Dynamics of Selenium Cycling in Deciduous Forests

Pamela Di Tullo1, Florence Pannier1, Maïté Bueno1, Isabelle Le Hecho1, Marie-Pierre Turpault2, Yves Thiry3
1 Laboratory of Bioinorganic Analytical and Environmental Chemistry (LCABIE), University of Pau and Pays de l’Adour/CNRS, UMR 5254, IPREM, Hélioparc, 2 Avenue du President Angot, 64053 Pau Cedex 9, France
2 INRA, Unité Biogéochimie des Ecosystèmes Forestiers 1138, 54280 Champenoux, France
3 Andra, Research and Development Division, Parc de la Croix Blanche, 1/7 rue Jean Monnet 92298 Châtenay –Malabry Cedex, France

O 8.7 in Trace element and metal biogeochemistry

14.07.2014, 15:55-16:15, H20

Selenium is a trace element with major environmental implications, due to the extremely narrow range between its essential and toxic concentrations. Furthermore, the significant inventory of 79Se within high level radioactive waste coupled with its long physical half-life makes the study of stable Se behavior (as a surrogate of 79Se) important for our understanding of the long-term safety of any radioactive waste repository.

Despite numerous studies during the last decades, uncertainties remain on Se biogeochemistry at the ecosystem scale. For example, no work on perennial vegetation and forest ecosystem involving an integrated dataset is to our knowledge published. This lack of reliable information may be due to well-known analytical problems linked to Se determinations at trace levels and the amount of samples to be collected and analyzed.

Thus, in order to enhance our understanding of long term Se fate in natural environment and our capacity of predictive modelling, dynamics of Se cycling were investigated in a temperate forest which is part of the Long-Term Environmental Observatory of Andra, built and monitored with INRA. Our study was carried out in a 50 years-old beech stand (France, Meuse/Haute-Marne region) located on a calcisol/brunisol. Firstly, Se inventories for major forest compartments (forest floor, soil, wood biomass, leaves and understory) and Se content in throughfall, litterfall and soil percolates at different depths were determined. Main annual fluxes used to quantitatively describe the biological cycling in forest ecosystem and input/output mass balances were established. In addition, with the aim to evaluate Se distribution at different soil depths and its overall mobility, parallel single extractions targeting operationally defined Se soil fractions were performed.

Based on our results the soil was shown to be an effective sink for Se originating from atmospheric deposition. The calculated stand budget for the whole soil column was positive for Se, accounting for 0.75± 0.31 g ha-1y-1. Annual Se uptake by trees and its further immobilization in ligneous compartments mobilize low amount of Se compared to the total soil inventory and its available pool. As a result, biological cycle related to above-ground biomass turnover seems to play a minor role in the global Se cycle although Se volatilization was not evaluated in this study.

The results obtained from selective extractions confirmed that organic matter greatly influence ambient Se retention through at least two main processes: (1) by acting as a selective sorbent for Se, most probably in association with microbial reductive reactions, (2) by sustaining the formation of organo-mineral associations, with a high affinity for Se. To conclude, the compilation of these data could be used to acquire insight into mechanism controlling the Se biogeochemistry on long time scale. With the current dataset a preliminary model for the Se cycle in forest ecosystem is also envisaged.

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last modified 2014-06-19