Feedbacks of plant identity and diversity on the diversity and community composition of rhizosphere microbiomes from a long‐term biodiversity experiment

Soil microbes are known to be key drivers of several essential ecosystem processes such as nutrient cycling, plant productivity and the maintenance of plant species diversity. However, how plant species diversity and identity affect soil microbial diversity and community composition in the rhizosphere is largely unknown. We tested whether, over the course of 11 years, distinct soil bacterial communities developed under plant monocultures and mixtures, and if over this time frame plants with a monoculture or mixture history changed in the bacterial communities they associated with. For eight species, we grew offspring of plants that had been grown for 11 years in the same field monocultures or mixtures (plant history in monoculture vs. mixture) in pots inoculated with microbes extracted from the field monoculture and mixture soils attached to the roots of the host plants (soil legacy). After 5 months of growth in the glasshouse, we collected rhizosphere soil from each plant and used 16S rRNA gene sequencing to determine the community composition and diversity of the bacterial communities. Bacterial community structure in the plant rhizosphere was primarily determined by soil legacy and by plant species identity, but not by plant history. In seven of the eight plant species the number of individual operational taxonomic units with increased abundance was larger when inoculated with microbes from mixture soil. We conclude that plant species richness can affect below‐ground community composition and diversity, feeding back to the assemblage of rhizosphere bacterial communities in newly establishing plants via the legacy in soil.     

Effects of cotton–maize rotation on soil microbiome structure

Verticillium wilt is a disastrous disease in cotton-growing regions in China. As a common management method, cotton rotation with cereal crops is used to minimize the loss caused by Verticillium dahliae. However, the correlation between soil microbiome and the control of Verticillium wilt under a crop rotation system is unclear. Therefore, three cropping systems (fallow, cotton continuous cropping, and cotton–maize rotation) were designed and applied for three generations under greenhouse conditions to investigate the different responses of the soil microbial community. The soil used in this study was taken from a long-term cotton continuous cropping field and inoculated with V. dahliae before use. Our results showed that the diversity of the soil bacterial community was increased under cotton–maize rotation, while the diversity of the fungal community was obviously decreased. Meanwhile, the structure and composition of the bacterial communities were similar even under the different cropping systems, but they differed in the soil fungal communities. Through microbial network interaction analysis, we found that Verticillium interacted with 17 bacterial genera, among which Terrabacter had the highest correlation with Verticillium. Furthermore, eight fungal and eight bacterial species were significantly correlated with V. dahliae. Collectively, this work aimed to study the interactions among V. dahliae, the soil microbiome, and plant hosts, and elucidate the relationship between crop rotation and soil microbiome, providing a new theoretical basis to screen the biological agents that may contribute to Verticillium wilt control.     

Plant identity strongly structures the root-associated fungal community in a diverse subtropical forest

Revealing the relationship between plants and root-associated fungi is very important in understanding diversity maintenance and community assembly in ecosystems. However, the community assembly of root-associated fungi of focal plant species along a subtropical plant species diversity gradient is less documented. Here, we examined root-associated fungal communities associated with five ectomycorrhizal (EM) plant species (Betula luminifera, Castanea henryi, Castanopsis fargesii, C. sclerophylla, and Quercus serrate) in a Chinese subtropical woody plant species diversity (1, 2, 4, 8, 16 and 24 species) experiment, using paired-end Illumina MiSeq sequencing of the ITS2 region. In total, we detected 1933 root-associated fungal operational taxonomic units (OTUs) at a 97% sequence similarity level. Plant identity had a significant effect on total and saprotrophic fungal OTU richness, but plant species diversity level had a significant effect on saprotrophic and pathogenic fungal OTU richness. The community composition of total, saprotrophic and EM fungi was structured by plant identity and plant species diversity level. However, the community composition of pathogenic fungi was only shaped by plant identity. This study highlights that plant identity has a stronger effect on the root-associated fungal community than plant species diversity level in a diverse subtropical forest ecosystem.     

贝加尔针茅草原土壤真菌群落结构对氮素和水分添加的响应

研究氮沉降和降雨变化对土壤真菌群落结构的互作效应,对未来预测多个气候变化因子对草地生态系统的交互作用具有重要意义。以施氮和灌溉模拟氮沉降和降雨增加,采用裂区设计,应用高通量测序技术,研究8个氮添加水平(0、15、30、50、100、150、200、300 kg N hm~(-2)a~(-1))和2个水分添加水平(不灌溉、模拟夏季增雨100 mm灌溉)对土壤真菌群落结构的影响。结果表明,氮素和水分添加后,土壤真菌群落中占优势的门类分别为接合菌门Zygomycota(22.0%—48.9%)、担子菌门Basidiomycota(7.8%—18.5%)、子囊菌门Ascomycota(9.4%—20.1%)、球囊菌门Glomeromycota(0.7%—3.1%)、壶菌门Chytridiomycota(0.1%—1.3%)。常规降雨条件下,随着氮添加水平升高,接合菌门相对丰度呈现出先升高后降低的趋势,N50处理最高;子囊菌门相对丰度在高氮添加时(N100—N300)呈升高趋势。而在氮素和水分同时添加条件下,随着氮添加水平升高,接合菌门相对丰度呈降低趋势,子囊菌门相对丰度变化则不明显。在相同的氮添加水平下,水分添加使接合菌门相对丰度增加,而担子菌门、子囊菌门、球囊菌门和壶菌门的相对丰度降低。在不同氮素和水分添加条件下,有5个土壤真菌门类11个真菌纲相对丰度变化显著。接合菌门的Mortierella属,担子菌门的Entolomataceae科和Geastrum属相对丰度变化极显著,可作为土壤真菌群落结构变化的指示种。PCo A分析结果也表明氮素和水分添加改变了土壤真菌群落结构。植物-土壤-微生物系统的结构方程模型结果表明,植物群落组成及植物物种丰富度的变化是土壤真菌群落结构发生变化的主要影响因素,土壤无机氮及p H的变化主要通过影响植物群落间接影响真菌群落,其对真菌群落的直接影响则较小。综上,氮素和水分添加改变了土壤真菌群落结构,且两者存在明显的互作效应,水分添加可改变氮添加对土壤真菌群落的影响。     

Short-term plant species impact on microbial community structure in soils with long-term agricultural history

The goal of our study was to capture the short-term effects of individual plant species on an established microbial community in a soil with a well-defined agricultural history. Using biochemical and molecular techniques we quantified the effects of plant species on changes in the soil microbial community over an 8-week time-course. We conducted a greenhouse experiment using field soil from a site that was managed as a Zea mays monoculture for over 50 years. The conditioned soil provided a baseline from which changes in microbial community composition through the effects of newly introduced plants could be determined. Within a short time frame (8 weeks), introduced plants influenced the soil microbial community in ways unique to each plant. Some species (Fagopyrm esculentum and xTriticosecale spp.) resulted in an increase of total microbial community richness, diversity and the stimulation of new microbial species not associated with the legacy vegetation. Other plants (Vicia villosa and Lolium multiflorum) tended to reduce community diversity. We suggest root surface area is good general predictor of rhizosphere microbial community diversity, but in some cases other plant traits may have dominant influence on plant-induced changes in microbial community composition.     

青枯病易感和钝感桑树品种根际土壤真菌群落结构比较

为研究青枯病易感和钝感桑树品种植株根际土壤真菌群落组成,该研究以ITS1F和ITS2R为引物,基于高通量测序技术对桑树青枯病易感品种(台湾长果桑,SM)和桑树青枯病钝感品种(桂桑12号,IM)植株根际土壤真菌群落结构进行分析。结果表明：(1)两个品种间指示真菌丰富度的ACE、Chao1指数及表征多样性的Shannon指数无显著差异,门分类水平,被孢霉门(Mortierellomycota)和球囊菌门(Glomeromycota)是青枯病钝感桑树品种植株根际土壤中特有的优势真菌门;而属分类水平,Apiotrichum、地丝菌属(Geotrichum)、足放线病菌属(Scedosporium)和腐质霉属(Humicola)等是青枯病易感桑树品种植株根际土壤中富集的特有优势真菌属。(2)青枯病易感桑树品种植株根际土壤中,缺失了被孢霉门、球囊菌门真菌,以及被孢霉属(Mortierella)、镰刀菌属(Fusarium)、曲霉菌属(Aspergillus)和毛壳菌属(Chaetomium)等具有生防功能的优势真菌门属,可能是其易感青枯病的重要原因。(3)根据真菌群落对同类环境资源的利用途径进行功...     

Influences of Plant Species Composition,Fertilisation and <Emphasis Type="Italic">Lolium perenne</Emphasis> Ingression on Soil Microbial Community Structure in Three Irish Grasslands

Semi-natural grassland soils are frequently fertilised for agricultural improvement. This practice often comes at a loss of the indigenous flora while fast-growing nitrogen-responsive species, such as Lolium perenne, take over. Since soil microbial communities depend on plant root exudates for carbon and nitrogen sources, this shift in vegetation is thought to influence soil microbial community structure. In this study, we investigated the influence of different plant species, fertilisation and L. perenne ingression on microbial communities in soils from three semi-natural Irish grasslands. Bacterial and fungal community compositions were determined by automated ribosomal intergenic spacer analysis, and community changes were linked to environmental factors by multivariate statistical analysis. Soil type had a strong effect on bacterial and fungal communities, mainly correlated to soil pH, as well as soil carbon and nitrogen status. Within each soil type, plant species composition was the main influencing factor followed by nitrogen fertilisation and finally Lolium ingression in the acidic upland and mesotrophic grassland. In the alkaline grassland, however, Lolium ingression had a stronger effect than fertilisation. Our results suggest that a change in plant species diversity strongly influences the microbial community structure, which may subsequently lead to significant changes in ecosystem functioning.     

Seed mucilage interacts with soil microbial community and physiochemical processes to affect seedling emergence on desert sand dunes

Seedling emergence is a critical stage in the establishment of desert plants. Soil microbes participate in plant growth and development, but information is lacking with regard to the role of microbes on seedling emergence. We applied the biocides (captan and streptomycin) to assess how seed mucilage interacts with soil microbial community and physiochemical processes to affect seedling emergence of Artemisia sphaerocephala on the desert sand dune. Fungal and bacterial community composition and diversity and fungal–bacterial interactions were changed by both captan and streptomycin. Mucilage increased soil enzyme activities and fungal–bacterial interactions. Highest seedling emergence occurred under streptomycin and mucilage treatment. Members of the phyla Firmicutes and Glomeromycota were the keystone species that improved A. sphaerocephala seedling emergence, by increasing resistance of young seedlings to drought and pathogen. Seed mucilage directly improved seedling emergence and indirectly interacted with the soil microbial community through strengthening fungal–bacterial interactions and providing favourable environment for soil enzymes to affect seedling emergence. Our study provides a comprehensive understanding of the regulatory mechanisms by which soil microbial community and seed mucilage interactively promote successful establishment of populations of desert plants on the barren and stressful sand dune.     

Positive legacy effect of previous legume proportion in a ley on the performance of a following crop of Lolium multiflorum

Aims

We investigated the legacy effects of a previous ley’s legume proportion on the performance of a following grass crop in a rotation.

Methods

In April 2015, a pure Lolium multiflorum L. crop was sown after the removal of legume containing swards (0–100% legumes), and was harvested four times over the following one-year period (3 times in 2015 and once the following April 2016). Labeled ¹⁵N fertilizer (50 kg N ha⁻¹) was applied during the 2nd and 3rd re-growth periods to determine N fluxes.

Results

Across the one-year period, a significant legume-legacy induced increase in biomass yield of L. multiflorum was observed over the entire range of previous legume proportions when compared against the non-legume ley, the effect being 2.15 and 1.73 t ha⁻¹ (P ≤ 0.001 each) in swards with 50% and 100% previous legume proportion, respectively, or up to +31%. The legume-legacy effect on biomass yield was most pronounced at the 1st harvest (June) and persisted into the 2nd harvest in August (P ≤ 0.05 both, over the entire range of previous legume proportion), though was no longer evident at the 3rd harvest (September). Importantly, the legume-legacy effect returned in the 4th harvest in April (P ≤ 0.05). Examining the source of N contributing to N yield confirmed that more N was derived from the soil at harvest 1 and 2 for previous legume containing leys (P ≤ 0.001) compared to those which contained no legumes, with a significant increase still seen for legume mixtures at harvest 3 (P ≤ 0.01).

Conclusions

The results demonstrate a sustained soil-transferred performance-enhancing legacy effect on a following crop in a rotation, with previous legume proportions of 50% having a comparable effect compared with that of a previous legume monoculture.

     

不同种植年限薰衣草根际土壤真菌群落结构的演变

土壤真菌群落结构和多样性对植物生长起着重要作用,了解种植年限对薰衣草根际土壤真菌群落结构的影响对薰衣草病害防治和增产的研究具有重要意义。探究不同种植年限薰衣草根际土壤真菌群落结构和多样性特征,及其随种植年限的演变规律。采集新疆伊犁霍城县种植年限1、3、5 a,以及未种植薰衣草土壤,对ITS序列进行Illumina高通量测序。对测序结果进行分析,比较各样品组真菌多样性和群落分布规律及与种植年限的关联。结果表明,Alpha多样性分析显示随着种植年限的增加Shannon指数逐渐降低,而Chao1指数先降低后增高。在种植薰衣草土壤中共检测到12个门,28个纲,72个目,146个科,236个属。在门水平上,子囊菌门(Ascomycota)、担子菌门(Basidiomycota)、球囊菌门(Glomeromycota)为优势菌门。随着种植年限的增加,子囊菌门相对丰度逐渐降低,担子菌门逐渐增高,而球囊菌门先增加后降低。在属水平上优势菌属为Xylodon、锐孔菌属(Oxyporus)、镰刀菌属(Fusarium)、枝孢属(Cladosporium),均属于植物致病菌。随着种植年限的增加,Xylodon...     

Effects of the endophyte Epichloë coenophiala on the root microbial community and growth performance of tall fescue in different saline-alkali soils

Soil salinization is detrimental to plant growth and yield in agroecosystems worldwide. Epichloë endophytes, a class of clavicipitaceous fungi, enhance the resistance of host plants to saline-alkali stress. This study explored the effects of the systemic fungal endophyte Epichloë coenophiala on the root microbial community and growth performance of tall fescue (Lolium arundinaceum) growing under different saline-alkali stress conditions. Structural equation modeling (SEM) was conducted to analyze the direct and indirect effects (mediated by root microbial community diversity and soil properties) of the endophyte on the growth of tall fescue under saline-alkali stress. The endophyte-infected plants produced higher shoot and root biomass compared to endophyte-free plants under saline-alkali stress (200 and 400 mM). Endophyte infection increased the fungal community diversity and altered its composition in the roots, decreasing the relative abundance of Ascomycota and increasing that of Glomeromycota. Furthermore, endophyte infection decreased the bacterial community diversity and the relative abundance of dominant Proteobacteria. SEM showed that endophyte infection increased the shoot and root biomass under saline-alkali stress (200 and 400 mM) by increasing the arbuscular mycorrhizal fungal diversity in the roots, and soil total nitrogen and phosphorus concentrations. Therefore, it is important to examine aboveground microbes as factors influencing plant growth in saline-alkali stress by affecting belowground microbes and soil chemical properties.     

Cascading effects of belowground predators on plant communities are density‐dependent

Soil food webs comprise a multitude of trophic interactions that can affect the composition and productivity of plant communities. Belowground predators feeding on microbial grazers like Collembola could decelerate nutrient mineralization by reducing microbial turnover in the soil, which in turn could negatively influence plant growth. However, empirical evidences for the ecological significance of belowground predators on nutrient cycling and plant communities are scarce. Here, we manipulated predator density (Hypoaspis aculeifer: predatory mite) with equal densities of three Collembola species as a prey in four functionally dissimilar plant communities in experimental microcosms: grass monoculture (Poa pratensis), herb monoculture (Rumex acetosa), legume monoculture (Trifolium pratense), and all three species as a mixed plant community. Density manipulation of predators allowed us to test for density‐mediated effects of belowground predators on Collembola and lower trophic groups. We hypothesized that predator density will reduce Collembola population causing a decrease in nutrient mineralization and hence detrimentally affect plant growth. First, we found a density‐dependent population change in predators, that is, an increase in low‐density treatments, but a decrease in high‐density treatments. Second, prey suppression was lower at high predator density, which caused a shift in the soil microbial community by increasing the fungal: bacterial biomass ratio, and an increase of nitrification rates, particularly in legume monocultures. Despite the increase in nutrient mineralization, legume monocultures performed worse at high predator density. Further, individual grass shoot biomass decreased in monocultures, while it increased in mixed plant communities with increasing predator density, which coincided with elevated soil N uptake by grasses. As a consequence, high predator density significantly increased plant complementarity effects indicating a decrease in interspecific plant competition. These results highlight that belowground predators can relax interspecific plant competition by increasing nutrient mineralization through their density‐dependent cascading effects on detritivore and soil microbial communities.     

Contrasting diurnal variations in soil organic carbon decomposition and root respiration due to a hysteresis effect with soil temperature in a Gossypium s. (cotton) plantation

Cool-season grasses commonly harbor fungal endophytes in their aerial tissues. However the effects of these symbionts on soil microbial communities have rarely been investigated. Our objective was to explore microbial community responses in soils conditioned by plants of the annual grass Lolium multiflorum with contrasting levels of infection with the endophyte Neotyphodium occultans. At the end of the host growing season, we estimated the functional capacity of soil microbial communities (via catabolic response profiles), the contribution of fungi and bacteria to soil activity (via selective inhibition with antibiotics), and the structure of both microbial communities by molecular analyses. Soil conditioning by highly infected plants affected soil catabolic profiles and tended to increase soil fungal activity. We detected a shift in bacterial community structure while no changes were observed for fungi. Soil responses became evident even without changes in host plant biomass or soil organic carbon or total nitrogen content, suggesting that the endophyte modified host rhizodepositions during the conditioning phase. Our results have implications for the understanding of the reciprocal interactions between above and belowground communities, suggesting that plant-soil feedbacks can be mediated by this symbiosis.     

黄河三角洲不同类型盐生植物土壤真菌群落结构特征

盐生植物种类及其所具有的不同耐盐调节方式影响着根际微生物群落的结构与组成。为明确不同类型盐生植物根际与非根际土壤中真菌群落结构与组成的差异及其与土壤环境间的相互关系,该研究采集了黄河三角洲地区芦苇、盐地碱蓬、獐毛3种不同类型盐生植物0～20 cm土层的根际和非根际土壤,通过高通量测序对其真菌群落多样性和结构进行了分析,以探究真菌群落特征与土壤理化因子间的关系。结果表明：(1)3种不同类型盐生植物根际土壤真菌群落丰富度显著大于各自非根际土,且獐毛根际土壤真菌群落丰富度显著大于芦苇和盐地碱蓬的根际土。(2)距离热图分析表明,芦苇和盐地碱蓬非根际土壤真菌群落间的相似性最大。(3)土壤真菌多样性和丰富度与土壤总碳、总氮、有效磷、pH呈正相关关系,与土壤盐分含量呈负相关关系。(4)3种不同类型盐生植物的根际与非根际土壤中,球囊菌门(Glomeromycota)均为绝对优势门,盾巨孢囊霉属(Scutellospora)为优势属。(5)RDA分析表明,土壤盐分含量是影响土壤真菌群落结构的重要因子,球囊菌门丰度与土壤总氮、总碳、有效磷、有机碳、pH呈正相关关系,与盐分呈负相关关系。(6)植物土壤真菌群...     

Responses of the soil fungal communities to the co‐invasion of two invasive species with different cover classes

• Soil fungal communities play an important role in the successful invasion of non‐native species. It is common for two or more invasive plant species to co‐occur in invaded ecosystems.
• This study aimed to determine the effects of co‐invasion of two invasive species (Erigeron annuus and Solidago canadensis) with different cover classes on soil fungal communities using high‐throughput sequencing.
• Invasion of E. annuus and/or S. canadensis had positive effects on the sequence number, operational taxonomic unit (OTU) richness, Shannon diversity, abundance‐based cover estimator (ACE index) and Chao1 index of soil fungal communities, but negative effects on the Simpson index. Thus, invasion of E. annuus and/or S. canadensis could increase diversity and richness of soil fungal communities but decrease dominance of some members of these communities, in part to facilitate plant further invasion, because high soil microbial diversity could increase soil functions and plant nutrient acquisition. Some soil fungal species grow well, whereas others tend to extinction after non‐native plant invasion with increasing invasion degree and presumably time. The sequence number, OTU richness, Shannon diversity, ACE index and Chao1 index of soil fungal communities were higher under co‐invasion of E. annuus and S. canadensis than under independent invasion of either individual species.
• The co‐invasion of the two invasive species had a positive synergistic effect on diversity and abundance of soil fungal communities, partly to build a soil microenvironment to enhance competitiveness of the invaders. The changed diversity and community under co‐invasion could modify resource availability and niche differentiation within the soil fungal communities, mediated by differences in leaf litter quality and quantity, which can support different fungal/microbial species in the soil.

     

Plant diversity and identity effects on predatory nematodes and their prey

There is considerable evidence that both plant diversity and plant identity can influence the level of predation and predator abundance aboveground. However, how the level of predation in the soil and the abundance of predatory soil fauna are related to plant diversity and identity remains largely unknown. In a biodiversity field experiment, we examined the effects of plant diversity and identity on the infectivity of entomopathogenic nematodes (EPNs, Heterorhabditis and Steinernema spp.), which prey on soil arthropods, and abundance of carnivorous non‐EPNs, which are predators of other nematode groups. To obtain a comprehensive view of the potential prey/food availability, we also quantified the abundance of soil insects and nonpredatory nematodes and the root biomass in the experimental plots. We used structural equation modeling (SEM) to investigate possible pathways by which plant diversity and identity may affect EPN infectivity and the abundance of carnivorous non‐EPNs. Heterorhabditis spp. infectivity and the abundance of carnivorous non‐EPNs were not directly related to plant diversity or the proportion of legumes, grasses and forbs in the plant community. However, Steinernema spp. infectivity was higher in monocultures of Festuca rubra and Trifolium pratense than in monocultures of the other six plant species. SEM revealed that legumes positively affected Steinernema infectivity, whereas plant diversity indirectly affected the infectivity of Heterorhabditis EPNs via effects on the abundance of soil insects. The abundance of prey (soil insects and root‐feeding, bacterivorous, and fungivorous nematodes) increased with higher plant diversity. The abundance of prey nematodes was also positively affected by legumes. These plant community effects could not be explained by changes in root biomass. Our results show that plant diversity and identity effects on belowground biota (particularly soil nematode community) can differ between organisms that belong to the same feeding guild and that generalizations about plant diversity effects on soil organisms should be made with great caution.     

Mixing tree species associated with arbuscular or ectotrophic mycorrhizae reveals dual mycorrhization and interactive effects on the fungal partners

1. Recent studies found that the majority of shrub and tree species are associated with both arbuscular mycorrhizal (AM) and ectomycorrhizal (EM) fungi. However, our knowledge on how different mycorrhizal types interact with each other is still limited. We asked whether the combination of hosts with a preferred association with either AM or EM fungi increases the host tree roots’ mycorrhization rate and affects AM and EM fungal richness and community composition.
2. We established a tree diversity experiment, where five tree species of each of the two mycorrhiza types were planted in monocultures, two‐species and four‐species mixtures. We applied morphological assessment to estimate mycorrhization rates and next‐generation molecular sequencing to quantify mycobiont richness.
3. Both the morphological and molecular assessment revealed dual‐mycorrhizal colonization in 79% and 100% of the samples, respectively. OTU community composition strongly differed between AM and EM trees. While host tree species richness did not affect mycorrhization rates, we observed significant effects of mixing AM‐ and EM‐associated hosts in AM mycorrhization rate. Glomeromycota richness was larger in monotypic AM tree combinations than in AM‐EM mixtures, pointing to a dilution or suppression effect of AM by EM trees. We found a strong match between morphological quantification of AM mycorrhization rate and Glomeromycota richness.
4. Synthesis. We provide evidence that the combination of hosts differing in their preferred mycorrhiza association affects the host''s fungal community composition, thus revealing important biotic interactions among trees and their associated fungi.

     

Dual inoculation of dark septate endophytes and Trichoderma viride drives plant performance and rhizosphere microbiome adaptations of Astragalus mongholicus to drought

Rhizosphere microbiome adapts their structural compositions to water scarcity and have the potential to mitigate drought stress of plants. To unlock this potential, it is crucial to understand community responses to drought in the interplay between soil properties, water management and exogenous microbes interference. Inoculation with dark septate endophytes (DSE) (Acrocalymma vagum, Paraboeremia putaminum) and Trichoderma viride on Astragalus mongholicus grown in the non-sterile soil was exposed to drought. Rhizosphere microbiome were assessed by Illumina MiSeq sequencing of the 16S and ITS2 rRNA genes. Inoculation positively affected plant growth depending on DSE species and water regime. Ascomycota, Proteobacteria, Actinobacteria, Chloroflexi and Firmicutes were the dominant phyla. The effects of dual inoculation on bacterial community were greater than those on fungal community, and combination of P. putaminum and T. viride exerted a stronger impact on the microbiome under drought stress. The observed changes in soil factors caused by inoculation could be explained by the variations in microbiome composition. Rhizosphere microbiome mediated by inoculation exhibited distinct preferences for various growth parameters. These findings suggest that dual inoculation of DSE and T. viride enriched beneficial microbiota, altered soil nutrient status and might contribute to enhance the cultivation of medicinal plants in dryland agriculture.     

Effect of nitrogen on interactions between ryegrass mosaic virus (RMV) and Lolium species in mixed swards

The effect of rate of nitrogen applied to three Lolium perenne-based field swards on the development of RMV symptoms and on the impact of the virus on the growth and composition of the swards was examined over two growing seasons. Glasshouse experiments investigated the competition between Lolium spp. in relation to RMV infection and rate of nitrogen. In general, there was a higher incidence of RMV symptoms at the higher rates of nitrogen but the relationship was not consistent in all experiments. RMV infection only significantly reduced the cumulative total yield of the sward that contained the very susceptible Lolium multiflorum cv. Barverdi and was treated with the higher rate of nitrogen. However, yield losses were consistently greater at the higher rates of nitrogen. RMV altered the relative yields of sward components and the effect was greatest after an extended period of severe frost at the end of 1995. There is evidence that the frost severely restricted the yield of RMV-infected L. multiflorum tillers in 1996. Where RMV reduced the yield of susceptible species, there was significant compensatory growth by less susceptible companion species, i.e. L. perenne and Phleum pratense, but the compensation was generally incomplete, resulting in reduced yields for virus-infected swards. RMV had no significant effect on the overall quality of the three field swards but for cv. Barverdi grown in the glasshouse it significantly reduced the percentage organic matter and water soluble carbohydrate, and D value. The reductions were greater at the higher rates of nitrogen.     

Contrasting primary successional trajectories of fungi and bacteria in retreating glacier soils

Early community assembly of soil microbial communities is essential for pedogenesis and development of organic legacies. We examined fungal and bacterial successions along a well‐established temperate glacier forefront chronosequence representing ~70 years of deglaciation to determine community assembly. As microbial communities may be heavily structured by establishing vegetation, we included nonvegetated soils as well as soils from underneath four plant species with differing mycorrhizal ecologies (Abies lasiocarpa, ectomycorrhizal; Luetkea pectinata, arbuscular mycorrhizal; Phyllodoce empetriformis, ericoid mycorrhizal; Saxifraga ferruginea, nonmycorrhizal). Our main objectives were to contrast fungal and bacterial successional dynamics and community assembly as well as to decouple the effects of plant establishment and time since deglaciation on microbial trajectories using high‐throughput sequencing. Our data indicate that distance from glacier terminus has large effects on biomass accumulation, community membership, and distribution for both fungi and bacteria. Surprisingly, presence of plants rather than their identity was more important in structuring bacterial communities along the chronosequence and played only a very minor role in structuring the fungal communities. Further, our analyses suggest that bacterial communities may converge during assembly supporting determinism, whereas fungal communities show no such patterns. Although fungal communities provided little evidence of convergence in community structure, many taxa were nonrandomly distributed across the glacier foreland; similar taxon‐level responses were observed in bacterial communities. Overall, our data highlight differing drivers for fungal and bacterial trajectories during early primary succession in recently deglaciated soils.