Distribution Of Microbial Community Structure,Interaction Network And Functional Profile Across Aggregates Of Rice Paddy Under Different Long-term Fertilization

Soil aggregation process is a key process of the soil ecosystem,which greatly affects the physical structure of soil,thereby changing the microbial community structure,and further affecting soil biodiversity and biogeochemical processes.Understanding the process of soil aggregation is the frontier of soil science in understanding soil properties,processes and functions.Paddy soil is a typical representative of Chinese agricultural civilization.Long-term cultivation of rice will lead to the accumulation of SOM,which in turn will lead to increased soil microbial abundance and activity.Lots of long-term field trials have shown that optimized management practices such as combined organic-inorganic application can significantly promote the formation of soil macroaggregates,and can also increase SOM accumulation and microbial activity,thereby improving soil fertility in rice paddy.In agricultural ecosystems,different fertilization can also affect soil aggregation process,distribution and stability by changing SOM and other physical and chemical properties as well as microbial community,thereby changing soil structure and quality,affecting soil functions and ecosystem services.Due to the differences in physical,chemical and biological characteristics of different sizes of aggregates,microbial diversity,community structure and functions will be directly affected.And different fertilization treatments can change the microbial community and functional distribution by changing the distribution and property of aggregates.Although soil carbon/nitrogen cycles,aggregates,and microorganisms have been the hotspots in soil science research,there are still few studies on the coupling relationship between soil microbial community and aggregates during the process of soil aggregation.Therefore,in this study,we chose a paddy soil under long-term fertilization as the research object,in order to systematically understand the distribution characteristics of microbial community structure,interaction network and functional groups at the aggregate scale of paddy soil under long-term fertilization management.This will provide scientific understanding for soil functions and ecological services,and also provide theoretical basis for the productivity improvement and sustainable development of paddy soil.In this study were used topsoil samples from a rice paddy under a long-term different fertilizer application trial initiated in 1987.Derived from paleo-alluvial-lacustrine deposit,the soil had developed as a Ferric-accumulic Stagnic Anthrosols following rice cultivation for thousands of years.The site was located in Jinjiaba Township,Wujiang District,Suzhou Municipality,Jiangsu Province,China.Lying in the central Tai Lake plain of China,the area is under a humid subtropical monsoon climate.The fertilizer application trial included no fertilizer(NF)as a control,chemical fertilizer only(CF),chemical fertilizer plus rice straw return(CFS)and plus pig manure(CFM).Separation of aggregate size fractions was performed using an optimized water dispersion and wet-sieving protocol.And bulk soil was separated in to large macroaggregates(L-Mac A,>2000 μm),macroaggregates(Mac A,2000-250 μm),microaggregates(Mic A,250-53 μm),fine microaggregates(F-Mic A,53-2μm)and fine clay(F-Clay,<2 μm).Mass proportion was 32.3-38.2% for F-Mic A,23.0-31.5% for Mac A,19.0-23.1% for Mic A,9.1-12.0% for L-Mac A and 4.9-7.5% for F-Clay,respectively.By analyzing soil properties and DNA sequencing data,we studied the the changes of soil carbon and nitrogen and microbial community in bulk soil and across soil aggregates of the rice paddy under different long-term fertilization,as well as the distribution of microbial interaction network and functional profile across soil aggregates.Futhermore,we focused on exploring the relationship between soil aggregates and microbial community.In this study,compared with NF,fertilization treatment increased soil organic carbon(SOC)and total nitrogen(TN)contents,but reduced soil p H and reached the bottom at 5.22 under CFS.CFS significantly increased soil microbial biomass carbon(MBC),while CF and CFM did not change significantly.Fertilization treatments,especially CFM(621.5 μm),also increased soil average mass diameter(MWD)and enhanced soil physical stability.Fertilization treatments,especially CFS,mainly increased in weakly mineral-bounded and metal ion-complexed organic C and N in soil.Aggregate size and fertilization treatment significantly affected aggregate mass proportion as well as the content and stock of SOC and TN,and size contributed greater than treatment.The mass proportion peaked in F-Mic A while depleted in F-Clay.Compared with NF,fertilization increased the proportion of Mac A,while the proportion of F-Mic A decreased.SOC and TN contents were highest in Mac A and F-Clay,and both were lowest in F-Mic A.Fertilization changed the SOC and TN contents mainly in Mac A or L-Mac A.Long-term fertilization increased the weakly mineral-bounded,metal ion-complexed and humic organic C and N components in the aggregates,and increased the ratio of the weakly mineral-bounded C to N.The content of the three components of C and N was lowest in F-Mic A,but no difference was found in other sizes.Greater than other treatments,CFS increased the weakly mineral-bounded and humic organic components in the aggregates,while CFM increased the metal ioncomplexed organic components in the aggregates.There was no significant difference in microbial abundance in bulk soil between different fertilization treatments.Compared with NF,CFS increased the alpha diversity of soil bacteria and fungi,while CFM reduced the alpha diversity of bacteria and fungi.Different long-term fertilization affected the composition of bacterial and fungal communities differently.Compared with NF,fertilization treatment increased the relative abundance of β-proteobacteria and Ignavibacteriae,but reduced Bacteroidetes,γ-proteobacteria and Chloroflexi;Acidobacteria,α-proteobacteria,Planctomycetes and Firmicutes were decreased under CF treatment,but increased under CFM and CFS treatments;The fungi of Ascomycota,Zygomycota and Chytridiomycota were increased by fertilization,while Basidiomycota was decreased.Long-term fertilization also significantly changed the community structure of bacteria and fungi in the paddy soil.Between treatments,CFM had the greatest impact on microbial community structure,while CFS had the least impact,and CF was in between.Moreover,the changes of microbial community structure under different fertilization treatments were significantly positively correlated with SOC and TN,but negatively correlated with p H.Meanwhile,aggregate size and fertilization treatment had significant effects on microbial abundance and B/F ratio in soil aggregates.Microbial abundance decreased with the decrease of aggregate size,and mainly enriched > 250 μm(especially fungi),while the B/F ratio increased with the decrease of size.Between treatments,microbial abundance mainly changed significantly in the F-Clay and F-Mic A.The microbial abundance in F-Clay increased under CF compared with NF,but decreased significantly several folds in CFS and CFM.While in F-Mic A,microbial abundance increased several folds after fertilization,and B/F decreased.In addition,fertilization increased the proportions of microbial abundance in Mac A macroaggregates compared with NF,but the proportions of fungal abundance in L-Mac A under CF and CFM were decreased.Fertilization had a significant effect on microbial diversity in aggregates,while aggregate size only affected fungal community diversity,and the effect of treatment was greater than size.Across aggregate sizes,the bacterial diversity of F-Mic A under CF was higher than that of F-Clay,while the fungal diversity of F-Mic A in CFM was higher than that of L-Mac A and Mac A.Fungal diversity showed a slight increase with the decreasing size.In addition,compared with NF,the microbial diversity under CFM treatment was reduced generally.In F-Clay,the composition of bacterial community distinctly changed compared to other size fractions,with lower Chloroflexi and higherγ-proteobacteria abundance.Between treatments,NF and CFS were similar in bacterial composition with higher Chloroflexi and lower β-proteobacteria abundance.Conversely,CF and CFM possessed lower Chloroflexi and higher β-proteobacteria abundance.For fungi,the relative abundance of Basidiomycota under CF and CFM was higher than under NF and CFS.Fertilization treatment and aggregate size had an independent and significant effect on the microbial community structure in aggregates,and the effect of size was slightly stronger than that of treatment,but the effect of size on the fungal community structure was weaker than treatment.The microbial community structure in aggregates was changed stronger and stronger with the decreasing size,and it was also strongly changed by different fertilization treatments.Compared with NF,from big to small,the intensity of changes in the microbial followed by CFM,CFS and CF.Moreover,the bacterial community structure is significantly related to SOC,TN,and C/N in aggregates,and C/N is the main driver,affecting the bacterial community structure.However,the fungal community structure was only significantly weakly related to SOC.In this study,the interaction networks of microbial communities in aggregates were distinctly different between aggregate size.In each size,the bacterial interactions were more complex and intensive than fungi,while the situation of bacterial-fungal interactions was in between,and the trends of positive/negative interactions within microbial community were similar.Moreover,microbial interactions in Mac A and Mic A were much more intensive than in sizes,while interaction intension in Mac A was even higher than in Mic A.The complexity of microbial interaction networks was not only related to microbial abundance and community structure,but also to soil spatial heterogeneity.Furthermore,the functional profiles of microbial communities in aggregates of rice paddy under long-term fertilization were differed by aggregate size.Bacterial communities within the greater aggregates possessed higher activities in carbon and nitrogen recycling and lower activities in bacterial migration,DNA metabolism and biochemical processes for surviving,in contrast,bacterial communities within the smaller size fractions(especially for F-Clay)were more active in cell motility,DNA metabolism and intracellular biochemical processed potential for surviving,but less active in assimilation of carbon and nitrogen sources.F-Clay tended to be dominated by K-strategists,while larger aggregates might possess both abundant r-strategists and K-strategists.However,larger size aggregates tended to possess abundant copiotrophs,while the smaller size aggregates(especially F-Clay)possessed more oligotrophs.In general,this study provided a general understanding of microbial community structure,individual interaction and functional profile across size fractions of aggregate of a paddy soil under long-term fertilization.Soil organic carbon,microbial abundance and community structure in soil aggregate fractions were shaped by the aggregate size,while the diversity of fungal community was mainly affected by fertilization.Compared with microaggregates,the microbial abundance associated with SOC accumulation was increased,meanwhile,the microbial networking and interaction were enhanced in macroaggregates,with higher abundances of r-strategists and copiotrophs with functions related to carbon and nitrogen metabolism.In addition,soil macroaggregates tended to contain less guilds of fungal parasite and litter saprotroph,suggesting better soil health with macro-aggregation promoted under combined organic/inorganic fertilization.This study confirmed that macroaggregates as micro-hot spots for microbial community,microbial functioning networking and potential plant defense.Of course,fertilization modified the mass and microbial proportion and in turn relative abundance of bacterial phylum and fungal species by macroaggregates.Consequently,spatial distribution or isolation of microbes in differently sized soil aggregates could underspin the soil fertility and health changes with fertilization in rice paddy.Therefore,the dynamics of microbial distribution,interaction and function within aggregate size fraction could matter soil functions for plant production and ecosystem health in rice paddy under pressure of food production and climate change nowadays.