Dynamic succession of soil microbial community during continuous cropping of Astragalus membranaceus Bge. var. mongholicus (Bge.)

Background. Continuous cropping disturbs the balance between the microbes beneficial to a plant and the pathogenic microorganisms in the rhizosphere soil, which has both a direct and indirect adverse effect on soil and plant health. It is highly significant to understand the mechanism of the obstacle found in continuous cropping and to search for a reasonable rotation model to solve the problem of continuous cropping. Astragalus membranaceus Bge. var. mongholicus (Bge.) (A. mongholicus) is a critical traditional Chinese herb, which is negatively affected by continuous cropping. Previous studies on the root rot pathogens of A. mongholicus have been conducted, while reports on the effects of A. mongholicus on the health of soil affected by continuous cropping are lacking. Methods. In this study, we observed the microbial community structure and the diversity of the rhizosphere soil under continuous cropping for 1, 3, and 6 years using the pyrosequencing approach, and compared this to bulk soil, using A. Mongholicus as the experimental material. The 16S rDNA and ITS amplicon sequencing techniques were used to detect the composition and diversity of bacteria and fungi in the rhizosphere soil and the bulk soil of A. Mongholicus. The diversity of the bacterial community and the structures of the rhizosphere and bulk soils were compared. The dynamics of the soil enzyme activity were also analyzed. Results. The results of this study illustrated that the continuous cropping of A. mongholicus caused a decline in the root dry weight, the ratio of root-top, and also influenced the growth of the root system of A. mongholicus. Continuous cropping and the sampling time shifts the diversity and structure of the microbial community in the rhizosphere soil of A. mongholicus, showing that the diversity of the microbial community in the A. mongholicus rhizosphere soil was decreased with an increase in the replanting years, while the structure of the microbial community deteriorated. The relative abundance of pathogenic fungi, Fusarium, Erysiphe, Rhizobiales, and Burkholderiales as well as bacteria related to nodulation were enriched in the A. mongholicus rhizosphere soil at different sampling stages. The beneficial bacteria decreased with the increasing years of continuous cropping during growth, which resulted in the microecological imbalance in the A. mongholicus rhizosphere, caused serious replanting diseases of continuous cropping. A decline in soil urease and invertase activities was observed after 6 years of continuous cropping. Our experimental results suggest that continuous cropping has a significant impact on soil bacterial and fungal community development, and that an increase in replanting years resulted in more negative impact on rhizosphere soil health and A. mongholicus growth.