1. Institute of Crop Science and Resource Conservation, Soil Science and Soil Ecology, Univ. of Bonn; Nussallee 13 53115 Bonn Germany
2. Agrosphere (IBG-3), Jülich Research Centre; 52425 Jülich Germany
3. U.S. Department of Agriculture-Agricultural Research Service, Soil and Water Management Research Unit; 1991 Upper Buford Circle St. Paul MN 55108