1. Departments of Soil, Water, and Climate (C.U.-S., K.E.Z., A.L., D.L.A.) and
2. Agronomy and Plant Genetics (C.U.-S., K.E.Z., M.R.-Y., A.L., C.P.V.), University of Minnesota, 1991 Upper Buford Circle, St. Paul, Minnesota 55108;
3. Centre de Investigación Sobre Fijación de Nitrógeno, Universidad Nacional Autónoma de México Apdo Postal 565–A, 62210 Cuernavaca Mor., Mexico (M.R.-Y.); and
4. United States Department of Agriculture-Agricultural Research Service, Plant Science Research Unit, 1991 Upper Buford Circle, St. Paul, Minnesota 55108 (C.P.V.)