1. Univ. of Zurich; Winterthurerstr. 190 Zurich 8057 Switzerland
2. INRA, Agrocampus Rennes, UMR Sol, Agronomie Spatialisation; 65 Rue de Saint-Brieuc, CS 84215 35042 Rennex Cedex France
3. Agriculture and Agri-Food Canada; 2560 Hochelaga Blvd. Ste. Foy QC G1V 2J3 Canada