Organic and Mineralogical Properties of the Rhizosphere That Promote Uranium Binding in Wetland Soils.

Subsurface contaminants move with groundwater flow and eventually resurface in wetlands.  While many contaminants can move readily through aquifers, the movement of most contaminants is strongly attenuated in wetlands because of the presence of geochemistry, microbiology, and hydrology conditions that promote contaminant binding.  The objective of this study was to evaluate the unique organic matter and mineralogical properties of the rhizosphere that may contribute to contaminant binding.  A three-year field study in a uranium-contaminated wetland was conducted to study the role of uranium immobilization in the rhizosphere.  Uranium was 15x greater in the rhizosphere then in the bulk wetland soil.  Contrary to expectation, XAS analysis indicated that rhizosphere uranium existed predominantly in the +6 oxidation state bound to OM and was not precipitated in the reduced +4 oxidation state.  The rhizosphere also had elevated carbon, nitrogen, manganese, and iron concentrations and total bacteria, Geobacter, and Anaeromyxobacter counts (as identified by qPCR).  Furthermore, the rhizosphere contained several organic molecules that were not identified in the non-rhizosphere soil; 54% of the >2200 ESI-FTICR-MS identified compounds were unique to the rhizosphere.  Rhizosphere organic matter (OM) contained several unique lipids, unsaturated hydrocarbons, and proteins not found in the non-rhizosphere OM.  XRD and Mössbauer analyses indicated that the rhizosphere had no ferrous iron containing minerals and had much greater ferrihydrite-like and nanogoethite phases than the bulk soil.  Together these finding have implications on the stewardship and long-term management of vegetation of contaminated wetlands.