Department Environmental Science and Energy Research, Weizmann Institute of Sciences, Rehovot 76100, Israel

Max Planck Institute for Chemistry, Biogeochemistry Department, P.O. Box 3060, 55020, Mainz, Germany

now at: Department of Environmental Sciences, The University of Toledo, Toledo, Ohio 43606-3390, USA

The Remote Sensing Laboratory, Jacob Blaustein Institute for Desert Research, Ben-Gurion University of the Negev, Sde Boker Campus, 84990, Israel

The Remote Sensing Laboratory, Jacob Blaustein Institute for Desert Research, Ben-Gurion University of the Negev, Sde Boker Campus, 84990, Israel

The Desertification and Restoration Ecology Research Center, Jacob Blaustein Institute for Desert Research, Ben-Gurion University of the Negev, Sde Boker Campus, 84990, Israel

now at: Department of Natural Resources, Agriculture Research Organization, Gilat Research Center, Ministry of Agriculture, Mobil Post Negev 85280, Israel

Max Planck Institute for Chemistry, Biogeochemistry Department, P.O. Box 3060, 55020, Mainz, Germany

Department Environmental Science and Energy Research, Weizmann Institute of Sciences, Rehovot 76100, Israel

Max Planck Institute for Chemistry, Biogeochemistry Department, P.O. Box 3060, 55020, Mainz, Germany