1. Department of Soil and Water Sciences, The Hebrew University of Jerusalem, P.O. Box 12, Rehovot 76100, Israel, Department of Plant, Soil and Insect Sciences, University of Massachusetts, Amherst, Massachusetts 01003, and Department of Remediation Research, UFZ - Centre for Environmental Research, Leipzig-Halle, Permoserstrasse 15, D-04318 Leipzig, Germany