Study On Assembly And Mechanism Of Soil Microbial Community In Alfalfa Planting Systems

Alfalfa（Medicago sativa L.）is a widely cultivated perennial legume crop and high-quality livestock feed.Intercropping and rotation systems have excellent ecological and economic benefits,which mainly based on the biological nitrogen fixation of alfalfa to enhance soil quality,thus affecting the growth of adjacent crops and improve crops yield.As for alfalfa planting systems,studying the soil microbial spatial distribution pattern and driving factors and revealing the generation and maintenance mechanism of soil microbial diversity are very important for in-depth understanding of the relationship between community stability and ecosystem functions and the sustainable management of agroecosystem.In this study,we collected soil samples from 114 sites in Northeast China to analyze the regional distribution characteristics and potential ecosystem functions of abundant and rare bacteria,responses to environmental changes and the driving mechanism of community construction in different alfalfa planting modes（crop monoculture and rotation）.Then,we revealed the relationship between fungal community assembly and potential functions and the important ecological role of core fungi in farmland ecosystem.Considering the close relationship between rhizosphere microorganisms and plant growth,we collected rhizosphere samples from 10 alfalfa fields in North China to explore the differences of rhizosphere soil microbial community composition and yield in alfalfa monoculture and rotation systems with different planting years.Additionally,we demonatrated the potential role of rhizosphere environmental changes on the relationship between community assembly process,microbial community composition and alfalfa yield.The main results are listed as follows:（1）In Northeast China,we detected significant distance-decay relationships for the abundant and rare sub-communities in both crop monoculture（CM）and rotation（CR）systems Significant（P<0.001）distance-decay relationships were detected for the abundant and rare sub-communities in both CM and CR systems.In addition,edaphic variables(CM_abundant=27.3%,CM_rare=9.1%;CR_abundant=33.3%,CR_rare=15.3%)largely contributed to the variation of sub-communities based on variance partitioning analysis（VPA）.Shannon and Chao1 indices were significantly（P<0.05）different between abundant and rare sub-communities in CM and CR systems.Both abundant(R_ANOSIM=0.065,P=0.004)and rare(R_ANOSIM=0.054,P=0.013)bacterial community were significantly（P<0.05）separated according to CM and CR systems.Based on Tax4Fun,for Proteobacteria,environmental information processing was the main potential function in CM,while metabolism was the one in CR system.Additionally,the potential functions of rare taxa were greater than that of abundant taxa both in CM and CR systems.Our study reveals regional distribution and diversity of abundant and rare bacterial communities in different cropping systems,which not only provide theoretical support for better understanding the roles of rare and abundant bacteria,but also offered new opportunity to improve agricultural ecosystem functions.（2）To compared with abundant sub-community,rare bacteria had closer phylogenetic clusters(ses.MNTD values:CM_abundant=-9.15,CM_rare=-11.29;CR_abundant=-9.35,CR_rare=-11.03)and higher phylogenyβ-diversity.The ecological preference of abundant bacteria to the same environmental gradient had a wider response threshold and stronger phylogenetic signal,and the environmental adaptability of abundant and rare bacteria in rotation system was greater than that of corresponding abundant and rare bacteria in CM system.The dispersal limitation(CM_abundant=80.46%,CR_abundant=87.66%)dominated the community assembly process of abundant taxa,while the rare bacterial community was mainly affected by variable selection(CM_rare=55.69%;CR_rare=51.46%).Soil p H was a determining factor that affect the deterministic and stochastic process balance of rare and abundant bacterial sub-communities.Our research expands the knowledge of environmental adaptation of abundant and rare bacteria to different planting patterns in farmland ecosystems,and enhances our understanding of the basic mechanisms of bacterial diversity generation and maintenance in the context of global environmental change.（3）There was a significant distance attenuation relationship between core and non-core fungal communities（P<0.001）in CM and CR systems.The richness and Shannon index of non-core fungi were significantly higher than core fungi（P<0.001）.Core(R_ANOSIM=0.07,P=0.003)and other(R_ANOSIM=0.05,P=0.01)taxa were significantly separated.Dispersal limitation dominates the community assembly process of core（CM=88.55%,CR=86.99%）and other（CM=78.59%,CR=79.93%）fungi,but the community assembly of core fungi has greater stochasticity.Cladophialophora,Preussia,Chaetomiaceae,Trichocladium,Stachybotrys and Hypocreales were 6 key microbiota in CM and CR systems.In addition,we found that compared with other fungi,and there was more connection between core fungal community assembly process and potential functions.These results could be employed to predict the diversity pattern and assembly process of soil fungi in different planting patterns by human disturbance.Additionally,our results would help to improve our understanding of the changes of ecosystem functions driven by soil microbial diversity in farmland ecosystems.（4）In North China,alfalfa monoculture yield for 6 years showed a significant（P<0.05）downward trend.Wheat-corn-alfalfa rotation generated higher（P<0.05）soil fertility index and alfalfa yield than wheat-alfalfa rotation and 2yr alfalfa monoculture.The alfalfa yield significantly（R²=0.583,P<0.001）increased with soil fertility.Significant differences were observed for bacteria(R_ANOSIM=0.889,P=0.001)and fungal(R_ANOSIM=0.955,P=0.001)communities in different planting years and regimes.Different ecological processes dominated bacterial and fungal communitie assembly.Blastococcus and Massilia were important biomarkers,which had a significant（P<0.05）positive correlation with alfalfa yield.Partial least squares path modeling（PLS-PM）analysis identified that planting regimes had positive impacts on alfalfa yield through edaphic variables,microbial genera and fungal community composition.Conversely,planting years had negative effects on alfalfa yield by indirectly regulating fungal genera and community composition through fungal community assembly.This study could help improve our understanding of the role and importance of rhizosphere soil microbial changes in regulating farmland soil processes and crop productivity.In conclusion,soil abundant and rare bacteria and core and other fungal communities in monoculture and rotation systems have obvious biogeographic distribution rather than random distribution,and their microbial community diversity characteristics are also significantly different in a large spatial range,which is closely related to the environmental adaptability of microbial taxa and the process of community assembly.Secondly,core microbial taxa may play an important ecological role in maintaining the complex relationship between soil fungal taxa in the farmland ecosystem,as well as in the underground nutrient cycle.In addition,the yield of alfalfa decreased significantly after more than 6 years of monoculture.The soil fertility and alfalfa yield under wheat-corn-alfalfa rotation were more dominant,and more common beneficial bacteria were enriched in its rhizosphere,which were closely related to alfalfa yield.In addition,the process of community assembly could have a potential relationship with alfalfa productivity by adjusting the differences of microbial genera and community composition.This would provide theoretical support for increasing crop yield and maintaining ecosystem stability through microbial regulation in the future.