国家自然科学基金项目21175011,21375008;北京市科学技术研究院青年骨干计划项目;“十二五”农村领域国家科技计划课题2011BAD26B040406.@@@@National Natural Science Foundation of China Nos.21175011,21375008;Youth Plan of Beijing Academy of Science and Technology;Twelfth Five-Year-Plan in National Science and Technology for the Rural Devel-opment No.2011BAD26B040406.