1. Department of Agronomy; Iowa State University; Ames IA 50011 USA
2. United States Department of Agriculture; Agricultural Research Service; Rangeland Resources Research Unit; Ft. Collins CO 80526 USA
3. United States Department of Agriculture; Agricultural Research Service; National Laboratory for Agriculture and the Environment; Ames IA 50011 USA
4. Department of Environmental Systems Science; Swiss Federal Institute of Technology; ETH-Zurich; Zurich CH 8092 Switzerland