Microbial contribution to organic chlorine forms over time in contrasted soils 1
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OSSWALD Aurélie , POSZWA Anne , BILLET David , BUENO Maïté , THIRY Yves , LEYVAL Corinne
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LIEC, UMR 7360 Université de Lorraine – CNRS, F-­‐54500 VANDOEUVRE LES NANCY ; LCABIE, UMR 5254, Technopôle Hélioparc Pau Pyrénées, F-­‐64053 PAU; 3
ANDRA, Direction R&D, Service transferts, 1-­‐7, rue Jean-­‐Monnet, F-­‐92298 Châtenay-­‐Malabry cedex. -­‐
Chlorine is a widespread element in terrestrial ecosystems, and chlorides (Cl ) is often considered as the main, conservative, form. However, recent studies (Bastviken et al. 2007; Redon et al. 2012) showed that chlorine is mostly present in soils as organically bound molecules and that organic chlorine (ClO) formation is primary mediated by microbial activities. But processes and rates of ClO formation and degradation in soils remain poorly understood. The aims of our study were (i) to quantify organic and inorganic chlorine contents in soils differing in their substrate and vegetation, then with contrasted microbiological and physico-­‐chemical 37
parameters, and (ii) to perform a Cl-­‐spiked soil experiment to estimate the rate of organic chlorine formation during incubations. Materials and methods, and experimental setup: Table 1. Soils localization, name (according to WRB classification) and cover. The organo-­‐mineral layers (A1 or Bure, Meuse (55), France Haut Jacques, Vosges, 88, France Ap, 0-­‐20cm) of 4 soils were Soil name (WRB) Calcaric cambisol Calcic cambisol Adystric cambisol Adystric cambisol collected in 2012, 2013 and 2014. Land use / vegetation Cultivated Beech forest Grassland Fir and beech forest Soil denomination in Calcareous cultivated Calcic forested Acid grassland Acid forested brown After sampling, microbiological this study brown soil brown soil brown soil soil parameters (enzyme activity, bacteria and fungi quantification) were determined and organic and inorganic chlorine contents in soils were measured by AOX after extractions based on Bastviken et al. (2007). Organic chlorine formation in the calcareous cultivated brown soil was studied in a 37
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tracer batch experiment. The soil was amended with a Na Cl solution (100mg/Kg Clinoganic) and incubated at 28°C for 15 (T1) and 90 (T2) days under 2 treatments: (i) sterilized (irradiation) soils and (ii) sterilized soils re-­‐inoculated by indigenous bacteria. Natural 35
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( Cl+ Cl) and tracer ( Cl) chlorine contents were determined in organic and inorganic fractions of incubated soils by ICP MS HR. Results and discussion: Fig 1. Total, extractable organic and non-­‐extractable organic chlorine contents measured in soils sampled in 2012, 2013, 2014 using AOX. -­‐ Extractable organic chlorine contents are very low (less than 1 mg/kg) compared to the non-­‐extractable organic chlorine contents. -­‐Total and non-­‐extractable organic chlorine contents of the acid forested brown soil were higher than in the other soils and gradually decreased over time. 37
Fig 2. Contents of tracer chlorine ( Cl) measured after incubation (T1 and T2) in the different calcareous forested brown soil fractions using ICP MS HR. Total tracer chlorine content measured is about 100 mg/kg, 37
which corresponds to the added levels of Cl in the spiked 37
solution. Some Cl from the tracer was measured in the organic fraction of soils at T1 and T2. This demonstrated that a 37
part of the spiked Cl, under an inorganic form at T0, has been transformed under an organic form during incubation. However, chlorine speciation is slightly and no significantly different between sterilized and re-­‐inoculated treatments. Conclusion: Chlorine contents measured in the sampled soils are in accordance with Redon et al. (2012). Nevertheless, microbial inoculation did not significantly modify organic chlorine contents formation in the experiment, which disagrees with Bastviken et al. (2007). Acknowledgement: This work is supported by the National Radioactive Waste Management Agency (ANDRA) in the frame of a PhD. We also thank the LCABIE laboratory and NEEDS project allowing us to achieve our measurements by ICP MS HR.