Major Role of Interactions between Organic Matter Biodegradability, Iron Reducing Bacteria and Ferric Oxides in the Availability and Partitioning of Metals in Soils.
Jacques Berthelin, Nouredine Bousserhine, Cecile Quantin, and Sebastien Stemmler. LIMOS , UMR 7137 ,CNRS _UHPNancy1, Faculté des Sciences, Boulevard des Aiguillettes , BP 239, Vandoeuvre les Nancy, 54506, France
Depending on the soil type, the horizon, the microsites (e.g. rhizospheric, non rhizospheric) large communities (10 2 to 10 8 per g dry soil) of cultivable iron reducing bacteria, able to dissolve ferric oxides have been observed in different soils. Among them, fermentative bacteria presented large efficient communities. Field analysis of soil solutions, of bacterial communities, of soil constituents on one hand and laboratory experiments on the other hand have shown that aero-anaerobic and anaerobic bacterial communities are strongly involved in the mobilization of iron, the weathering of ferric oxides and oxyhydroxides not only in waterlogged soils but also in temporarily hydromorphic soils. Significant correlation have been determined between bacterial degradation (oxidation) of soil organic matter and bacterial reduction and dissolution of soil ferric oxyhydroxides. Different yields have been calculated between organic matter biodegradation - mineralization and ferric reduction that underline the fundamental role of soil organic matter quality and biodegradability. Bacterial dissolution of Fe and Mn oxides bearing metals and involving iron reducing bacteria promotes solubilization of different metals in substitution in the mineral lattice (Co, Ni, Cr...). The metal substitution has different effects, depending on the nature of the metal and the substitution index: e.g. aluminium substitution decreased but manganese or cobalt have no effect on goethite dissolution kinetics. Different parameters can be involved in the control of these processes and are discussed. Such bacterial weathering of oxyhydroxides depend strongly on the availability and biodegradability of organic matter . It has a significant impact on the partitioning and distribution of the major and trace elements into the different geochemical compartments (water soluble, exchangeable, associated to organic matter , to Mn or Fe oxides...) of the soil solid phases. After such weathering the relative metal content (Fe, Mn, Co, Ni, Cr...) increases in the most labile compartments (e.g. water soluble, exchangeable...). It also increased in the amorphous and poorly crystallized Fe oxides phase and is modified in the organic compartment. This can be related to “amorphisation” of minerals during bacterial weathering associated to organic matter biodegradation . Under aero-anaerobic and anaerobic conditions, bacterial degradation of soil organic matter in interaction with bacterial Fe and Mn reduction can drastically influence the mobility of metals and their partitioning into the different compartments of the solid phases. Such processes are important to be known and quantified to determine metal behavior and cycling, to improve the definition of their availability and risk assesment and also to propose basis and models of geomicrobiological functioning and recommandation for management and development of environmental technologies.