Protist diversity and community complexity in the rhizosphere of switchgrass are dynamic as plants develop

Targeted manipulation of food webs in the plant rhizosphere

Trophic interactions between micro- and macro-organisms structure food webs in the plant rhizosphere. These interactions affect the plant-associated microbiota and nutrient dynamics, and influence plant health and performance. In this forum article we discuss the need for, and challenges associated with, targeted manipulation of soil food webs toward the development of multitrophic synthetic communities.

Rhizosphere-colonizing bacteria persist in the protist microbiome

Soils contain diverse predatory protists that affect the abundance and behavior of rhizosphere bacteria, including bacteria that may benefit plant health. Protists harbor their own bacterial microbiomes, and we previously observed that plants inoculated with protists harbored rhizosphere bacteria similar to those in the protist inoculum. To determine how protist microbiomes affect the rhizosphere, we profiled the bacteria of eight diverse rhizosphere protist isolates after 2 years of laboratory culture. We then compared the protist culture microbiomes to maize rhizosphere communities 6 weeks after protist inoculation. Introduction of protists enriched 13 protist-associated bacterial amplicon sequence variants (ASVs) in the rhizosphere, which comprised ~10% of the rhizosphere bacterial community. Additional bacterial ASVs ranked highly in abundance in both rhizosphere (top 100) and protist (top 20) microbiomes; together, a median 47% of the protist microbiome was enriched or in high rank abundance in the rhizosphere. Inoculation with three out of eight protist cultures positively affected root biomass traits, but a protist mixture had no effect, indicating that the impact of protist-associated bacteria on plant growth is context dependent. Isolates of protist-associated bacteria had both positive and negative effects on protist growth in culture, suggesting that the bacteria use multiple strategies to survive in proximity to predators. This study demonstrates that even after long-term laboratory culture, rhizosphere protist cultures host bacteria that can colonize the rhizosphere of maize. The findings also identify diverse groups of rhizosphere-colonizing bacteria that persist among protist predators, which suggests that these bacteria could associate with or benefit from protists in the soil.

IMPORTANCE

Understanding the impact of predatory protists on the plant microbiome will be essential to deploy protists in sustainable agriculture. This study shows that eight rhizosphere protist isolates hosted diverse and distinct bacterial communities and that a large proportion of these bacteria could be found colonizing the maize root environment 6 weeks after protists were inoculated onto seedlings. This study demonstrates that certain bacteria from the maize rhizosphere can persist for years in protist cultures and retain the ability to colonize rhizosphere soil, suggesting that protists might support the survival of these rhizosphere bacteria in the absence of the plant.

Diversified Soil Types Differentially Regulated the Peanut (Arachis hydropoaea L.) Growth and Rhizosphere Bacterial Community Structure

Peanut (Arachis hydropoaea L.) demonstrates a prominent adaptability to diverse soil types. However, the specific effects of soil types on peanut growth and bacterial communities remain elusive. This study conducted a thorough examination of the agronomic traits, the corresponding physicochemical properties, and bacterial structure of rhizosphere soil in acidic (AT), neutral (NT), and saline-alkali (ST) soils, elucidating the internal relationship between soil type and peanut yield. Our results showed that different soil types exhibited significant differences in peanut yield, with ST demonstrating the lowest yield per plant, showing an 85.05% reduction compared to NT. Furthermore, available phosphorus content, urease, and invertase activities were substantially reduced in both ST and AT, particularly in ST by 95.35%, 38.57%, and 62.54%, respectively. Meanwhile, metagenomic sequencing unveiled a notable decline in Bradyrhizobium and Streptomyces in these soils, which is crucial for soil improvement. Further metabolic pathway analysis revealed that the reduction in pathways related to soil remediation, fertility improvement, and stress response in AT and ST may lead to slower peanut growth. In conclusion, peanuts cultivated in acidic and saline-alkali soils can increase yield via implementing soil management practices such as improving soil quality and refining micro-environments. Our study provides practical applications for enhancing peanut yield in low- to medium-yield fields.

Microbial community dynamics in the soil-root continuum are linked with plant species turnover during secondary succession

Grazing exclusion and land abandonment are commonly adopted to restore degraded ecosystems in semiarid and arid regions worldwide. However, the temporal variation in the soil- versus root-associated microbiome over plant species turnover during secondary succession has rarely been quantified. Using the chronosequence restored from fenced grassland and abandoned farmlands on the Loess Plateau of China, we characterized the dynamics of the soil- and root-associated microbiome of host plant with different dominance statuses during secondary succession from 0 to 40 years. Our results revealed that the root microhabitat, the host plant and their interactions were the main contributors to the bacterial community shift (R2 = 15.5%, 8.1%, and 22.3%, respectively), and plant interspecies replacement had a greater effect on the shift in the root-associated microbial community than intraspecies replacement did during succession. The root-associated bacterial community of pioneer plants was particularly responsive to succession, especially the endosphere community. Endosphere microbial diversity was positively correlated with host plant coverage change, and the diversity and abundance of taxon recruitment into the endosphere of pioneer plants from the surrounding environment decreased as succession progressed. The community assembly processes also indicated that the endosphere microbiota are strongly selected in younger host plants, whereas stochastic processes dominate in aged host plants. Our study provides evidence of the unique response of the root-associated microbiome to the replacement of plant species during secondary succession, and the function of endosphere microbes should be considered when studying plant-microbe feedback.

National-scale distribution of protists associated with sorghum leaves and roots

Protists, as integral constituents of the plant microbiome, are posited to confer substantial benefits to plant health and performance. Despite their significance, protists have received considerably less attention compared to other constituents of the plant microbiome, such as bacteria and fungi. To investigate the diversity and community structure of protists in sorghum leaves and roots, we employed amplicon sequencing of the eukaryotic 18S rRNA gene in 563 leaf and root samples collected from 57 locations across China. We found significant differences in the diversity and community structure of protists in sorghum leaves and roots. The leaf was taxonomically dominated by Evosea, Cercozoa and Ciliophora, while the root was dominated by Endomyxa, Cercozoa and Oomycota. The functional taxa of protists exhibited notable differences between leaves and roots, with the former being predominantly occupied by consumers and the latter by parasites. The community composition of protists in the leaf was predominantly influenced by mean annual precipitation, whereas soil pH played a more significant role in the root. The present study identified the most abundant and distributed protists in sorghum leaves and roots and elucidated the underlying factors that govern their community structure. The present study offers a novel perspective on the factors that shape plant-associated protist communities and their potential roles in enhancing the functionality of plant ecosystems.

Differential responses of soil bacteria, fungi and protists to root exudates and temperature

The impact of climate warming on soil microbes has been well documented, with studies revealing its effects on diversity, community structure and network dynamics. However, the consistency of soil microbial community assembly, particularly in response to diverse plant root exudates under varying temperature conditions, remains an unresolved issue. To address this issue, we employed a growth chamber to integrate temperature and root exudates in a controlled experiment to examine the response of soil bacteria, fungi, and protists. Our findings revealed that temperature independently regulated microbial diversity, with distinct patterns observed among bacteria, fungi, and protists. Both root exudates and temperature significantly influenced microbial community composition, yet interpretations of these factors varied among prokaryotes and eukaryotes. In addition to phototrophic bacteria and protists, as well as protistan consumers, root exudates determined to varying degrees the enrichment of other microbial functional guilds at specific temperatures. The effects of temperature and root exudates on microbial co-occurrence patterns were interdependent; root exudates primarily simplified the network at low and high temperatures, while responses to temperature varied between single and mixed exudate treatments. Moreover, temperature altered the composition of keystone species within the microbial network, while root exudates led to a decrease in their number. These results emphasize the substantial impact of plant root exudates on soil microbial community responses to temperature, underscoring the necessity for future climate change research to incorporate additional environmental variables.

Rhizosphere microbial community construction during the latitudinal spread of the invader Chromolaena odorata

The colonization of alien plants in new habitats is typically facilitated by microorganisms present in the soil environment. However, the diversity and structure of the archaeal, bacterial, and fungal communities in the latitudinal spread of alien plants remain unclear. In this study, the rhizosphere and bulk soil of Chromolaena odorata were collected from five latitudes in Pu' er city, Yunnan Province, followed by amplicon sequencing of the soil archaeal, bacterial, and fungal communities. Alpha and beta diversity results revealed that the richness indices and the structures of the archaeal, bacterial, and fungal communities significantly differed along the latitudinal gradient. Additionally, significant differences were observed in the bacterial Shannon index, as well as in the structures of the bacterial and fungal communities between the rhizosphere and bulk soils. Due to the small spatial scale, trends of latitudinal variation in the archaeal, bacterial, and fungal communities were not pronounced. Total potassium, total phosphorus, available nitrogen, available potassium and total nitrogen were the important driving factors affecting the soil microbial community structure. Compared with those in bulk soil, co-occurrence networks in rhizosphere microbial networks presented lower complexity but greater modularity and positive connections. Among the main functional fungi, arbuscular mycorrhizae and soil saprotrophs were more abundant in the bulk soil. The significant differences in the soil microbes between rhizosphere and bulk soils further underscore the impact of C. odorata invasion on soil environments. The significant differences in the soil microbiota along latitudinal gradients, along with specific driving factors, demonstrate distinct nutrient preferences among archaea, bacteria, and fungi and indicate complex microbial responses to soil nutrient elements following the invasion of C. odorata.

Intercropping enhances maize growth and nutrient uptake by driving the link between rhizosphere metabolites and microbiomes

Intercropping leads to different plant roots directly influencing belowground processes and has gained interest for its promotion of increased crop yields and resource utilization. However, the precise mechanisms through which the interactions between rhizosphere metabolites and the microbiome contribute to plant production remain ambiguous, thus impeding the understanding of the yield-enhancing advantages of intercropping. This study conducted field experiments (initiated in 2013) and pot experiments, coupled with multi-omics analysis, to investigate plant-metabolite-microbiome interactions in the rhizosphere of maize. Field-based data revealed significant differences in metabolite and microbiome profiles between the rhizosphere soils of maize monoculture and intercropping. In particular, intercropping soils exhibited higher microbial diversity and metabolite chemodiversity. The chemodiversity and composition of rhizosphere metabolites were significantly related to the diversity, community composition, and network complexity of soil microbiomes, and this relationship further impacted plant nutrient uptake. Pot-based findings demonstrated that the exogenous application of a metabolic mixture comprising key components enriched by intercropping (soyasapogenol B, 6-hydroxynicotinic acid, lycorine, shikimic acid, and phosphocreatine) significantly enhanced root activity, nutrient content, and biomass of maize in natural soil, but not in sterilized soil. Overall, this study emphasized the significance of rhizosphere metabolite-microbe interactions in enhancing yields in intercropping systems. It can provide new insights into rhizosphere controls within intensive agroecosystems, aiming to enhance crop production and ecosystem services.

Bacterial assembly and succession patterns in conventional and advanced drinking water systems: From source to tap

Bacteria are pivotal to drinking water treatment and public health. However, the mechanisms of bacterial assembly and their impact on species coexistence remain largely unexplored. This study explored the assembly and succession of bacterial communities in two full-scale drinking water systems over one year. We observed a decline in bacterial biomass, diversity, and co-occurrence network complexity along the treatment processes, except for the biological activated carbon filtration stage. The conventional plant showed higher bacterial diversity than the advanced plant, despite similar bacterial concentrations and better removal efficiency. The biological activated carbon filter exhibited high phylogenetic diversity, indicating enhanced bacterial metabolic functionality for organic matter removal. Chlorination inactivated most bacteria but favored some chlorination-resistant and potentially pathogenic species, such as Burkholderia, Bosea, Brevundimonas, and Acinetobacter. Moreover, the spatiotemporal dynamics of the bacterial continuum were primarily driven by stochastic processes, explaining more than 78% of the relative importance. The advanced plant's bacterial community was less influenced by dispersal limitation and more by homogeneous selection. The stochastic process regulated bacterial diversity and influenced the complexity of the species co-occurrence network. These findings deepen our understanding of microbial ecological mechanisms and species interactions, offering insights for enhancing hygienic safety in drinking water systems.

The abundant fraction of soil microbiomes regulates the rhizosphere function in crop wild progenitors

The rhizosphere influence on the soil microbiome and function of crop wild progenitors (CWPs) remains virtually unknown, despite its relevance to develop microbiome-oriented tools in sustainable agriculture. Here, we quantified the rhizosphere influence-a comparison between rhizosphere and bulk soil samples-on bacterial, fungal, protists and invertebrate communities and on soil multifunctionality across nine CWPs at their sites of origin. Overall, rhizosphere influence was higher for abundant taxa across the four microbial groups and had a positive influence on rhizosphere soil organic C and nutrient contents compared to bulk soils. The rhizosphere influence on abundant soil microbiomes was more important for soil multifunctionality than rare taxa and environmental conditions. Our results are a starting point towards the use of CWPs for rhizosphere engineering in modern crops.

Quality variation and salt-alkali-tolerance mechanism of Cynomorium songaricum: Interacting from microbiome-transcriptome-metabolome

Addressing soil salinization and implementing sustainable practices for cultivating cash crops on saline-alkali land is a prominent global challenge. Cynomorium songaricum is an important salt-alkali tolerant medicinal plant capable of adapting to saline-alkali environments. In this study, two typical ecotypes of C. songaricum from the desert-steppe (DS) and saline-alkali land (SAL) habitats were selected. Through the integration of multi-omics with machine learning, the rhizosphere microbial communities, genetic maps, and metabolic profiles of two ecotypes were created and the crucial factors for the adaptation of C. songaricum to saline-alkali stress were identified, including 7 keystone OTUs (i.e. Novosphingobium sp., Sinorhizobium meliloti, and Glycomyces sp.), 5 core genes (cell wall-related genes), and 10 most important metabolites (i.e. cucurbitacin D and 3-Hydroxybutyrate) were identified. Our results indicated that under saline-alkali environments, the microbial competition might become more intense, and the microbial community network had the simple but stable structure, accompanied by the changes in the gene expression related to cell wall for adaptation. However, this regulation led to the reduction in active ingredients, such as the accumulation of flavonoids and organic acid, and enhanced the synthesis of bitter substances (cucurbitacin D), resulting in the decrease in the quality of C. songaricum. Therefore, compared to the SAL ecotype, the DS was more suitable for the subsequent development of medicinal and edible products of C. songaricum. Furthermore, to explore the reasons for this quality variation, we constructed a comprehensive microbial-genetic-metabolic regulatory network, revealing that the metabolism of C. songaricum was primarily influenced by genetic factors. These findings not only offer new insights for future research into plant salt-alkali tolerance strategies but also provide a crucial understanding for cultivating high-quality medicinal plants.

Effects of the invasion of Ralstonia solanacearum on soil microbial community structure in Wuhan, China

This study investigated the change in the microbiome of tomato rhizosphere soils after the invasion of Ralstonia solanacearum and analyzed the correlation between microbes and soil physicochemical properties. Diversity analyses of the bacteria in healthy and diseased rhizosphere soil samples (HRS and DRS) revealed that HRS had a higher species diversity and were compositionally different from DRS (P ≤ 0.05). Substantial differences in the relative abundance of Actinobacteria (37.52% vs 28.96%, P ≤ 0.05) and Proteobacteria (29.20% vs 35.59%, P ≤ 0.05) were identified in HRS and DRS, respectively. Taxonomic composition analysis showed ten differentially abundant genera, and seven of them (Gaiella, Roseisolibacter, Solirubrobacter, Kribbella, Acidibacter, Actinomarinicola, and Marmoricola) are more abundant in HRS. Soil pH and enzyme activities were negatively correlated with the abundance of R. solanacearum. The contents of total nitrogen (TN), total phosphorus (TP), total potassium (TK), alkaline nitrogen (alkaline N), available phosphorus (AP), available potassium (AK), NO3-N(NN), NH4+-N (AN), and organic matter (OM) were all significantly increased in DRS. The composition and richness of protozoa in the samples show significant differences. Cephalobus, Acrobeles, Heteromita, norank_Tylenchida, and Rotylenchulus were enriched in DRS. Microbial interaction networks revealed that the HRS networks were more complex than the DRS networks. Overall, the results of this study demonstrate that healthy soil has a more complex microbial community structure and higher enzyme activity, and the invasion of R. solanacearum damages the soil microbial system.IMPORTANCEHow does the invasion of Ralstonia solanacearum affect tomato rhizosphere bacteria and protozoa? Which microbial changes can affect the growth of R. solanacearum? To date, most research studies focus on bacteria, with little research on protozoa, and even less on the synergistic effects between protozoa and bacteria. Here, we analyzed the correlation between tomato rhizosphere bacterial and protozoan communities and soil physicochemical properties during the invasion of R. solanacearum. We found that the diversity and abundance of rhizosphere microorganisms in healthy rhizosphere soil samples (HRS) were significantly higher than those in diseased rhizosphere soil samples (DRS), and there were significant changes in soil pH and enzyme activity. Overall, in this study, the analysis of microbial changes during the invasion of R. solanacearum provides a theoretical basis for the prevention and control of bacterial wilt.

Different long-term fertilization regimes affect soil protists and their top-down control on bacterial and fungal communities in Mollisols

Soil protists represent a vastly diverse component of soil microbial communities and significantly contribute to biogeochemical cycling. However, how different fertilization regimes impact the protistan communities and their top-down control on bacteria and fungi remain largely unknown. Here, using high-throughput sequencing, we investigated the differences in protist communities and their relationships with bacterial and fungal communities in Mollisols of Northeast China that were subjected to chemical and organic fertilization over 30 years. The results showed that manure addition rather than chemical fertilization significantly increased protistan alpha diversity and changed protistan community structure. Manure amendments markedly increased the relative abundances of protistan consumers (such as Cercozoa) and reduced the proportion of phototrophic protists (such as Chlorophyta). Soil pH was the most influential factor driving microbial communities, and protists were less sensitive to environmental disturbances than bacteria and fungi. Protistan communities exhibited more stronger relationships with bacterial communities than fungal communities, and Chlorococcum was the most important contributor in regulation of microbial taxa and functional genes. Furthermore, manure addition slightly simplified the microbial network, and chemical plus manure fertilization improved network stability with the highest robustness. Manure addition specifically mitigated the negative interactions between protists and bacteria while reinforced the positive interactions between protists and fungi. This study advanced our knowledge about the roles of protistan groups in regulating microbial communities and ecosystem functions associated with chemical and organic fertilization.

Urban greenspaces shape soil protist communities in a location-specific manner

The impacts of urbanization on aboveground biodiversity are well studied, and its impact on soil microorganisms are also receiving increased attention. However, the impact of urbanization on the soil protists are hardly investigated. Here, we studied how urbanization and distinct urban greenspaces affect protist communities. We used amplicon sequencing of the18 S rRNA gene of samples from five types of urban greenspaces (parks, greenbelts, industrial areas, residential areas and hospital lawns), neighboring natural forests and agricultural ecosystems in Ningbo, China. We found that urban greenspaces harbored higher protist α-diversity than forests, while protist β-diversity increased from agricultural systems to urban greenspaces to forests. Among the studied driving factors, soil bacterial α- and β-diversity best predicted phagotrophic protist α- and β-diversity in urban greenspaces, while differences in α- and β-diversity of phototrophic protists were best explained by soil carbon-to-nitrogen ratio and fungal β-diversity, respectively. Abiotic factors i.e., total phosphorus and carbon-to-nitrogen ratio, best predicted the α- and β-diversity of protist parasites in urban greenspaces, respectively. The results revealed that the composition and drivers of protist communities vary between functional groups and urban ecosystems. Overall, our findings contribute to a better understanding of drivers of soil protist communities and indicate that soil protist communities and associated soil functions could be managed in predictable ways in urban greenspaces.

Arbuscular mycorrhizal fungi increased peanut (Arachis hypogaea L.) yield by changing the rhizosphere microbial community structure in saline-alkali soil

Arbuscular mycorrhizal fungi (AMF) have demonstrated the potential to enhance the saline-alkali tolerance in plants. Nevertheless, the extent to which AMF can ameliorate the tolerance of salt-sensitive plants to alkaline conditions necessitates further investigation. The current study is primarily centered on elucidating the impact of AMF on the growth of the Huayu22 (H22) when cultivated in saline-alkaline soil. We leveraged DNA of rhizosphere microorganisms extracted from saline-alkali soil subjected to AMF treatment and conducted high-throughput sequencing encompassing 16S rRNA gene and ITS sequencing. Our findings from high-throughput sequencing unveiled Proteobacteria and Bacillus as the prevailing phylum and genus within the bacterial population, respectively. Likewise, the predominant fungal phylum and genus were identified as Ascomycota and Haematonectria. It is noteworthy that the relative abundance of Proteobacteria, Actinobacteria, Chloroflexi, Bacteroidetes, and Ascomycota exhibited significant increments subsequent to AMF inoculation. Our investigation into soil enzyme activity revealed a remarkable surge post-AMF inoculation. Notably, the amounts of pathogen growth inhibitory enzymes and organic carbon degrading enzymes rise, as predicted by the putative roles of microbial communities. In saline-alkali soil, inoculation of AMF did boost the yield of H22. Notable improvements were observed in the weight of both 100 fruits and 100 grains, which increased by 20.02% and 22.30%, respectively. Conclusively, this study not only provides a theoretical framework but also furnishes empirical evidence supporting the utilization of AMF as a viable strategy for augmenting the yield of salt-sensitive plants grown in alkaline conditions.

Rhizosphere shapes the associations between protistan predators and bacteria within microbiomes through the deterministic selection on bacterial communities

The assembly of bacterial communities in the rhizosphere is well-documented and plays a crucial role in supporting plant performance. However, we have limited knowledge of how plant rhizosphere determines the assembly of protistan predators and whether the potential associations between protistan predators and bacterial communities shift due to rhizosphere selection. To address this, we examined bacterial and protistan taxa from 443 agricultural soil samples including bulk and rhizosphere soils. Our results presented distinct patterns of bacteria and protistan predators in rhizosphere microbiome assembly. Community assembly of protistan predators was determined by a stochastic process in the rhizosphere and the diversity of protistan predators was reduced in the rhizosphere compared to bulk soils, these may be attributed to the indirect impacts from the altered bacterial communities that showed deterministic process assembly in the rhizosphere. Interestingly, we observed that the plant rhizosphere facilitates more close interrelationships between protistan predators and bacterial communities, which might promote a healthy rhizosphere microbial community for plant growth. Overall, our findings indicate that the potential predator-prey relationships within the microbiome, mediated by plant rhizosphere, might contribute to plant performance in agricultural ecosystems.

Precipitation Drives Soil Protist Diversity and Community Structure in Dry Grasslands

Protists are essential components of soil microbial communities, mediating nutrient cycling and ecosystem functions in terrestrial ecosystems. However, their distribution patterns and driving factors, particularly, the relative importance of climate, plant and soil factors, remain largely unknown. This limits our understanding of soil protist roles in ecosystem functions and their responses to climate change. This is particularly a concern in dryland ecosystems where soil microbiomes are more important for ecosystem functions because plant diversity and growth are heavily constrained by environmental stresses. Here, we explored protist diversity and their driving factors in grassland soils on the Tibetan Plateau, which is a typical dryland region with yearly low temperatures. Soil protist diversity significantly decreased along the gradient of meadow, steppe, and desert. Soil protist diversity positively correlated with precipitation, plant biomass and soil nutrients, but these correlations were changed by grazing. Structural equation and random forest models demonstrated that precipitation dominated soil protist diversity directly and indirectly by influencing plant and soil factors. Soil protist community structure gradually shifted along meadow, steppe and desert, and was driven more by precipitation than by plant and soil factors. Soil protist community compositions were dominated by Cercozoa, Ciliophora and Chlorophyta. In particular, Ciliophora increased but Chlorophyta decreased in relative abundance along the gradient of meadow, steppe and desert. These results demonstrate that precipitation plays more important roles in driving soil protist diversity and community structure than plant and soil factors, suggesting that future precipitation change profoundly alters soil protist community and functions in dry grasslands.

Biological Interactions and Environmental Influences Shift Microeukaryotes in Permafrost Active Layer Soil Across the Qinghai-Tibet Plateau

Permafrost active layer soils are harsh environments with thaw/freeze cycles and sub-zero temperatures, harboring diverse microorganisms. However, the distribution patterns, assembly mechanism, and driving forces of soil microeukaryotes in permafrost remain largely unknown. In this study, we investigated microeukaryotes in permafrost active layer across the Qinghai-Tibet Plateau (QTP) using 18S rRNA gene sequencing. The results showed that the microbial eukaryotic communities were dominated by Nematozoa, Ciliophora, Ascomycota, Cercozoa, Arthropoda, and Basidiomycota in terms of relative abundance and operational taxonomic unit (OTU) richness. Nematozoa had the highest relative abundance, while Ciliophora had the highest OTU richness. These phyla had strong interactions between each other. Their alpha diversity and community structure were differently influenced by the factors associated to location, climate, and soil properties, particularly the soil properties. Significant but weak distance-decay relationships with different slopes were established for the communities of these dominant phyla, except for Basidiomycota. According to the null model, community assemblies of Nematozoa and Cercozoa were dominated by heterogeneous selection, Ciliophora and Ascomycota were dominated by dispersal limitation, while Arthropoda and Basidiomycota were highly dominated by non-dominant processes. The assembly mechanisms can be jointly explained by biotic interactions, organism treats, and environmental influences. Modules in the co-occurrence network of the microeukaryotes were composed by members from different taxonomic groups. These modules also had interactions and responded to different environmental factors, within which, soil properties had strong influences on these modules. The results suggested the importance of biological interactions and soil properties in structuring microbial eukaryotic communities in permafrost active layer soil across the QTP.

Movement of protistan trophic groups in soil-plant continuums

Protists, functionally divided into consumers, phototrophs, and parasites act as integral components and vital regulators of microbiomes in soil-plant continuums. However, the drivers of community structure, assembly mechanisms, co-occurrence patterns, and the associations with human pathogens and different protistan trophic groups remain unknown. Here, we characterized the phyllosphere and soil protistan communities associated with three vegetables under different fertilization treatments (none and organic fertilization) at five growth stages. In this study, consumers were the most diverse soil protist group, had the role of inter-kingdom connector, and were the primary biomarker for rhizosphere soils which were subjected to decreasing deterministic processes during plant growth. In contrast, phototrophs had the greatest niche breadth and formed soil protistan hubs, and were the primary biomarkers for both bulk soils and the phyllosphere. Parasites had minimal input to microbial co-occurrence networks. Organic fertilization increased the relative abundance (RA) of pathogenic protists and the number of pathogen-consumer connections in rhizosphere soils but decreased protistan richness and the number of internal protistan links. This study advances our understanding of the ecological roles and potential links between human pathogens and protistan trophic groups associated with soil-plant continuums, which is fundamental to the regulation of soil-plant microbiomes and maintenance of environmental and human health.

Community assembly of comammox Nitrospira in coastal wetlands across southeastern China

Complete ammonia oxidizers (comammox Nitrospira) are ubiquitous in coastal wetland sediments and play an important role in nitrification. Our study examined the impact of habitat modifications on comammox Nitrospira communities in coastal wetland sediments across tropical and subtropical regions of southeastern China. Samples were collected from 21 coastal wetlands in five provinces where native mudflats were invaded by Spartina alterniflora and subsequently converted to aquaculture ponds. The results showed that comammox Nitrospira abundances were mainly influenced by sediment grain size rather than by habitat modifications. Compared to S. alterniflora marshes and native mudflats, aquaculture pond sediments had lower comammox Nitrospira diversity, lower clade A.1 abundance, and higher clade A.2 abundance. Sulfate concentration was the most important factor controlling the diversity of comammox Nitrospira. The response of comammox Nitrospira community to habitat change varied significantly by location, and environmental variables accounted for only 11.2% of the variations in community structure across all sites. In all three habitat types, dispersal limitation largely controlled the comammox Nitrospira community assembly process, indicating the stochastic nature of these sediment communities in coastal wetlands. IMPORTANCE Comammox Nitrospira have recently gained attention for their potential role in nitrification and nitrous oxide (N2O) emissions in soil and sediment. However, their distribution and assembly in impacted coastal wetland are poorly understood, particularly on a large spatial scale. Our study provides novel evidence that the effects of habitat modification on comammox Nitrospira communities are dependent on the location of the wetland. We also found that the assembly of comammox Nitrospira communities in coastal wetlands was mainly governed by stochastic processes. Nevertheless, sediment grain size and sulfate concentration were identified as key variables affecting comammox Nitrospira abundance and diversity in coastal sediments. These findings are significant as they advance our understanding of the environmental adaptation of comammox Nitrospira and how future landscape modifications may impact their abundance and diversity in coastal wetlands.

Niche and ecosystem preference of earliest diverging fungi in soils

Within the supergroup Rotosphaeromycetes, or "Holomycota"/"Nucletmycea", there are several well-recognised unicellular clades in the earliest diverging fungi (EDF). However, we know little about their occurrence. Here, we investigated EDF in the rhizosphere and bulk soils from cropland, forest, orchard, and wetland ecosystems around the Beijing-Hebei area, China, to illustrate their niche and ecosystem preference. More than 500 new operational taxonomic units (OTUs) of EDF were detected based on the 18S rRNA genes. Microsporida and Aphelida constitute dominant groups, whereas Rozellosporida was quite rare. Although the EDF community was site-specific, the soil chemical characteristics, vegetation, and other eukaryotic microorganisms were the key factors driving the occurrence of EDF. Moreover, the stochastic process consisted the most of the EDF community assembly.

Long-term chemical fertilization results in a loss of temporal dynamics of diazotrophic communities in the wheat rhizosphere

Diazotrophs are potential bacterial biofertilizers with efficacy for plant nutrition, which convert atmospheric N₂ into plant available nitrogen. Although they are known to respond strongly to fertilization, little is known about the temporal dynamics of diazotrophic communities throughout plant developmental under different fertilization regimes. In this study, we investigated diazotrophic communities in the wheat rhizosphere at four developmental stages under three long-term fertilization regimes: no fertilizer (Control), chemical NPK fertilizer only (NPK), and NPK fertilizer plus cow manure (NPKM). Fertilization regime had greater effect (explained of 54.9 %) on diazotrophic community structure than developmental stage (explained of 4.8 %). NPK fertilization decreased the diazotrophic diversity and abundance to one-third of the Control, although this was largely recovered by the addition of manure. Meanwhile, Control treatment resulted in significant variation in diazotrophic abundance, diversity, and community structure (P = 0.001) depending on the developmental stage, while the NPK fertilization resulted in the loss of temporal dynamics of the diazotrophic community (P = 0.330), which could be largely recovered by the addition of manure (P = 0.011). Keystone species identified in this study were quite different among the four developmental stages under Control and NPKM treatment but were similar among stages under NPK treatment. These findings suggest that long-term chemical fertilization not only reduces diazotrophic diversity and abundance, but also results in a loss of temporal dynamics of rhizosphere diazotrophic communities.

Guild and Niche Determination Enable Targeted Alteration of the Microbiome

Microbiome science has greatly contributed to our understanding of microbial life and its essential roles for the environment and human health1-5. However, the nature of microbial interactions and how microbial communities respond to perturbations remains poorly understood, resulting in an often descriptive and correlation-based approach to microbiome research6-8. Achieving causal and predictive microbiome science would require direct functional measurements in complex communities to better understand the metabolic role of each member and its interactions with others. In this study we present a new approach that integrates transcription and translation measurements to predict competition and substrate preferences within microbial communities, consequently enabling the selective manipulation of the microbiome. By performing metatranscriptomic (metaRNA-Seq) and metatranslatomic (metaRibo-Seq) analysis in complex samples, we classified microbes into functional groups (i.e. guilds) and demonstrated that members of the same guild are competitors. Furthermore, we predicted preferred substrates based on importer proteins, which specifically benefited selected microbes in the community (i.e. their niche) and simultaneously impaired their competitors. We demonstrated the scalability of microbial guild and niche determination to natural samples and its ability to successfully manipulate microorganisms in complex microbiomes. Thus, the approach enhances the design of pre- and probiotic interventions to selectively alter members within microbial communities, advances our understanding of microbial interactions, and paves the way for establishing causality in microbiome science.

Effects of time-space conversion on microflora structure, secondary metabolites composition and antioxidant capacity of Codonopsis pilosula root

In order to study the relationship between medicinal plant Codonopsis pilosula phenotype, secondary metabolites, antioxidant capacity and its rhizosphere soil nutrients, root-related microorganisms under seasonal and geographical changes, high-throughput sequencing technology was used to explore the bacterial community structure and variation in rhizosphere soil and root endosphere from six regions of Dingxi City, Gansu Province during four seasons. Secondary metabolites composition and antioxidant capacities of C. pilosula root collected successively from four seasons were determined. The chemical properties, nutrient content and enzyme activities of rhizosphere of C. pilosula were significantly different under different temporal and spatial conditions. All soil samples were alkaline (pH 7.64-8.42), with water content ranging from 9.53% to 19.95%, and electrical conductivity varied widely, showing obvious time-scale effects. Different time scales were the main reasons for the diversity and structure of rhizosphere bacterial community of C. pilosula. The diversity and richness of rhizosphere bacterial community in autumn and winter were higher than those in spring and summer, and bacterial community structure in spring and summer was more similar to that in autumn and winter. The root length and diameter of C. pilosula showed significant time gradient difference under different spatiotemporal conditions. Nutrition and niche competition lead to significant synergistic or antagonistic interactions between rhizosphere bacteria and endophytic bacteria, which invisibly affect soil properties, abundance of functional bacteria and even yield and quality of C. pilosula. Soil properties, rhizosphere bacteria and endophytic bacteria directly promoted root phenotype, stress resistance and polysaccharide accumulation of C. pilosula.

Diversity and assembly of root-associated microbiomes of rubber trees

INTRODUCTION

Understanding the diversity and assembly of the microbiomes of plant roots is crucial to manipulate them for sustainable ecosystem functioning. However, there are few reports about microbial communities at a continuous fine-scale of roots for rubber trees.

METHODS

We investigate the structure, diversity, and assembly of bacterial and fungal communities for the soil (non-rhizosphere), rhizosphere, and rhizoplane as well as root endosphere of rubber trees using the amplicon sequencing of 16S ribosomal ribonucleic acid (rRNA) and Internally Transcribed Spacer (ITS) genes.

RESULTS

We show that 18.69% of bacterial and 20.20% of fungal operational taxonomic units (OTUs) in the rhizoplane derived from the endosphere and 20.64% of bacterial and 20.60% of fungal OTUs from the soil. This suggests that the rhizoplane microbial community was a mixed community of soil and endosphere microbial communities and that microorganisms can disperse bidirectionally across different compartments of the plant root. On the other hand, in the absence of an enrichment or depletion of core bacterial and fungal OTUs in the rhizosphere, little differences in microbial composition as well as a more shared microbial network structure between the soil and the rhizosphere support the theory that the rhizosphere microbial community is a subset of the soil community. A large number of functional genes (such as nitrogen fixation and nitrite reduction) and more enriched core OTUs as well as a less stable but more complex network structure were observed in the rhizoplane of rubber tree roots. This demonstrated that the rhizoplane is the most active root compartment and a hotspot for plant-soil-environment interactions. In addition, bacterial and fungal communities in the rhizoplane were more stochastic compared to the rhizosphere and soil.

DISCUSSION

Our study expands our understanding of root-associated microbial community structure and function, which may provide the scientific basis for sustainable agriculture through biological process management.

Plant associated protists-Untapped promising candidates for agrifood tools

The importance of host-associated microorganisms and their biotic interactions for plant health and performance has been increasingly acknowledged. Protists, main predators and regulators of bacteria and fungi, are abundant and ubiquitous eukaryotes in terrestrial ecosystems. Protists are considered to benefit plant health and performance, but the community structure and functions of plant-associated protists remain surprisingly underexplored. Harnessing plant-associated protists and other microbes can potentially enhance plant health and productivity and sustain healthy food and agriculture systems. In this review, we summarize the knowledge of multifunctionality of protists and their interactions with other microbes in plant hosts, and propose a future framework to study plant-associated protists and utilize protists as agrifood tools for benefiting agricultural production.

A Reproducible and Tunable Synthetic Soil Microbial Community Provides New Insights into Microbial Ecology

Microbial soil communities form commensal relationships with plants to promote the growth of both parties. The optimization of plant-microbe interactions to advance sustainable agriculture is an important field in agricultural research. However, investigation in this field is hindered by a lack of model microbial community systems and efficient approaches for building these communities. Two key challenges in developing standardized model communities are maintaining community diversity over time and storing/resuscitating these communities after cryopreservation, especially considering the different growth rates of organisms. Here, a model synthetic community (SynCom) of 16 soil microorganisms commonly found in the rhizosphere of diverse plant species, isolated from soil surrounding a single switchgrass plant, has been developed and optimized for in vitro experiments. The model soil community grows reproducibly between replicates and experiments, with a high community α-diversity being achieved through growth in low-nutrient media and through the adjustment of the starting composition ratios for the growth of individual organisms. The community can additionally be cryopreserved with glycerol, allowing for easy replication and dissemination of this in vitro system. Furthermore, the SynCom also grows reproducibly in fabricated ecosystem devices (EcoFABs), demonstrating the application of this community to an existing in vitro plant-microbe system. EcoFABs allow reproducible research in model plant systems, offering the precise control of environmental conditions and the easy measurement of plant microbe metrics. Our results demonstrate the generation of a stable and diverse microbial SynCom for the rhizosphere that can be used with EcoFAB devices and can be shared between research groups for maximum reproducibility. IMPORTANCE Microbes associate with plants in distinct soil communities to the benefit of both the soil microbes and the plants. Interactions between plants and these microbes can improve plant growth and health and are therefore a field of study in sustainable agricultural research. In this study, a model community of 16 soil bacteria has been developed to further the reproducible study of plant-soil microbe interactions. The preservation of the microbial community has been optimized for dissemination to other research settings. Overall, this work will advance soil microbe research through the optimization of a robust, reproducible model community.

New insights into engineered plant-microbe interactions for pesticide removal

Over the past decades, rapid industrialization along with the overutilization of organic pollutants/pesticides has altered the environmental circumstances. Moreover, various anthropogenic, xenobiotics and natural activities also affected plants, soil, and human health, in both direct and indirect ways. To counter this, several conventional methods are currently practiced, but are uneconomical, noxious, and is yet inefficient for large-scale application. Plant-microbe interactions are mediated naturally in an ecosystem and are practiced in several areas. Plant growth promoting rhizobacteria (PGPR) possess certain attributes affecting plant and soil consequently performing decontamination activity via a direct and indirect mechanism. PGPR also harbors indispensable genes stimulating the mineralization of several organic and inorganic compounds. This makes microbes potential candidates for contributing to sustainably remediating the harmful pesticide contaminants. There is a limited piece of information about the plant-microbe interaction pertaining predict and understand the overall interaction concerning a sustainable environment. Therefore, this review focuses on the plant-microbe interaction in the rhizosphere and inside the plant's tissues, along with the utilization augmenting the crop productivity, reduction in plant stress along with decontamination of pesticides/organic pollutants in soil for sustainable environmental management.

Nitrogen, manganese, iron, and carbon resource acquisition are potential functions of the wild rice Oryza rufipogon core rhizomicrobiome

BACKGROUND

The assembly of the rhizomicrobiome, i.e., the microbiome in the soil adhering to the root, is influenced by soil conditions. Here, we investigated the core rhizomicrobiome of a wild plant species transplanted to an identical soil type with small differences in chemical factors and the impact of these soil chemistry differences on the core microbiome after long-term cultivation. We sampled three natural reserve populations of wild rice (i.e., in situ) and three populations of transplanted in situ wild rice grown ex situ for more than 40 years to determine the core wild rice rhizomicrobiome.

RESULTS

Generalized joint attribute modeling (GJAM) identified a total of 44 amplicon sequence variants (ASVs) composing the core wild rice rhizomicrobiome, including 35 bacterial ASVs belonging to the phyla Actinobacteria, Chloroflexi, Firmicutes, and Nitrospirae and 9 fungal ASVs belonging to the phyla Ascomycota, Basidiomycota, and Rozellomycota. Nine core bacterial ASVs belonging to the genera Haliangium, Anaeromyxobacter, Bradyrhizobium, and Bacillus were more abundant in the rhizosphere of ex situ wild rice than in the rhizosphere of in situ wild rice. The main ecological functions of the core microbiome were nitrogen fixation, manganese oxidation, aerobic chemoheterotrophy, chemoheterotrophy, and iron respiration, suggesting roles of the core rhizomicrobiome in improving nutrient resource acquisition for rice growth. The function of the core rhizosphere bacterial community was significantly (p < 0.05) shaped by electrical conductivity, total nitrogen, and available phosphorus present in the soil adhering to the roots.

CONCLUSION

We discovered that nitrogen, manganese, iron, and carbon resource acquisition are potential functions of the core rhizomicrobiome of the wild rice Oryza rufipogon. Our findings suggest that further potential utilization of the core rhizomicrobiome should consider the effects of soil properties on the abundances of different genera. Video Abstract.

Rhizosphere Soil Microbial Community Under Ice in a High-Latitude Wetland: Different Community Assembly Processes Shape Patterns of Rare and Abundant Microbes

The rhizosphere soil microbial community under ice exhibits higher diversity and community turnover in the ice-covered stage. The mechanisms by which community assembly processes shape those patterns are poorly understood in high-latitude wetlands. Based on the 16S rRNA gene and ITS sequencing data, we determined the diversity patterns for the rhizosphere microbial community of two plant species in a seasonally ice-covered wetland, during the ice-covered and ice-free stages. The ecological processes of the community assembly were inferred using the null model at the phylogenetic bins (taxonomic groups divided according to phylogenetic relationships) level. Different effects of ecological processes on rare and abundant microbial sub-communities (defined by the relative abundance of bins) and bins were further analyzed. We found that bacterial and fungal communities had higher alpha and gamma diversity under the ice. During the ice-free stage, the dissimilarity of fungal communities decreased sharply, and the spatial variation disappeared. For the bacterial community, homogeneous selection, dispersal limitation, and ecological processes (undominated processes) were the main processes, and they remained relatively stable across all stages. For the fungal community, during the ice-covered stage, dispersal limitation was the dominant process. In contrast, during the ice-free stage, ecological drift processes were more important in the Scirpus rhizosphere, and ecological drift and homogeneous selection processes were more important in the Phragmites rhizosphere. Regarding the different effects of community assembly processes on abundant and rare microbes, abundant microbes were controlled more by homogeneous selection. In contrast, rare microbes were controlled more by ecological drift, dispersal limitation, and heterogeneous selection, especially bacteria. This is potentially caused by the low growth rates or the intermediate niche breadths of rare microbes under the ice. Our findings suggest the high diversity of microbial communities under the ice, which deepens our understanding of various ecological processes of community assembly across stages and reveals the distinct effects of community assembly processes on abundant and rare microbes at the bin level.

Brassica napus Bacterial Assembly Processes Vary with Plant Compartment and Growth Stage but Not between Lines

Holobiont bacterial community assembly processes are an essential element to understanding the plant microbiome. To elucidate these processes, leaf, root, and rhizosphere samples were collected from eight lines of Brassica napus in Saskatchewan over the course of 10 weeks. We then used ecological null modeling to disentangle the community assembly processes over the growing season in each plant part. The root was primarily dominated by stochastic community assembly processes, which is inconsistent with previous studies that suggest of a highly selective root environment. Leaf assembly processes were primarily stochastic as well. In contrast, the rhizosphere was a highly selective environment. The dominant rhizosphere selection process leads to more similar communities. Assembly processes in all plant compartments were dependent on plant growth stage with little line effect on community assembly. The foundations of assembly in the leaf were due to the harsh environment, leading to dominance of stochastic effects, whereas the stochastic effects in the root interior likely arise due to competitive exclusion or priority effects. Engineering canola microbiomes should occur during periods of strong selection assuming strong selection could promote beneficial bacteria. For example, engineering the microbiome to resist pathogens, which are typically aerially born, should focus on the flowering period, whereas microbiomes to enhance yield should likely be engineered postflowering as the rhizosphere is undergoing strong selection. IMPORTANCE In order to harness the microbiome for more sustainable crop production, we must first have a better understanding of microbial community assembly processes that occurring during plant development. This study examines the bacterial community assembly processes of the leaf, root, and rhizosphere of eight different lines of Brassica napus over the growing season. The influence of growth stage and B. napus line were examined in conjunction with the assembly processes. Understanding what influences the assembly processes of crops might allow for more targeted breeding efforts by working with the plant to manipulate the microbiome when it is undergoing the strongest selection pressure.

Rootstock-scion combination contributes to shape diversity and composition of microbial communities associated with grapevine root system

To alleviate biotic and abiotic stresses and enhance fruit yield, many crops are cultivated in the form of grafted plants, in which the shoot (scion) and root (rootstock) systems of different species are joined together. Because (i) the plant species determines the microbial recruitment from the soil to the root and (ii) both scion and rootstock impact the physiology, morphology and biochemistry of the grafted plant, it can be expected that their different combinations should affect the recruitment and assembly of plant microbiome. To test our hypothesis, we investigated at a field scale the bacterial and fungal communities associated with the root system of seven grapevine rootstock–scion combinations cultivated across 10 different vineyards. Following the soil type, which resulted in the main determinant of the grapevine root microbial community diversity, the rootstock–scion combination resulted more important than the two components taken alone. Notably, the microbiome differences among the rootstock–scion combinations were mainly dictated by the changes in the relative abundance of microbiome members rather than by their presence/absence. These results reveal that the microbiome of grafted grapevine root systems is largely influenced by the combination of rootstock and scion, which affects the microbial diversity uptaken from soil.

Water deficit affects inter-kingdom microbial connections in plant rhizosphere

The frequency and severity of drought are increasing due to anthropogenic climate change and are already limiting cropping system productivity in many regions around the world. Few microbial groups within plant microbiomes can potentially contribute towards the fitness and productivity of their hosts under abiotic stress events including water deficits. However, microbial communities are complex and integrative work considering the multiple co-existing groups of organisms is needed to better understand how the entire microbiome responds to environmental stresses. We hypothesize that water deficit stress will differentially shape bacterial, fungal, and protistan microbiome composition and influence inter-kingdom microbial interactions in the rhizospheres of corn and sugar beet. We used amplicon sequencing to profile bacterial, fungal, and protistan communities in corn and sugar beet rhizospheres grown under irrigated and water deficit conditions. The water deficit treatment had a stronger influence than host species on bacterial composition, whereas the opposite was true for protists. These results indicate that different microbial kingdoms have variable responses to environmental stress and host factors. Water deficit also influenced intra- and inter-kingdom microbial associations, wherein the protist taxa formed a separate cluster under water deficit conditions. Our findings help elucidate the influence of environmental and host drivers of bacterial, fungal, and protistan community assembly and co-occurrence in agricultural rhizosphere environments.

18S rRNA gene amplicon sequencing combined with culture-based surveys of maize rhizosphere protists reveal dominant, plant-enriched and culturable community members

Protists play important roles in shaping the microbial community of the rhizosphere and defining these roles will require the study of protist isolates. However, there is still a limited understanding of how well protist isolation efforts can capture the diversity and composition of rhizosphere protistan communities. Here, we report a simultaneous isolation and 18S rRNA gene amplicon sequencing survey describing the protist diversity of maize rhizospheres in two climatically and pedologically distinct sites. We demonstrated that the maize rhizosphere exerted significant and site-dependent effects on the protistan community structure and defined a set of core and rhizosphere-enriched protists. From the same root samples, we generated a library of 103 protist isolates representing 46 18S rRNA gene sequence variants from six eukaryotic supergroups. While cultured isolates represented a small proportion of total protist diversity recovered by sequencing, they included taxa enriched in rhizosphere soils across all samples, encompassing 9% of all core sequence variants. The isolation approach also captured 17 protists not detected through 18S rRNA gene amplicon sequencing. This study demonstrated that maize roots select for distinct protistan communities, and established a diverse protist culture collection that can be used for future research linking protists to rhizosphere status and plant health.

N-induced root exudates mediate the rhizosphere fungal assembly and affect species coexistence

Root exudates play essential roles in shaping root-associated microbial communities in plant-soil systems. However, knowledge regarding the influence of root exudates on soil communities, particularly concerning their assembly processes and species coexistence patterns, remains limited. In this study, we performed a 20-month pot experiment using a nitrogen (N) addition gradient (0, 2.5, 5, 7.5, 10, and 15 g N m^-2 yr^-1), amplicon sequencing, and metabolomics to investigate the effect of short-term N addition on the assembly process and species coexistence of fungal communities, as well as their association with root exudates in the rhizosphere and bulk soils around Bothriochloa ischaemum. The results demonstrated that short-term N addition led to distinct differences in the diversity, composition, assembly process, and co-occurrence networks of fungal communities in the rhizosphere and bulk soils. The diversity of fungal communities in the rhizosphere soil increased with the rate of N input and peaked at N10 treatment; this could be correlated with the increased abundance in long-chain organic acids (LCOAs). However, above the threshold N rate of 10 g N m^-2 yr^-1, diversity decreased probably because of the high N-induced inhibitory effect on root exudates (i.e., LCOAs). N addition increased the relative abundance of Sordariomycetes in the rhizosphere and decreased the relative abundance of Mortierellomycetes in the bulk soil, while enhancing the abundance of pathotrophs in both bulk and rhizosphere soils. The rhizosphere fungal community was dominated by a stochastic process at a low N input (N0 and N2.5) and by deterministic processes at a high N input (N10 and N15), which is opposite to the trends in the bulk soil. These fungal assembly processes determine the coexistence of fungal species; deterministic processes lead to less interconnected networks in rhizosphere soils that harbor a more complex network than the bulk soil. Associations between the assembly process and species coexistence in the rhizosphere of B. ischaemum were closely related to the changes in root exudates, such as amino acids, short-chain organic acids, and phenols, which were stimulated by N addition. Collectively, our study emphasizes the key roles of root exudates in the establishment of fungal communities in the plant-soil system and furthers our understanding of plant-microbe interactions.

Community RNA-Seq: multi-kingdom responses to living versus decaying roots in soil

Roots are a primary source of organic carbon input in most soils. The consumption of living and detrital root inputs involves multi-trophic processes and multiple kingdoms of microbial life, but typical microbial ecology studies focus on only one or two major lineages. We used Illumina shotgun RNA sequencing to conduct PCR-independent SSU rRNA community analysis ("community RNA-Seq") and simultaneously assess the bacteria, archaea, fungi, and microfauna surrounding both living and decomposing roots of the annual grass, Avena fatua. Plants were grown in 13CO2-labeled microcosms amended with 15N-root litter to identify the preferences of rhizosphere organisms for root exudates (13C) versus decaying root biomass (15N) using NanoSIMS microarray imaging (Chip-SIP). When litter was available, rhizosphere and bulk soil had significantly more Amoebozoa, which are potentially important yet often overlooked top-down drivers of detritusphere community dynamics and nutrient cycling. Bulk soil containing litter was depleted in Actinobacteria but had significantly more Bacteroidetes and Proteobacteria. While Actinobacteria were abundant in the rhizosphere, Chip-SIP showed Actinobacteria preferentially incorporated litter relative to root exudates, indicating this group's more prominent role in detritus elemental cycling in the rhizosphere. Our results emphasize that decomposition is a multi-trophic process involving complex interactions, and our methodology can be used to track the trajectory of carbon through multi-kingdom soil food webs.

Bridging Microbial Functional Traits With Localized Process Rates at Soil Interfaces

In this review, we introduce microbially-mediated soil processes, players, their functional traits, and their links to processes at biogeochemical interfaces [e.g., rhizosphere, detritusphere, (bio)-pores, and aggregate surfaces]. A conceptual view emphasizes the central role of the rhizosphere in interactions with other biogeochemical interfaces, considering biotic and abiotic dynamic drivers. We discuss the applicability of three groups of traits based on microbial physiology, activity state, and genomic functional traits to reflect microbial growth in soil. The sensitivity and credibility of modern molecular approaches to estimate microbial-specific growth rates require further development. A link between functional traits determined by physiological (e.g., respiration, biomarkers) and genomic (e.g., genome size, number of ribosomal gene copies per genome, expression of catabolic versus biosynthetic genes) approaches is strongly affected by environmental conditions such as carbon, nutrient availability, and ecosystem type. Therefore, we address the role of soil physico-chemical conditions and trophic interactions as drivers of microbially-mediated soil processes at relevant scales for process localization. The strengths and weaknesses of current approaches (destructive, non-destructive, and predictive) for assessing process localization and the corresponding estimates of process rates are linked to the challenges for modeling microbially-mediated processes in heterogeneous soil microhabitats. Finally, we introduce a conceptual self-regulatory mechanism based on the flexible structure of active microbial communities. Microbial taxa best suited to each successional stage of substrate decomposition become dominant and alter the community structure. The rates of decomposition of organic compounds, therefore, are dependent on the functional traits of dominant taxa and microbial strategies, which are selected and driven by the local environment.

Fighting climate change: soil bacteria communities and topography play a role in plant colonization of desert areas

Global warming has exacerbated desertification in arid regions. Exploring the environmental variables and microbial communities that drive the dynamics of geographic patterns of desert crops is important for large-scale standardization of crops that can control desertification. Here, predictions based on future climate data from CMIP6 show that a steady expand in the suitable production areas for three desert plants (Cistanche deserticola, Cynomorium songaricum and Cistanche salsa) under global warming, demonstrating their high adaptability to future climate change. We examined the biogeography of three desert plant soil bacteria communities and assessed the environmental factors affecting the community assembly process. The α-diversity significantly decreased along elevated latitudes, indicating that the soil bacterial communities of the three species have latitude diversity patterns. The neutral community model evaluated 66.6% of the explained variance of the bacterial community in the soil of desert plants and Modified Stochasticity Ratio <0.5, suggesting that deterministic processes dominate the assembly of bacterial communities in three desert plants. Moreover, topography (longitude, elevation) and precipitation as well as key OTUs (OTU4911: Streptomyces eurythermus and OTU4672: Streptomyces flaveus) drive the colonization of three desert plants. This research offers a promising solution for desert management in arid areas under global warming.

Taxonomic Compositions and Co-occurrence Relationships of Protists in Bulk Soil and Rhizosphere of Soybean Fields in Different Regions of China

As the main consumers of bacteria and fungi in farmed soils, protists remain poorly understood. The aim of this study was to explore protist community assembly and ecological roles in soybean fields. Here, we investigated differences in protist communities using high-throughput sequencing and their inferred potential interactions with bacteria and fungi between the bulk soil and rhizosphere compartments of three soybean cultivars collected from six ecological regions in China. Distinct protist community structures characterized the bulk soil and rhizosphere of soybean plants. A significantly higher relative abundance of phagotrophs was observed in the rhizosphere (25.1%) than in the bulk soil (11.3%). Spatial location (R 2 = 0.37-0.51) explained more of the variation in protist community structures of soybean fields than either the compartment (R 2 = 0.08-0.09) or cultivar type (R 2 = 0.02-0.03). The rhizosphere protist network (76 nodes and 414 edges) was smaller and less complex than the bulk soil network (147 nodes and 880 edges), indicating a smaller potential of niche overlap and interactions in the rhizosphere due to the increased resources in the rhizosphere. Furthermore, more inferred potential predator-prey interactions occur in the rhizosphere. We conclude that protists have a crucial ecological role to play as an integral part of microbial co-occurrence networks in soybean fields.