1. Environmental Geochemistry Group, LGIT, University of Grenoble and CNRS, BP 53, 38041 Grenoble, Cedex 9, France, Laboratoire d'Ecologie Microbienne de la Rhizosphere, UMR CNRS/CEA, no. 163 CEA DEVM Centre de Cadarache, 13108 Saint-Paul lez Durance, Cedex, France, Universite Aix Marseille 3, CNRS, CEREGE, 13545 Aix en Provence, France, and Lawrence Berkeley National Laboratory, Advanced Light Source, MS 6-2100, Berkeley, California 94720