1. Department of Mathematics, Northeast Forestry University, Harbin 150040, China
2. Engineering Research Center of Agricultural Microbiology Technology, Ministry of Education, Harbin 150080, China
3. Heilongjiang Provincial Key Laboratory of Ecological Restoration and Resource Utilization for Cold Region, Harbin 150080, China
4. School of Mathematical Science, Heilongjiang University, Harbin 150080, China