1. Resource and Environment College Northeast Agricultural Univ. Harbin 150030 China
2. Dep. Agronomy and Horticulture Univ. of Nebraska‐Lincoln Lincoln NE 68583
3. Institute of Chemical and Environmental Engineering Harbin Univ. of Science and Technology Harbin 150040 China
4. USDA‐Agricultural Research Service Agroecosystem Management Research Unit Lincoln NE 68583