Microbial-mediated Carbon Sequestration Mechanism Under Long-term Conservation Tillage Practices

Farmland ecosystems act as the most active carbon pool and play an key role in maintaining the global carbon balance.Conservation tillage is considered as one of the most effective practices to improve soil carbon storage and soil health in arid regions,and the effect of tillage practices on soil C dynamics is mediated by soil microbes.Therefore,it is meanfuling to clarify the mechanism of organic carbon sequestration involving by soil microbes under different tillage practices.In this study,based on a long-term field experiment of dry-fed spring maize in Shouyang,Shanxi（started in 2003）,and the samples were collected in 2019.We used methods such as phospholipid fatty acid technology,fluorescent microplate enzyme technology,high-throughput sequencing technology and co-occurrence network analysis to study the drivers of changes in microbial community composition;microbial community traits regulate SOC sequestration mechanisms;the relationship between microbial community composition and soil pore structural and the effect of soil microbial community composition on soil aggregate organic carbon storage,and further to explore carbon sequestration mechanism mediated by soil microbes under long-term conservation tillage practices.The main results of this research were as follows:（1）It was clarified that the main driving factors of soil microbial community composition under reduced tillage and no-tillage.Tillage practices significantly affected microbial communities composition in bulk soil and rhizosphere during the growth period of spring maize（seedling stage,jointing stage,silking stage and mature stage）by improving soil physicochemical properties..RT-RI and NT-RM significantly increased the number of Gram-positive bacteria,Gram-negative bacteria and fungi in the rhizosphere soil.At 0–10 cm,both RT-RI and NT-RM increased the number of gram-negative bacteria,gram-positive bacteria,fungi,fungi/bacteria,and actinomycetes,which were mainly driven by soil water content,organic carbon content and soil bulk density.However,at 10–25 cm,the effect of RT-RI on the above-mentioned microbial groups were significantly greater than that of NT-RM,and it was significantly correlated with soil organic carbon,total nitrogen,p H and soil temperature.（2）It was elucidated that the relationship between microbial community diversity and SOC sequestration under reduced tillage and no-tillage,and revealed the microbial key taxa of carbon sequestration.We found that the initial and current of average SOC stocks was 19 and 28 Mg ha^-1 at0–25 cm,respectively.RT-RI and NT-RM increased SOC accumulation at 0–10 cm by 116%and 131%compared with CT-RR,respectively,and RT-RI increased SOC accumulation at 10–25 cm by 159%.Changes in co-occurrence network analysis revealed that NT-RM resulted in a stable bacterial network,whereas both RT-RI and NT-RM produced a stable fungal network relative to CT-RR.Microbial diversity and keystone taxa correlated positively with SAR under conservation tillage practices.Using a Zi-Pi plot,we identified several keystone OTUs,five of which（i.e.Cytophagales,JG30-KF-CM45,Sphingobacteriales,Thelephorales and Pleosporales）showed a positive correlation with SAR.（3）It was found that there was a synergistic positive effect between soil pore structure and microbial community composition under reduced tillage and no-tillage.The total microbial biomass and key taxa significantly affected soil pore structure traits.Compared with CT-RR,both RT-RI and NT-RM significantly improved total porosity,specific surface area and degree of anisotropy.In addition,RT-RI improved critical pore diameter and connection probability;however,NT-RM had no significant effect on connection probability.The microbial community composition（PLFAs,16S and ITS）differed significantly among tillage practices.Compared with CT-RR,RT-RI and NT-RM increased the total microbial biomass,bacterial and fungal biomass and gene abundance of cbb M,however,decreased the gene abundance of cbb L.Soil pore structure traits and gram-negative bacteria,Gram-positive bacteria,general bacterial monomers,and phylum-level of Actinobacteria,Bacteroidete,Gemmatimonadetes,Planctomycetes,Verrucomicrobia and Ascomycota,Basidiomycota,Glomeromycota,Kickxellomycota have mutual promotion effects.（4）It was revealed that the mechanism of soil aggregate SOC physical protection and microbial carbon sequestration synergistically improving soil organic carbon under reduced tillage and no-tillage.RT-RI and NT-RM enhance SOC concentration and storage in cropland,and the SOC storage in macroaggregates（>250μm）was higher than that in microaggregates（<250μm）,which promoted the accumulation and distribution of organic carbon in macroaggregates.The distribution of different carbon components in aggregates was related to soil microbial community composition,and fungal community composition（16:1w5c and 18:2w6,9c）can improve POC in LM（>2000μm）and SM（250–2000μm）fraction,the bacterial community composition was closely related to the m SOC content in the M（<250μm）fraction.Soil aggregate stability promotes SOC sequestration through both direct effects and indirect effects mediated by microorganisms.Overall,conservation tillage practices could drive soil nutrient changes,increase SOC content,and improve pore structure by reducing soil disturbance and increasing residue input,thereby affecting the distribution of microbial community traits（microbial diversity,community composition,and key taxa）.Moreover,the improving microbial community diversity,key taxa（i.e.Cytophagales,JG30-KF-CM45,Sphingobacteriales,Thelephorales and Pleosporales）of biological carbon sequestration,soil aggregate stability and physical protection of aggregates together promote SOC sequestration.