Soil Microbial Community Compositions And Functions Under Conservation Tillage

Soil microbial system which is large and complicated play a key role in the decomposition of soil organic matter,elements cycling and soil-borne diseases.Conservation tillage changes the habitat of soil microorganisms by reducing soil disturbance and residues return,thereby affecting soil microbial community.Systematic studies on the influences of conservation tillage(NT)on soil microbial community composition,especially functional compositions are relatively few.Therefore,based on the conservation tillage experiment in Northeast China,space-time variations of soil fungal PLFA content,diversity and community structure under conservation tillage were clarified utilizing the phospholipid fatty acid method and amplicon sequencing,and soil microbial functions were furtherly studied using shotgun metagenomics sequencing.The main conclusions were as follows:(1)Cropping and tillage effects on soil fungal PLFA contents,diversity and community structure were greater than sampling time.(2)Compared with the continuous maize(Zea mays L.)system,the average of soil fungal PLFA content during the growing seasons(April,June and August)under the maize-soybean(Glycine max Merr.)rotation system decreased by 60.1%(0-5cm soil depth)and 63.8%(5-15 cm soil depth),while the average of soil fungal diversity index and evenness index increased by 22.0%(0-5 cm soil depth),19.8%(0-5 cm soil depth)and 40.0%(5-15 cm soil depth),65.0%(5-15 cm soil depth)respectively.Compared with CT(conventional tillage),there was no significant effect of NT on the soil fungal biomass and diversity in 5-15 cm soil depth,but the average of the soil fungal PLFA content during the growing season in 0-5 cm soil depth under NT(no-tillage)significantly increased by 43.3%,richness index significantly increased by 9.8%,and the soil fungal community was more stable.(3)Cropping system and tillage played the most important roles in shaping soil fungal community at both soil depths.At 0-5 cm soil depth,there was an obvious consistent trend of enrichment from Ascomycota in rotation system to Basidiomycota in continuous maize system over the growing season.At 5-15 cm soil depth,there was an obvious consistent trend of enrichment from saprotrophs in CT to symbiotrophs in NT over the growing season.NT favored symbiotrophs for all cropping systems,depths and sampling times.(4)There were significant differences in soil microbial community structure(including fungi,bacteria and archaea,etc.)under three tillage regimes,namely no-tillage with residue(maize straw)return(NT),moldboard plow without residue return(CT)and moldboard plow with residue return(MP).More species differences were showed between NT and CT.Compared with other treatments,NT exhibited higher relative abundances of some genera,such as Bradyrhizobium,Sorangium and Reyranella that were significantly positively correlated with dissolved organic carbon;and CT exhibited higher relative abundances of some genera that can adapt to carbon restriction conditions,such as Blastococcus,unclassified?p?Chloroflexi and Actinoplanes.(5)There were divergences in soil microbial functional composition(KO categories level)among three tillage regimes.Compared with other treatments,the relative abundances of some genes related to amino acid metabolism,fatty acid degradation and carbon metabolism were higher in CT.The stable and nutrient-rich environment under NT was more conducive to some functional genes related to ABC transporter and quorum sensing involved in efficient substrate transport such as ions,carbohydrates,amino acids and peptides,and Agrobacterium tumefaciens quorum sensing.There were fewer significantly different soil functional categories between NT and MP than those of other tillage treatments.(6)CAZy database annotated results showed that some significant differences on CAZy families such as glycoside hydrolases(GH102?GH5?38 and GH13?17),glycosyl transferases(GT39)and polysaccharide lyases(PL17 and PL5?1)under different tillage treatments.Moreover,total abundances of these differential CAZy genes in NT and MP were higher than that in CT.Based on KEGG database,we found that there were divergences in soil C-cycling functional gene composition under different tillage regimes(Adonis,R²=0.45;p=0.006).A total of 76 C-cycling functional genes(>0.1%)differed significantly under different tillage treatments,the clustering heatmaps of genes related to carbon fixation,methane metabolism and carbohydrate metabolism showed that MP and CT were similar,clustered together;NT was grouped separately,and the abundance of most up-regulated genes in NT were significantly positively correlated with soil DOC,soil bulk density or soil water content.Contribution analysis showed that Thaumarchaeota contributed the most to the soil C-cycling function,Acidobacteria,Crenarchaeota,Planctomycetes and Nitrospira also had important contribution.Besides,based on 6-year average,soil CO₂ emissions in NT was significantly lower than that in MP and CT in early time of growing season,specifically NT decreased soil CO₂ emissions over MP by 28%(May),11%(June),23%(July)and decreased by 31%(May),19%(June),7%(July)over CT.(7)Soil dissolved organic carbon(DOC)and soil water content induced by tillage were significant factors affecting soil microbial community structure;soil bulk density and soil DOC were significant factors affecting soil entire microbial functional compositions and C-cycling functional gene composition.