1. State Key Lab. of Soil Erosion and Dryland Farming on the Loess Plateau; Northwest A&F Univ.; Yangling 712100 China
2. Institute of Plant Nutrition and Soil Science, Christian-Albrechts-Univ. zu Kiel; Olshausenstr. 40 24118 Kiel Germany
3. Dep. of Agronomy; Iowa State Univ.; Ames IA 50011-1010
4. College of Resources and Environment of Southwest Univ.; Chongqing 400715 China