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

Abstract

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 (Cs) 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 (Pb) and Plutonium-239+240 (Pu) 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 yr for Cs,Pb and Pu, 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 yr for Cs,Pb and Pu, respectively). These rates, evaluated at sites with an elevated soil erosion risk exceed the respective soil production rates. Among the three FRN methods used,Pu 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, Pb produced comparable results, with a wide range of erosion rates sensitive to changes in grassland management. Pb can then be as well considered as a suitable soil tracer to investigate alpine agroecosystems.

Rapid decrease of soil erosion rates with soil formation and vegetation development in periglacial areas

“…This also accords with other studies in the Swiss Alps, e.g. of Zollinger et al (2015) and Meusburger et al (2018). Alewell et al (2013), however, found slightly higher values in the Urseren valley and Val Piora (15 and 30 km southeast of Sustenpass), suggesting that a minor fraction of the Pu at these localities may have originated from Chernobyl.…”

“…The distribution of Pu is spatially more homogenous (owing to the mostly global fallout origin) and its extremely low mobility in the soil. For these reasons, an increasing number of studies have used the 239+240 Pu method in the past decade, especially in the Alps (Gabrieli et al, 2011; Zollinger et al, 2015; Meusburger et al, 2016, 2018). In addition, the mathematical models used to trace soil erosion using Pu have been steadily improved (Lal et al, 2013; Arata et al, 2016).…”

“…To date, most studies using FRN as a tracer for soil erosion have focused on agricultural soils (Schimmack et al, 2002; Calitri et al, 2019) or pastures (Tims et al, 2010; Lal et al, 2013; Alewell et al, 2013; Meusburger et al, 2018). In order to assess whether soil erosion rates are still sustainable, they need to be compared to the rates of soil production (Meusburger and Alewell, 2014; Alewell et al, 2015).…”

Promoting the use of isotopic techniques to combat soil erosion: An overview of the key role played by the SWMCN Subprogramme of the Joint FAO/IAEA Division over the last 20 years

“…Since the start of the CRP D1.50.17, some preliminary and promising findings contributing to refining the FRN and CSSI techniques have already been obtained and published by participants of the CRP: the Department of Environmental Sciences (University of Basel, Switzerland) in collaboration with the SWMCN Laboratory developed a “universal” conversion model, called MODERN (i.e., Modelling Deposition and Erosion rates with RadioNuclides), to assess soil redistribution magnitudes from FRN measurements (Arata et al, ). MODERN is the only conversion model that can be used for 137 Cs, 210 Pb ex , and 7 Be as well as for the new soil tracer 239+240 Pu; plutonium isotopes (i.e., 239+240 Pu) have been tested and validated relative to other more mature radioisotopic approaches for deriving soil erosion rates under various upland agro‐environments in Switzerland (Arata et al, ; Meusburger et al, ) and also in South Korea (Meusburger et al, ); the development of a cost‐effective sampling strategy when using CSSI techniques to reduce and optimize analytical labour (Mabit et al, ), and the use of artificial mixtures to confirm the accuracy and reliability of the CSSI mixing models, a new proposal for converting isotopic proportion into soil proportion using the FA concentrations instead of the total % C org has been made (Alewell, Birkholz, Meusburger, Schindler Wildhaber, & Mabit, ); through their studies in South West England, Taylor, Keith‐Roach, Iurian, Mabit, and Blake () reported that the use of cosmogenic 7 Be as a soil erosion and/or sediment tracer requires an accurate knowledge of its temporal fallout dynamics as its deposition flux can be highly variable across months and seasons; a study performed in the Madagascar highlands (see next section) highlighted that the combined use of 137 Cs and 210 Pb ex allowed evaluating the effectiveness of ancient terracing practices to protect soil against erosion. For the first time, this pilot Malagasy FRN investigation highlighted that despite low expected 137 Cs activity, this method can still be used with success in African countries located in the Southern Hemisphere.…”

Fast recovery of lead from hydrochloric acid via a novel silica-supported anion exchange resin for the determination of 210Pb in environmental samples

“…The measurement of 210 Pb is significant for environmental studies, as it provides quantitative information on the flux of 222 Rn and its daughter nuclides in the atmosphere [4][5][6]. Such measurements are applied in uranium exploration, monitoring radionuclide migration in the uranium series, and luminescence dating of sediments [7][8][9]. 210 Pb and its daughter 210 Po have also aroused great public concern because they account for 35% of the total internal irradiation dose resulting from naturally occurring radionuclides.…”