| Soil aggregates are the basic units of soil structure and an important indicator of soil quality.Indeed,the size,arrangement and stability of aggregates are particularly crucial for determining many soil physicochemical and biological characteristics.Soil aggregate formation is a dynamic process,and the microbial community is assumed to be an important factor controlling this process.The activities of soil microorganisms are to a large extent responsible for the formation and stabilization of soil aggregation.Compared with bacteria,fungi appear to form and stabilize soil aggregates through a wider variety of mechanisms.Nevertheless,there are few studies to date on variations in fungal communities in aggregate size classes,as most previous studies only focused on fungal community variations in whole soil.Agronomic managements can significantly influence soil structure as well as soil microbial community and thus soil quality.In contrast to the well-studied effects of agronomic managements(e.g.,fertilization and crop rotation)on aggregate distribution and on the microbial communities found in whole soil,data on the changes in microbial communities(especially fungi)and their activities within different microenvironments(i.e.,aggregate sizes)due to agronomic managements are currently lacking.However,such knowledge is essential for understanding the course of both aggregate formation and nutrient cycling,which has important implications for the implementation of improving soil fertility and the analytical representation of soil biogeochemical processes across large scales of practical interest regarding dynamic changes in soil quality.Here,we present a comprehensive investigation of the changes in specific microbial taxa,along with the variability in aggregate distribution,under long-term different agronomic managements(fertilization and corn-tobacco rotation).And further,the differential effects of bacteria and fungi on the formation of soil aggregates were discussed for the first time by incubation experiments.We used real time PCR,high-throughput sequencing and Biolog microplate method to explore 1)the responses of soil bacteria and fungi to different fertilization treatments under the regulation of soil aggregates,2)the responses of soil bacteria and fungi to corn-tobacco rotation under the regulation of soil aggregates,3)the dynamic process of small size soil particles(<0.25mm)aggregated by bacteria and fungi to form large size aggregates(>0.25mm)in a laboratory incubation experiment and 4)the dynamic process of large size soil aggregates(1?5 mm)disaggregated by bacteria and fungi to form small size aggregates(<1 mm)in a laboratory incubation experiment.This study aims to decipher the mechanism of microbial effects on soil aggregate distribution The following results are obtained:1.Significant impacts of fertilization(ie.,no fertilizer(CK),nitrogen application alone(N),inorganic fertilizers(NPK)and inorganic fertilizer with manure supplements(NPKM))on soil aggregate distribution and distinct changes in fungal and bacterial communities in soil aggregates were observed,and the fungal community exhibited more marked changes compared with the bacterial community.Soil carbon and nitrogen stores were the most effective soil factors for characterizing changes in microbial communities.Lower abundant(with the relative abundance less than 6%)bacterial phyla played an important role in the differentiation of microbial communities due to fertilization in soil aggregates,and microbes in smaller aggregates were more sensitive to fertilization than those in larger ones.Nitrogen application(N)alone decreased carbon storage,whereas increased the abundance of N2O-producing fungi(e.g.,Nectriaceae,Mortierellaceae,Trichocomaceae,etc.)in both>2 mm and 0.25?2 mm aggregates,denitrifying bacteria(e.g.,?-,?-and?-Proteobacteria)in<0.25 mm aggregates and nitrifying bacteria(e.g.,Nitrospira)in 0.25?2 mm aggregates.The active N dynamics influenced by both bacteria and fungi under N treatment,as well as the low fungal counts and soil carbon contents observed under N treatment,likely contributed to soil hardening.In contrast,inorganic plus organic amendment(NPKM)treatments increased the abundance of Agaricomycetes fungi in all sizes of aggregates,and improved the storage of soil carbon as well as microbial counts in soil,all of which contributed to to the well-aggregated soil in NPKM treatment.2.Different crops(corn and tobacco)substantially impacted the distribution of soil aggregates and microbial communities in soil aggregates.When compared with tobacco soil,corn soil exhibited higher proportion of>2 mm aggregates and more soil carbon contents;corn soil also possessed more microbial counts and r-strategists bacteria in>0.25 mm aggregates,as well as more Glomeromycota fungi in<0.25 mm aggregates.The significant shift in fungal communities associated with all size aggregates was related to changes in soil total carbon(TC),the C/N ratio and pH,whereas,the significant shift in bacterial community was only observed in>2 mm aggregates,with close relation with TC.Both bacteria in>2 mm aggregates and fungi in 0.25?2 mm aggregates actively responded to corn-tobacco rotation;the indicator bacterial taxa associated with>2 mm aggregates were mainly members of dominant(20?30%)bacterial phyla(e.g.,Proteobacteria and Acidobacteria),and the indicator fungal taxa associated with 0.25?2 mm aggregates covered almost all classified fungal phyla.The sum of changes in both bacterial and fungal communities in response to corn-tobacco rotation was larger in 0.25?2 mm aggregates(with variation degree 17.51)than in>2 mm aggregates(with variation degree 9.95)and in<0.25 mm aggregates(with variation degree 13.43).3.A significant amount of newly and rapidly formed>0.25 mm soil aggregates from<0.25 mm soil particles in the presence of soil microorganisms were observed,reaching a maximum within 30?60 days.These newly formed aggregates(i.e.,increased by 9%?20%compared with those at day 7)remained stable for a considerable period of time(60?220 days),and the microbial activity remained unchanged at the same time.Inoculation of fungi alone did not significantly promote the formation of>0.25 mm aggregates from<0.25 mm soil particles.Manure supplements significantly promoted the reproduction of both bacteria and fungi in soil,and the active metabolism of bacterial community in rich soil was important and necessary for the effective increase of>0.25 mm aggregates(in the case of the absence of physical factors).However,the metabolism rate of the carbon source for fungi did not significantly contribute to the formation of>0.25 mm aggregates from<0.25 mm soil particles.With manure supplements,bacteria strongly reduced the carbon source metabolic activity of the coexisting fungi but contributed to the elevated carbon-source utilization ability of fungi(e.g.,the ability to utilize complex carbon source compounds,such as alpha,beta-cyclodextrin)at the same time,both of which promoted(ie.by 12.41%)the formation of>0.25 mm aggregates in a shorter period(e.g.,less than 30 days)and maintained the constant distribution of soil aggregates compared with the situation under the presence of bacteria or fungi alone.In addition,the counts of bacteria and fungi are in reciprocal dynamic changes under the condition of the coexistence of both bacteria and fungi,and the types of carbon source utilization for bacteria and fungi changed over time.4.The microbial activity in soil is required for the disaggregation of soil aggregates with the absence of external disturbance.Soil microbes significantly and rapidly(7 days)decreased(i.e.by 43%?59%)<1 mm aggregates to form more>1 mm soil aggregates,and also significantly increased(i.e.by 35%?61%)<1 mm aggregates accompanied by the disaggregation of>1 mm aggregates within a certain period(e.g.,7?30 days).It seemed that the microbial role in disaggregation of>1 mm aggregates was weaker than in the aggregation of<1 mm aggregates.Although the carbon source metabolism rate of fungi in soils inoculated with fungi alone was higher than that of fungi in soils inoculated with bacteria and fungi,inoculating soil with fungi alone reduced the stability of large size aggregates;bacteria reduced the carbon source metabolic activity of fungi in coexistence,but promoted the ability of fungi to utilize more kinds of carbon sources as well as contributed to the stability of soil aggregates,which together contributed to the maintance of stability of>1 mm aggregates for a long period of time.Moreover,the carbon-source utilization rates of fungi significantly affected the formation of>1 mm aggregates from<1 mm aggregates,in contrast with that of the coexisting bacteria.When compared with the variation of aggregate sizes,different microbial groups colud lead to more significant differences in the carbon source utilization of microorganisms in different soil samples.When compared with manure supplements,the inoculation of bacteria more significantly changed the carbon source utilization type of the coexisting fungi,and promoted the carbon-source utilization ability of fungi.In this research,when compared with planting tobacco,planting corn can significantly increase the proportion of large size aggregates and soil carbon storage,with more microbial counts and r-strategists bacteria in>0.25 mm aggregates as well as more Glomeromycota fungi in<0.25 mm aggregates.Both manure supplements and nitrogen application alone can significantly increase the proportions of large size aggregates.Whereas,the low soil carbon storage,the active N dynamics influenced by both bacteria and fungi,as well as the low fungal counts observed under nitrogen application alone management likely contribute to soil hardening.Manure supplements can significantly increase the abundance of Agaricomycetes fungi which taxa contribute a lot in soil aggregation and stability,and also increase microbial counts as well as soil carbon storage;meanwhile,bacteria strongly reduced the carbon source metabolic activity of the coexisting fungi but contributed to the elevated carbon-source utilization ability of fungi in soil.All the changes happened in soils under manure supplement management contribute to the well-aggregated soil structure and stabilize the ecological function of soil microorganisms.These findings provide us with the better knowledge of the biological factors affecting both soil aggregate formation and stabilization,which is of great significance to control soil organic carbon storage and cultivate soil fertility. |
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