Higher Diversity of Soybean-Nodulating Rhizobia Populations in Grassland in Black Soil.

　　Higher diversity of Soybean-nodulating Rhizobia populations in grassland in Black Soil 　　Xiao Zeng Han*, Jun Yaa 　　Key Laboratory of Mollisols Agroecology, Northeast Institute of Geography and Agroecology, Chinese Academy of Sciences, Harbin, 150081, China. 　　The soybean nodulating rhizobial genetic diversity after long-term arable cultivation, including soybean-, maize-, and wheat-monoculture (S-S, M-M, and W-W) and maize/soybean/wheat rotation (M/S/W), was compared with that under permanent grassland (GL) in Heilongjiang, China. All the isolates from different treatments were classified into four Bradyrhizobium genospecies by multilocus sequence analysis of 6 housekeeping genes. Conclusively, 1) Unique community structure of soybean rhizobia, dominated by the novel genospecies Bradyrhizobium sp. I, sp. II and sp. III, exists in the studied fields. 2) The higher diversity of the plant species in the GL fields helped to maintain the higher diversity of soybean rhizobia. 3) Cultivation of soybean increased the population size, but decreased the diversity of soybean rhizobia. 4) In monoculture systems, soybean favored Bradyrhizobium sp. I., while maize and wheat favored Bradyrhizobium sp. III. 6) OC, AP and pH were main soil parameters positively correlated with the population sizes and the groups of Bradyrhizobium sp. I, sp. II and B. japonicum USDA 6, but negatively correlated with Bradyrhizobium III. These results not only clearly revealed that the natural vegetation restoration and crop culture system affected the diversity and population size of rhizobia, but also demonstrated an interesting interactions between the rhizobia and plants of no legumes, which offered novel information for the biogeography of soybean rhizobia, for interactions among the rhizobia and their biotic and abiotic environmental factors, and for the evolution of rhizobial populations adapted to the significant changes of soil properties. 　　 Keywords: Interaction, Bradyrhizobium, Soybean, Maize, Wheat, Diversity