Perennial grassland led to more complex bacterial communities as well as inter-domain networks across three continuous cropping systems
Authors: Zhu Ruifen , Liu Chang , Xu Yuandong , Sun Wanbin , Chen Jishan 
Published: March 2024

Abstract
continuous cropping is the main strategy to accelerate vegetation restoration and improve rhizosphere microbial community in northeast of China. However, it is largely unknown that the response of rhizosphere microbial community structures, specific microbial taxa, and co-occurrence patterns to continuous cropping with varieties. In this study, three different continuous cropping systems (Corn, Alfalfa and Sheepgrass) collected from the rhizosphere microbial community (bacterial and fungal) were analyzed by application of an Illumina HiSeq high-throughput sequencing technique. The results showed that the α and β-diversity indices of the fungal communities were significantly different across the three continuous cropping systems. In addition, the analysis showed that the dominant bacterial phyla were Proteobacteria (25.6% (Alfalfa), 25.6% (Corn), 25.6% (Sheepgrass)), Acidobacteria (19.5%, 19.4%, 19.5%), Actinobacteria (11.3%, 11.3%, 11.3%) and Gemmatimonadetes (9.9%, 9.8%, 9.9%), whereas Ascomycota (37% (Alfalfa), 40% (Corn), 39% (Sheepgrass)), Basidiomycota (10%, 10%, 11%), and Glomeromycota (4%, 4%, 3%) represented the most abundant fungal taxa. Through the analysis of the molecular ecological network (MENA), Alfalfa has the largest number of nodes and edges in the bacterial network and Sheepgrass has the largest number of nodes and edges in the fungal network. However, the highest ratio of positive to negative links in Alfalfa was more than three times higher than that in the others (Alfalfa = 6.22, Corn = 1.48 and Sheepgrass = 2.04). The fungal networks showed a different trend with a higher ratio of positive to negative links in Sheepgrass than in Corn. The fungal network also revealed an increase in the ratio of positive to negative links in Alfalfa (6.22), similar to bacterial networks. To distinguish differences in taxa co-occurring among the continuous cropping systems, we compared the number of positive and negative links within and between microbial groups. Microbial networks were dominated by bacteria-fungi associations (10.39%) for Alfalfa, bacteria-fungi associations (8.42%) for Corn, and bacteria-fungi associations (12.64%) for Sheepgrass. Bacteria-bacteria associations represented 64.68% from Alfalfa, 46.47% from Corn and 22.61% from Sheepgrass of the total number of links, respectively. In contrast, fungi - fungi associations represented 24.93%, 45.11% and 64.75% of the total number of links, respectively. However, the positive to negative link ratio for the bacteria-bacteria associations was the highest in Sheepgrass (Alfalfa = 1.89, Corn = 3.27 and Sheepgrass = 5.37). Associations across the two groups were mainly characterized by a higher number of links in Sheepgrass (perennial grasslands). This highlights that perennial grassland systems led to more complex bacterial as well as inter-domain networks, which can have important implications for the contribution of microbes to soil ecosystem. Inter-domain networks also revealed the predominant role of the fungi as key taxa in soil microbiome networks across all continuous cropping systems.