Excess Lead-210 and Plutonium-239+240: Two suitable radiogenic soil erosion tracers for mountain grassland sites

The expected growing population and challenges associated with globalisation will increase local food and feed demands and enhance the pressure on local and regional upland soil resources. In light of these potential future developments it is necessary to define sustainable land use and tolerable soil loss rates with methods applicable and adapted to mountainous areas. Fallout-radionuclides (FRNs) are proven techniques to increase our knowledge about the status and resilience of agro-ecosystems. However, the use of the Caesium-137 (¹³⁷Cs) method is complicated in the European Alps due to its heterogeneous input and the timing of the Chernobyl fallout, which occurred during a few single rain events on partly snow covered ground. Other radioisotopic techniques have been proposed to overcome these limitations. The objective of this study is to evaluate the suitability of excess Lead-210 (²¹⁰Pb_ex) and Plutonium-239+240 (^239+240Pu) as soil erosion tracers for three different grassland management types at the steep slopes (slope angles between 35 and 38°) located in the Central Swiss Alps. All three FRNs identified pastures as having the highest mean (± standard deviation) net soil loss of -6.7 ± 1.1, -9.8 ± 6.8 and -7.0 ± 5.2 Mg ha^-1 yr^-1 for ¹³⁷Cs, ²¹⁰Pb_ex and ^239+240Pu, respectively. A mean soil loss of -5.7 ± 1.5, -5.2 ± 1.5 and-5.6 ± 2.1 was assessed for hayfields and the lowest rates were established for pastures with dwarf-shrubs (-5.2 ± 2.5, -4.5 ± 2.5 and -3.3 ± 2.4 Mg ha^-1 yr^-1 for ¹³⁷Cs, ²¹⁰Pb_ex and ^239+240Pu, respectively). These rates, evaluated at sites with an elevated soil erosion risk exceed the respective soil production rates. Among the three FRN methods used, ^239+240Pu appears as the most promising tracer in terms of measurement uncertainty and reduced small scale variability (CV of 13%). Despite a higher level of uncertainty, ²¹⁰Pb_ex produced comparable results, with a wide range of erosion rates sensitive to changes in grassland management. ²¹⁰Pb_ex can then be as well considered as a suitable soil tracer to investigate alpine agroecosystems.