Molecular Analysis Of The Changes Of Soil Microbial Communities In Continuous Cropping Of Soybean In The Black Soil Of Northeast China

Heilongjiang Province is the main producing region of soybean in China,and the soybean planting area accounts for about 40% of the national planting area.The soybean planting area in Heilongjiang Province has gradually increased under the adjustment of the national crop planting structure and the implementation of the soybean industry revitalization plan in recent years.The long-term experiment and multi-point field investigation confirmed that soybean yield was significantly decreased by 10-30% under continuous cropping for 1-3 years.The occurrence of continuous cropping obstacle in Northeast China has becoming an unavoidable practical problem.Therefore,the study on the shifts of microbial community composition,diversity distribution and structures between continuous cropping of soybean and rotation with maize is expected to provide scientific basis for understanding the mechanisms of soybean continuous cropping obstacle.This study was based on long-term experiment in the black soil region of Northeast China,the abundance,diversity and community composition of bacteria,fungi and archaea in the bulk and rhizosphere soils under short-term continuous cropping of soybean for 3 years(CC3)and 5 years(CC5),long-term continuous cropping of soybean for 13 years(CC13),and soybean-corn rotation for 5 years(CR5)were analyzed by qPCR and high-throughput sequencing.The main results are as follows:(1)Compared with CC3 and CC5,CC13 and CR5 significantly increased soil pH and nutrient contents,such as nitrogen,phosphorus and potassium in bulk soils.CC13 and CR5 significantly increased bacterial abundance and alpha diversity both in the bulk and rhizosphere soils.PCoA and RDA analysis showed that continuous cropping of soybean significantly changed the bacterial community structures in bulk and rhizosphere soils.This differentiation was mainly affected by soil pH and nutrient contents,with soil pH was the most important factor regulating the changes of soil bacterial community structures.CC13 and CR5 significantly increased the relative abundance of beneficial bacteria Bradyrhizobium sp.and Gemmatimonas sp.,but decreased the relative abundance of potential pathogenic bacteria Burkholderia thailandensis and Burkholderia mallei,which may have a positive effect on improving soil environment.In addition,the results of network analysis showed that CC13 and CR5 enhanced the stability of bacterial network structure compared with CC3 and CC5.(2)Compared with CC3 and CC5,CC13 and CR5 significantly increased and decreased fungal abundance in bulk and rhizosphere soils,respectively.The years of continuous cropped soybean had no significant effect on fungal alpha diversity,but the fungal alpha diversity was significantly increased in CR5.The continuous cropping years and cropping system significantly affected the fungal community structures in both the bulk and rhizosphere soils.The results of RDA analysis showed that soil C/N was the main factor driving the variations of soil fungal communities.Besides,CC13 and CR5 significantly reduced the relative abundance of root rot pathogens Fusarium oxysporum,while significantly increased the relative abundance of antagonistic Epicoccum nigrum,and beneficial fungi Trichosporon sp.and Mortierella sp.In addition,the network structure of fungal communities was more complex in CC13 and CR5 than CC3 and CC5,indicating that long-term continuous cropping of soybean and soybean-corn rotation increased the interaction among fungi.(3)Compared with continuous cropping systems,crop rotation significantly reduced the alpha diversity of rhizosphere soil archaeal communities.The results of PCoA and RDA analysis showed that the community structure of soil archaea was affected by cropping systems and continuous cropping years,and soil C/N was a primary factor driving the changes of soil archaea communities.Moreover,compared with bacterial and fungal communities,the soil archaea communities were relatively simple with 5 archaea phyla were detected,namely,Thaumarchaeota,Euryarchaeota,Crenarchaeota,Nanoarchaeota and Unclassified.Thaumarchaeota and Nitrososphaeria are dominant archaea phylum and class in both the bulk and rhizosphere soils.And the relative abundance of Nitrososphaeria with ammonia oxidation were remarkably increased in CC13 and CR5,suggesting that long-term continuous cropping of soybean and soybean-corn rotation may promote the soil nitrogen cycling process.This study demonstrated that soil microbial abundance,diversity and community structure composition were significantly changed by continuous cropping years and cropping systems.The structure variations of bacterial communities were mainly affected by soil pH,while that of fungal and archaeal communities were mainly restricted by soil C/N.Soil microbial community structure and network topological properties of CC13 were similar to those of CR5,but were remarkably different with CC3 and CC5.Meanwhile,CC13 significantly increased the relative abundance of soil beneficial microorganisms and decreased the relative abundance of pathogenic microorganisms,indicating that the disease suppressive soils might be developed after long-term continuous cropping,and the recovery of biological factor is the main reason to alleviate the soybean continuous cropping obstacles.This research benefits for us to understand the mechanism of soybean continuous cropping obstacles,and is also important for us to protect and utilize the valuable soil resources from view of microbial aspects.