Effects Of Intercropping And Inoculating Rhizobias On Soybean Soil Bacterial Community Structure

In Chinese agricultural production, Soybean intercropping with Gramineae has a long history, not only can eliminate the problem of "nitrogen repression" in the soybean field, improving the nitrogen-fixing ability, but also can achieve double production. Therefore, this paper will combine soybean intercropping with Gramineae and inoculation with Sinorhizobium fredii WGF03 and deletion mutants AexoD, and study the effects of two factors on soybean growth and bacterial community structure.Results of plant physiological indicators show that:(1) comparing with monocropping soybean, intercropping soybean with sugarcane has the abilities to promote soybean growth and symbiotic nitrogen fixation,also the soybean plant height, number of nodules, nodules’weight, and nodules enzyme activity have increased, so the intercropping system can promote soybean growth and symbiotic nitrogen fixation capacity. (2) The most significant promotion effect is in inoculation of wild type WGF03. Combined effects of intercropping system and inoculation factors promote the growth of soybean together. Plant height at flowering and maturity increased by 7.29% and 5.3%, and the nodule numbers and nodule weight increased 44.95% ～ 70.89%, the nitrogenase activity are highest in the four treatments, reaching 28.99 μmol· g-1 · h-1.After inoculated mutant AexoD, the increasing trend was not obvious, even in the the number of flowering nodules, nodules’ weight and flowering nodule nitrogen fixation enzyme activity, inhibitory effect occurs, reduced 34.23% ～ 51.02%, which illustrates that the influence of this mutant exceeds intercropping system, playing a negative inhibition, thus, we suggesting that exoD gene can influence soybean plant growth, especially in nodules.Through analysing nodule endophytic bacterial community structure, we found that:(1) nodules endophytic bacterias in the flowering have a larger numbers, Similarity index between treatment groups range from 84.3 to 92.3,the differences between the treatment groups are larger; in maturity,the amounts decrease, Similarity index between treatment groups range from 90.3 to 95, and the differences between the treatment groups decrease. (2) Between monoculture and intercropping system methods, diversity and similarity have no significant difference, which is because in the intercropping system, sugarcane’s roots are far away from soybean nodules, nodules bacterial community structure aes not affected. (3) In flowering, inoculation treatment significantly reduced the kinds and the numbers of endophytic bacteria in nodules. Cyanobacteria which is dominant in flowring help foreign Rhizobium for biological nitrogen fixation. In maturity, the indigenous rhizobia ultimately dominant due to inoculated Rhizobium less competitive, so that the bacterial community structures become little difference among the four treatment groups, and the number of Cyanophyta also drastically reduced.Analysing soybean rhizosphere and non-rhizosphere soil bacterial community, we know that:(1) Soil bacterias are more abundant than in the corresponding nodules bacterias, and most of the rhizosphere soil bacterias are PGPR bacterias, which can promote the growth of plant and the absorption of nutrition. (2) Two analytical methods all draw a conclusion that the kind and the number of rhizosphere bacterial community are all larger than non-rhizosphere bacteria. (3) The similarity data showed by high-throughput sequencing method indicate that W4, W5, W6 have high similarities, but W1, W2, W3 are different from each other because of intercropping system and inoculation treatment. All indicate that in the intercropping system, non-rhizosphere community structure are affected in the reason of sugarcane roots stretch to soybean non-rhizosphere zone. (4) In rhizosphere soil, the diversity of inoculating WGF03 is larger than non-inoculation group, while the mutant bacteria in the rhizosphere and non-rhizosphere soil did not exhibit differences. We can speculate that the wild-type strain WGF03 has strong adaptability, and has a greater impact on the surrounding soil bacteria, their secreted a large number of extracellular polysaccharides or other substances attract bacterias to the rhizosphere part.