, but it merits further study at many locations with different soils, topography, and management practices.