1. College of Agronomy and Biotechnology China Agricultural Univ., Key Lab. of Farming System, Ministry of Agriculture of China Beijing 100193 China
2. Nanjing Research Institute for Agricultural Mechanization Ministry of Agriculture of China Nanjing 210014 China
3. Soil and Fertilizer Institute of Hunan Province Changsha 410125 China
4. Carbon Management and Sequestration Center, School of Environment and Natural Resources The Ohio State Univ. Columbus OH 43210