Bacterial FeIII reduction enhances the dissociation of Fe oxyhydroxides - organic matter associations in podzolic Bhs soil horizons

Vermeire, Marie-Liesse [UCL] Bonneville, S [ULB, Brusseles, Belgium] Stenuit, Benoît [UCL] Delvaux, Bruno [UCL] Cornélis, Jean-Thomas [ULG, AGRO BIOTECH, Gembloux]

Fe oxyhydroxides in soils can bind large amounts of dissolved organic matter (DOM) because of their specific surface area and variable charge surface. The formation of DOM–Fe oxide association is a major process of soil organic matter (SOM) protection against heterotrophic respiration, as well as a control of Fe oxyhydroxide reactivity and evolution. FeIII oxyhydroxides can undergo reductive dissolution in anoxic conditions without any biotic control. Yet it is now evidenced that microbial FeIII reduction primarily controls iron redox chemistry, and is one of the most significant event in soils. However, little is known on the impact of dissimilatory FeIII reduction on the fate of the Fe-SOM association in soils. We incubated during 96h three soil samples from Bhs horizons in the presence of Shewanella putrefaciens, a well-known dissimilatory FeIII-reducing bacteria. We measured the kinetics and extent of the release of DOM, Fe2+ and total Fe in solution. The three selected Bhs horizons are 270, 330 and 550 yr-old and were collected in a podzolic chronosequence. The soil free Fe content amounts to 2.648, 21.632 and 26.114 g kg-1 of soils, respectively in 270 yr-Bhs, 330 yr-Bhs and 550 yr-Bhs. The contents of both the short-range-order (SRO) Fe phases and mineral-protected C also increase with age from 2.271, 11.645 to 24.847 g kg-1 (SRO), and 0.8, 13.2, 15.9 g kg-1 (C) respectively in 270 yr-Bhs, 330 yr-Bhs and 550 yr-Bhs. We show that the release of dissolved Fe2+ and C increases by 2.8-fold and 1.7-fold, respectively, in the presence of Shewanella putrefaciens compared to control experiment. Furthermore, the kinetics of C release strongly follows the one of Fe2+ release. The concentration of DOM is positively correlated with the Fe2+ concentration in the presence of S. putrefaciens. Our results, showing the release of both Fe2+ and DOM, suggest that the dissimilatory FeIII-reduction during soil anoxic events can have a s