Impact Of No Tillage System On Soil Microbial Community Structure Of Cropland In Mollisol

The Mollisol in Northeast China is fertile.However,due to the continuous high intensity utilization in recent decades,the fertility of Mollisol has declined and other related problems have rised.Conservation tillage is a better alternative.No tillage(NT)has been proved to improve soil microbial community.However,due to the limitation of research technology,the mechanism of the effect of NT on soil microbial community is still not clear.Therefore,based on 15 years of long-term location experiment,using high-throughput sequencing and structural equation modeling(SEM),our objectives were to compare the soil microbial community at 0-5 and 5-20cm depths in NT and conventional tillage(CT),and to explore the interactions among tillage,soil and microbial community.The study aims to provide theoretical support for the construction of efficient,ecological and sustainable conservation tillage technology system in Northeast China.The main conclusions are as follows:(1)The results showed that the content of chemical properties p H,SOC,TN,DOC,DON,NO₃-N,NH₄-N,TP,AP,TK AK,and microbial properties MBC,MBN,BR,q MIC,q CO₂,and soil organic carbon components LF,POC,f POC,MOC at 0-5cm depth were increaed by NT treatment.In addition,NT increased soil organic carbon storage.To sum up,we suggest that NT cropland should be ploughed every few years to mix the topsoil and subsoil.This study provides theoretical guidance for soil improvement in northeast cold area.(2)NT5 significantly increased the abundance of soil bacteria.The number of shared OTU between two soil layers in CT was more than that in NT.The dominant phyla of bacteria in NT and CT include Proteobacteria,Actinobacteria,Acidobacteria,Chloroflexi and Gemmatimonadetes.The Chao 1 and Shannon of NT5 were significantly higher than those of NT20.NMDS analysis showed that the bacterial communities of NT and CT had their own cluster distribution,and the diversity and composition of bacterial communities in topsoil were significantly different from those in subsoil.The indicator species of NT5 is lautropia.There were significant changes in extracellular structure,transcription,coenzyme transport and metabolism,secondary metabolite biosynthesis,transport and catabolism,cell cycle control,cell division,chromosome division,energy production and conversion,lipid transport and metabolism,and Cytoskeleton under NT.The relative abundance of C and N related dominant functional bacteria Arenimonas,Devosia,Ferruginibacter,Flavobacterium,Hyphomicrobium,Mesorhizobium,Oryzihumus,Pedomicrobium,Segetibacter and Tumebacillus were significantly different between NT5 and NT20,but not between CT5 and CT20.The SEM showed that tillage and soil depth together explained 88%of the diversity and 82%of the composition of bacterial community,respectively.BD,p H and SOC had significant indirect effects on the bacterial diversity and composition.In conclusion,this study suggested that in NT soil,if unqine 0-20 cm layer was taken,the diversity and composition of bacterial community between the upper and lower layers of soil might be neglected.The results of this study deepen the understanding of the role of conservation tillage in changing soil bacterial community structure and the understanding of soil physical and chemical properties driving bacterial community structure.(3)The fungal gene copy number of NT20 was significantly lower than that in other treatments.The dominant phyla of fungi in NT and CT include Ascomycota,Zygomycota,Basidiomycota,Rozellomycota,and the dominant genera include Mortierella,Guehomyces and Fusarium Trichoderma.The Chao 1 and Shannon in NT5 were significantly higher than those in NT20,but there was no difference between CT5 and CT20.The results showed that the fungal community composition of 0-5 cm and 5-20 cm depth under NT and CT could form a good cluster distribution.NT changed the diversity and composition of soil fungi,and the diversity and composition of fungi changed with soil depth.The indicator species of NT5 is Coniochaetales?sp.Most of the relative abundance of potential pathogenic fungi in NT5 treatment were significantly higher than that in NT20,but there was no significant difference between CT5 and CT20,and there was no consistent trend of potential beneficial fungi.The abundance of soil pathogenic fungi was the highest in NT5.SEM showed that tillage and soil depth could explain 64%diversity and 95%composition of fungal community,respectively.Tillage and soil depth indirectly changed the diversity and composition of soil fungi by affecting SOC,p H and BD.(4)The relative abundance of Proteobacteria at phylum level of soil diazotrophic was the highest in NT and CT soils.At the genus level,the relative abundance of Bradyrhizobium was the highest,followed by unclassified bacteria and Geobacter.The difference of Bradyrhizobium in all treatments was not significant.Tillage and soil depth had no significant effect on the copy number of nif H gene,Chao 1 and Shannon.PCo A analysis showed that there were significant differences in the composition of soil diazotrophic community between NT and CT.All soil physical and chemical characteristics were not significantly correlated with copy number of nif H gene,Chao 1 and Shannon,but soil bulk density was significantly correlated with PC1.Mantel test showed that soil diazotrophic community was significantly correlated with soil bulk density.The results of this study highlight the important role of soil bulk density in the composition of soil diazotrophic community under NT,and deepen our understanding of the effect of tillage methods and soil depth on soil diazotrophic community.