The work was financially supported by the Ministry of Education,
People’s Republic of China as a grand fostering project (grant
no. 707011). The authors also thank the National Natural Science
Foundation of China for a key project (grant no. 40930739), and
the Ministry of Science and Technology, People’s Republic of China
as the Hi-tech Research and Development Program (863) of China
(grant no. 2006AA06Z386), and the Sino-Russian Joint Research
Center on Natural Resources and Eco-Environmental Sciences.