Biogeochemical cycle of radium: Insights into alkaline-earth cations budget and evidence of hidden vegetation uptake

Abstract

The migration of radionuclides in the environment represents a major current issue, especially radium (Ra) that hold one of the main radiation exposure hazards among Naturally Occurring Radioactive Materials. This study aims to assess the biogeochemical radium cycling within a forested ecosystem and to highlight the relevance of Ra isotopes for understanding calcium and other alkaline earth cations behavior. This study was carried out at the INRAE-Andra experimental beech-forested site of Montiers (Meuse, France). ²²⁶Ra and ²²⁸Ra concentrations were measured in various compartments of the ecosystem: soil solutions, vegetation and soil samples. In-situ experiment of litter decomposition was also performed.

Variations in the ²²⁸Ra/²²⁶Ra isotopic ratio across ecosystem compartments allowed to distinguish between ²²⁸Ra-rich weathering inputs and ²²⁸Ra-poor biological return fluxes, thereby enabling to trace the radium cycle. Radium exported by soil solutions predominantly originates from litter decomposition which begins releasing radium only 12–18 months after litterfall. Conversely, the (²²⁸Ra/²²⁶Ra) ratio in fine roots implied that trees absorbed Ra primarily from mineral soil weathering rather than from litter-derived sources. As a result, approximately 95 % of litter-derived Ra remains in the soil rather than being reabsorbed by vegetation, challenging traditional assumptions of significant reabsorption in cation budget models.

Flux calculations showed vegetation uptake and return fluxes dominate over drainage or atmospheric inputs fluxes. (²²⁸Ra/²³²Th) radioactive disequilibrium within the soil enabled estimation of Ra leaching flux and subsequent vegetation uptake. Results revealed that 7–23 % of the absorbed Ra is incorporated into annual, non-perennial biomass, while 12–43 % is allocated to new biomass increments. Consequently, 34–80 % of the Ra absorbed by vegetation appears to accumulate in unexpected, presumably long-lived, perennial tree structures such as sapwood. These results imply sustained accumulation over multiple years in perennial tissues. While this immobilization of Ra within beech trees may not be generalizable to all tree species, it highlights the need for specific species of detailed analyses across all tree compartments - including radial variations in sapwood - to avoid underestimating vegetation uptake, particularly for alkaline-earth cations such as calcium.