Abstract: Polymerase chain reaction-denaturing gradient gel electrophoresis and sequence analysis were adopted to assess the nifH gene diversity and community structure of soil nitrogen-fixing bacteria in five types of grassland (Filifolium sibiricum steppe, Stipa baicalensis steppe, Leymus chinensis steppe, S. grandis steppe, and S. kryrowi steppe) in Hulunbeier. The nifH gene diversity of nitrogen-fixing bacteria was the highest in L. chinensis steppe, followed by in S. kryrowi, S. grandis, S. baicalensis, and F. sibiricum steppes. The community composition of nitrogen-fixing bacteria differed significantly with grassland type. In L. chinensis steppe, the community composition of nitrogen-fixing bacteria had high similarity with that in S. grandis steppe, but the similarity among other three grassland types was lower than 60%. Phylogenetic analysis showed that most nitrogen-fixing bacteria were  Alpha-proteobacteria of Proteobacteria, belonging to Bradyrhizobium, Mesorhizobium, Rhodopseudomonas, and Azospirillum, respectively. In the 5 grassland types, free-living nitrogen-fixing bacteria and associative nitrogen-fixer were the major ecotypes in S. kryrowi steppe, while symbiotic nitrogen-fixer was the dominant ecotype in other four grassland types.

Key words: Cotton, Compound population, Cropping pattern, LAI, Leaf area duration