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In soil, arbuscular mycorrhizal fungi (AMF) meet the roots of both host and presumed nonhost plants, but the interactional mechanisms of AMF with and functional relevance for nonhost plants is little known. Here we show AMF can colonize an individually grown nonhost plant, Arabidopsis thaliana, and suppress the growth of Arabidopsis and two nonhost Brassica crops. This inhibitory effect increased with increasing AMF inoculum density, and was independent of AMF species or nutrient availability. 13C isotope labeling and physiological analyses revealed no significant carbon‒phosphorus exchange between Arabidopsis and AMF, indicating a lack of nutritional function in this interaction. AMF colonization activated the danger‐associated peptide Pep‐PEPR signaling pathway, and caused clear defense responses in Arabidopsis. The impairment of Pep‐PEPR signaling in nonhost plants greatly compromised AMF‐triggered defensive responses and photosynthesis suppression, leading to higher colonization rates and reduced growth suppression upon AMF inoculation. Pretreatment with Pep peptide decreased AMF colonization, and largely substituted for AMF‐induced growth suppression in nonhosts, confirming that the Pep‐PEPR pathway is a key participant in resistance to AMF colonization and in mediating growth suppression of nonhost plants. This work greatly increases our knowledge about the functional relevance of AMF and their mechanisms of interactions with nonhost plants. Summary Statement: There is no significant carbon and phosphorus exchange between AMF and colonized nonhost plant Arabidopsis thaliana. AMF activated the danger‐associated peptide Pep‐PEPR signaling pathway, which acted as a key participant in resistance to AMF colonization and in mediating growth suppression of nonhost plants. [ABSTRACT FROM AUTHOR]

Mineral‐associated organic matter is an integral part of soil carbon pool. Biological processes contribute to the formation of such organo‐mineral complexes when soil microbes, and in particular soil fungi, deposit a suite of extracellular metabolic compounds and their necromass on the mineral surfaces. While studied in bulk, micro‐ to nanoscale fungal–mineral interactions remain elusive. Of particular interest are the mutual effects at the interface between the fungal exometabolites and proximal mineral particles. In this work, we have grown saprotrophic and symbiotic fungi in contact with two soil minerals with contrasting properties: quartz and goethite, on top of X‐ray transparent silicon nitride membrane windows and analyzed fungal hyphae by synchrotron‐based scanning transmission X‐ray microscopy in combination with near edge X‐ray fine structure spectroscopy at C(K) and Fe(L) absorption edges. In the resultant chemical maps, we were able to visualize and differentiate organic compounds constituting the fungal cells, their extracellular metabolites, and the exometabolites adsorbing on the minerals. We found that the composition of the exometabolites differed between the fungal functional guilds, particularly, in their sugar to protein ratio and potassium concentration. In samples with quartz and goethite, we observed adsorption of the exometabolic compounds on the mineral surfaces with variations in their chemical composition around the particles. Although we did not observe clear alteration in the exometabolite chemistry upon mineral encounters, we show that fungal–mineral interaction result in reduction of Fe(III) in goethite. This process has been demonstrated for bulk systems, but, to our knowledge, this is the first observation on a single hypha scale offering insight into its underlying biological mechanisms. This demonstrates the link between processes initiated at the single‐cell level to macroscale phenomena. Thus, spatially resolved chemical characterization of the microbial–mineral interfaces is crucial for an increased understanding of overall carbon cycling in soil. [ABSTRACT FROM AUTHOR]

This study aimed to analyze and quantify the effect of different ratios of vermicompost, phosphate rock, and sulfur on P solubilization and release by Pseudomonas fluorescens Ur21, and to identify optimal levels of those variables for an efficient biofertilizer. Twenty experiments were defined by surface response methodology based on a central composite design (CCD), and the effects of various quantities of vermicompost, phosphate rock, and sulfur (encoded by −1, 0, or +1) on P solubilization was explored. The results show that the CCD model had high efficiency for predicting P solubilization (R2 = 0.9035). The strongest effects of the included variables on the observed P solubilization were linear effects of sulfur and organic matter (vermicompost), a quadratic effect of phosphate rock, and an interactive effect of organic matter × phosphate rock. Statistical analysis of the coefficients in the CCD model revealed that vermicompost, vermicompost × phosphate rock, and phosphate rock × phosphate rock treatments increased P solubilization. The optimal predicted composition for maximal P solubilization by P. fluorescens Ur21 (at 1684.39 mg·kg−1, with more than 90% of the added phosphate dissolved) was 58.8% vermicompost, 35.3% phosphate rock, and 5.8% sulfur. ANOVA analysis confirmed the model's accuracy and validity in terms of F value (10.41), p value (<0.001), and non-significant lack of fit. [ABSTRACT FROM AUTHOR]

Carrion is a frequent but overlooked source of nutrients to the soil. The decomposition of carrion is accelerated by invertebrate scavengers, but the impact of the scavengers on below‐ground biota and its functions is scarcely known.We conducted a laboratory experiment to investigate the effects of the burying beetle Nicrophorus vespilloides on the soil community of a temperate broadleaved forest. We assembled microcosms from soil collected from an oak woodland and treated them with mouse Mus musculus carcasses and mating pairs of burying beetles (♀+♂) in a factorial design with control soils. We sampled independent replicates over time to investigate how the beetles affect soil microarthropods and microbial biomass (bacteria and fungi) in relation to soil pH and organic matter content.The beetle treatment initially reduced the total microbial biomass and abundance of major groups of microarthropods relative to the control soil. At the same time, organic matter increased in the beetle treatment and then dropped to the pre‐beetle level (i.e. soil baseline) at the end of the beetle breeding cycle (2 weeks). The rapid temporal changes in organic matter were mimicked by the relative abundances of the dominant microarthropod groups, with Oribatida relatively more abundant than Collembola and predaceous mites in the beetle treatment. The overall final effect of the beetle (relative to the laboratory control) on microarthropods was negative but the beetle kept these variables within the levels observed for freshly collected soil (baseline), while the final effect on pH was positive, and most likely driven by the surplus of nutrients from the carcass and biochemical changes triggered by the decomposition process.In nature, scavenging invertebrates are widespread. Our study demonstrates that beetles breeding in carcasses regulate the dynamics of key components of the soil food web, including microbial biomass, changes in the relative abundances of dominant microarthropods and soil organic matter and pH. Given the abundance of these beetles in nature, the study implies that the distribution of these beetles is a key driver of variation in soil nutrient cycling in woodlands. A free Plain Language Summary can be found within the Supporting Information of this article. [ABSTRACT FROM AUTHOR]

Here, we present our collective musings on soil research challenges and opportunities and, in particular, the points raised by Philippe Baveye (Baveye, 2020a, 2020b) and Johan Bouma (Bouma, 2020) on I bypass i and I hyperbole i in soil science. Furthermore, developing a healthy and constructive post-publication peer-review system, where bypasses and hyperbolic approaches can be identified and discussed, would ultimately boost publication quality and contribute to a more open discourse in soil science. [Extracted from the article]

The socio-economic values of fertile and carbon-rich Dark Earth soils are well described from the Amazon region. Very recently, Dark Earth soils were also identified in tropical West Africa, with comparable beneficial soil properties and plant growth-promoting effects. The impact of this management technique on soil microbial communities, however, is less well understood, especially with respect to the ecologically relevant group of arbuscular mycorrhizal (AM) fungi. Thus, we tested the hypotheses that (1) improved soil quality in African Dark Earth (AfDE) will increase soil microbial biomass and shift community composition and (2) concurrently increased nutrient availability will negatively affect AM fungal communities. Microbial communities were distinct in AfDE in comparison to adjacent sites, with an increased fungal:bacterial ratio of 71%, a pattern mainly related to shifts in pH. AM fungal abundance and diversity, however, did not differ despite clearly increased soil fertility in AfDE, with 3.7 and 1.7 times greater extractable P and total N content, respectively. The absence of detrimental effects on AM fungi, often seen following applications of inorganic fertilizers, and the enhanced role of saprobic fungi relevant for mineralization and C sequestration support previous assertions of this management type as a sustainable alternative agricultural practice. [ABSTRACT FROM AUTHOR]

We examined combined effects of biochar, arbuscular mycorrhizal (AM) fungi and salinity on plant growth and physiology to test whether and how biochar influences AM fungi mediated growth and nutrition enhancements, and whether and how biochar provides amelioration in salt stressed soils. We carried out a full three-factorial greenhouse experiment with Lactuca sativa ; and a second study with a wider range of biochar and salt additions to examine physicochemical effects on soil parameters. Biochar together with AM fungal inoculation resulted in an additional plant yield increase compared to each alone under non-saline conditions. In parallel with increased plant growth, we found increased uptake of P and Mn with AM fungi and biochar addition, but to a lesser extent than biochar-induced growth promotion. Both factors, but especially biochar alleviated salinity-caused growth depressions, and improved Na/K ratio in salinity stressed plants. Reduced Na uptake of plants and reduced conductivity in biochar-ameliorated soils suggest that a likely mechanism involves ion adsorption to biochar surfaces. Our results suggest that plants depend on symbiotic microorganisms to fully exploit biochar benefits in soils, suggesting avenues for joint management in agriculture. Biochar may be advantageous in saline soils, but long-term studies are required before recommendations should be given. [ABSTRACT FROM AUTHOR]

Biochar application to soils has potential to simultaneously improve soil fertility and store carbon to aid climate change mitigation. While many studies have shown positive effects on plant yields, much less is known about the synergies between biochar and plant growth promoting microbes, such as mycorrhizal fungi. We present the first evidence that arbuscular mycorrhizal (AM) fungi can use biochar as a physical growth matrix and nutrient source. We used monoxenic cultures of the AM fungus Rhizophagus irregularis in symbiosis with carrot roots. Using scanning electron microscopy we observed that AM fungal hyphae grow on and into two contrasting types of biochar particles, strongly attaching to inner and outer surfaces. Loading a nutrient-poor biochar surface with nutrients stimulated hyphal colonization. We labeled biochar surfaces with 33P radiotracer and found that hyphal contact to the biochar surfaces permitted uptake of 33P and its subsequent translocation to the associated host roots. Direct access of fungal hyphae to biochar surfaces resulted in six times more 33P translocation to the host roots than in systems where a mesh prevented hyphal contact with the biochar. We conclude that AM fungal hyphae access microsites within biochar, that are too small for most plant roots to enter (<10 μm), and can hence mediate plant phosphorus uptake from the biochar. Thus, combined management of biochar and AM fungi could contribute to sustainable soil and climate management by providing both a carbon-stable nutrient reservoir and a symbiont that facilitates nutrient uptake from it. [ABSTRACT FROM AUTHOR]

Arbuscular mycorrhizal fungi (AMF) have a key role in plant phosphate (Pi) uptake by their efficient capture of soil phosphorus (P) that is transferred to the plant via Pi transporters in the root cortical cells. The activity of this mycorrhizal Pi uptake pathway is often associated with downregulation of Pi transporter genes in the direct Pi uptake pathway. As the total Pi taken up by the plant is determined by the combined activity of mycorrhizal and direct pathways, it is important to understand the interplay between these, in particular the actual activity of the pathways. To study this interplay we modulated the delivery of Pi via the mycorrhizal pathway in Pisum sativum by two means: (1) Partial downregulation by virus‐induced gene silencing of PsPT4, a putative Pi transporter gene in the mycorrhizal pathway. This resulted in decreased fungal development in roots and soil and led to reduced plant Pi uptake. (2) Changing the percentage of AMF‐colonized root length by using non‐, half‐mycorrhizal or full‐mycorrhizal split‐root systems. The combination of split roots, use of 32P and 33P isotopes and partial silencing of PsPT4 enabled us to show that the expression of PsPT1, a putative Pi transporter gene in the direct pathway, was negatively correlated with increasing mycorrhizal uptake capacity of the plant, both locally and systemically. However, transcript changes in PsPT1 were not translated into corresponding, systemic changes in actual direct Pi uptake. Our results suggest that AMF have a limited long‐distance impact on the direct pathway. [ABSTRACT FROM AUTHOR]

Information about topics discussed at the 7th International Conference on Mycorrhiza (ICOM 7) held in New Delhi, India from January 6-11, 2013 is presented. The conference highlighted the importance of addressing scientific findings within an applied context to improve sustainable agriculture and strengthen field implementations. Topics include the evolution of the mycorrhizal symbiosis, the importance of biotic and abiotic factors, and applied aspects of mycorrhizal research.

Glomalin is a glycoprotein produced by arbuscular mycorrhizal (AM) fungi, and the soil fraction containing glomalin is correlated with soil aggregation. Thus, factors potentially influencing glomalin production could be of relevance for this ecosystem process and for understanding AM fungal physiology. Previous work indicated that glomalin production in AM fungi may be a stress response, or related to suboptimal mycelium growth. We show here that environmental stress can enhance glomalin production in the mycelium of the AM fungus Glomus intraradices. We applied NaCl and glycerol in different intensities to the medium in which the fungus was grown in vitro, causing salinity stress and osmotic stress, respectively. As a third stress type, we simulated grazing on the extraradical hyphae of the fungus by mechanically injuring the mycelium by clipping. NaCl caused a strong increase, while the clipping treatment led to a marginally significant increase in glomalin production. Even though salinity stress includes osmotic stress, we found substantially different responses in glomalin production due to the NaCl and the glycerol treatment, as glycerol addition did not cause any response. Thus, our results indicate that glomalin is involved in inducible stress responses in AM fungi for salinity, and possibly grazing stress. [ABSTRACT FROM AUTHOR]

Abstract: Arbuscular mycorrhizal (AM) fungi are root symbionts that enhance plant growth and improve soil fertility and soil structure in drylands. Even though AM fungi are obligate biotrophs, organic matter (OM) can stimulate their growth, but the mechanisms behind this are still unknown. Here, we compared the effect of nutrient patches of different OM sources to intrinsic components of OM such as inorganic nutrient supply and an improved soil water-holding capacity (WHC; via application of hydrophilic polymers), on AM fungal growth. Fatty acids extracted from in-growth mesh bags incubated in the field were used as biomarkers for AM fungi and other soil microbes. We found an enhancement of AM fungal growth in certain nutrient patches. Two out of three OM types stimulated AM fungal growth strongly, and also the addition of inorganic nutrients enhanced AM fungi, though to a lesser extent than OM. Enhanced soil WHC, on the other hand, did not influence AM fungal growth. AMF were more strongly enhanced by the mineral nutrients relative to other soil organisms. Intrinsic nutrients might be an important factor for AMF growth stimulation in OM additions, but there was no evidence that nutrients alone can explain this phenomenon. [Copyright &y& Elsevier]

Abstract: We investigated element accumulation in vesicles of the arbuscular mycorrhizal (AM) fungus Glomus intraradices, extracted from the roots of inoculated leek plants. The elemental composition (elements heavier than Mg) was quantified using particle-induced X-ray emission (PIXE), in combination with scanning transmission ion microscopy (STIM). In vesicles, P was the most abundant of the elements analysed, followed by Ca, S, Si and K. We analysed 12 vesicles from two root systems and found that the variation between vesicles was particularly high for P and Si. The P content related positively to Si, Zn and K, while its relation to Cl fitted to a negative power function. Vesicle transects showed that P and K were present in central parts, while Ca was present mainly near the vesicle surfaces. The results showed that P is an important part (0.5% of the dry weight) of the vesicle content and that the distribution of some elements, within mycelia, may be strongly correlated. [Copyright &y& Elsevier]

The exchange of carbohydrates and mineral nutrients in the arbuscular mycorrhizal (AM) symbiosis must be controlled by both partners in order to sustain an evolutionarily stable mutualism. Plants downregulate their carbon (C) flow to the fungus when nutrient levels are sufficient, while the mechanism controlling fungal nutrient transfer is unknown. Here, we show that the fungus accumulates nutrients when connected to a host that is of less benefit to the fungus, indicating a potential of the fungus to control the transfer of nutrients. We used a monoxenic in vitro model of root organ cultures associated with Glomus intraradices, in which we manipulated the C availability to the plant. We found that G. intraradices accumulated up to seven times more nutrients in its spores, and up to nine times more in its hyphae, when the C pool available to the associated roots was halved. The strongest effect was found for phosphorus (P), considered to be the most important nutrient in the AM symbiosis. Other elements such as potassium and chorine were also accumulated, but to a lesser extent, while no accumulation of iron or manganese was found. Our results suggest a functional linkage between C and P exchange. [ABSTRACT FROM AUTHOR]

We investigated element accumulation in the arbuscular mycorrhizal fungus Glomus intraradices. Fungal spores and mycelia growing in monoxenic cultures were analyzed. The elemental composition was quantified using particle-induced X-ray emission (PIXE) in combination with scanning transmission ion microscopy. In the spores, Ca and Fe were associated mainly with the spore wall, while P and K showed patchy distributions and their concentrations were correlated. Excess of P in the hyphal growth medium increased the P and Si concentrations in spores and increased the K/Ca ratio in spores. Increased P availability decreased the concentration of Zn and Mn in spores. We concluded that the availability of P influences the uptake and accumulation of several elements in spores. It is demonstrated that PIXE analysis is a powerful tool for quantitative analysis of elemental accumulation in fungal mycelia. [ABSTRACT FROM AUTHOR]

ABSTRACT Established ecological theory has focused on unitary organisms, and thus its concepts have matured into a form that often hinders rather than facilitates the ecological study of modular organisms. Here, we use the example of filamentous fungi to develop concepts that enable integration of non‐unitary (modular) organisms into the established community ecology theory, with particular focus on its spatial aspects. In doing so, we provide a link between fungal community ecology and modern coexistence theory (MCT). We first show how community processes and predictions made by MCT can be used to define meaningful scales in fungal ecology. This leads to the novel concept of the unit of community interactions (UCI), a promising conceptual tool for applying MCT to communities of modular organisms with indeterminate clonal growth and hierarchical individuality. We outline plausible coexistence mechanisms structuring fungal communities, and show at what spatial scales and in what habitats they are most likely to act. We end by describing challenges and opportunities for empirical and theoretical research in fungal competitive coexistence. [ABSTRACT FROM AUTHOR]

A letter to the editor is presented related to the fungi arbuscular mycorrhizal.

Hedgerows structure agricultural landscape worldwide but little is known on their soil communities. In the intensively managed agricultural landscapes of Ireland and the UK, hedgerows protect soil from degradation and are thought to provide a reservoir of soil biodiversity for the farmed fields. But hedgerows are currently threatened by emergent tree diseases spread with rising volumes of trade in plants. This study analysed the effect of the most dominant hedgerow tree species on the composition and diversity of soil communities, which we estimated with oribatid mites in the context of an imminent ash dieback outbreak in Northern Ireland. We hypothesised that the environmental heterogeneity created by different tree species and good tree health are beneficial to the abundance and diversity of these soil communities. We found that communities differ, both in terms of species composition and relative abundances, between different trees in the trimmed hedgerows, and between untrimmed and trimmed hedgerows. The presence of trees was associated with more variable species composition. The analysis of phospholipid fatty acids (PLFA) showed that pasture and adjacent hedgerows were dominated by bacteria but there was no clear relationship between bacterial biomass and oribatid abundance or richness. However, changes in bacterial PLFAs were correlated positively with changes in oribatid community composition. Soil under ash trees which were diseased supported a higher species richness and greater abundance of oribatids than healthy trees. We conclude that the presence of different tree species in hedgerows adds to the overall diversity of soil hedgerow communities and hypothesise that the structural heterogeneity created by tree canopy drives increased diversity. Ill health in ash trees is currently positively affecting oribatid communities, possibly due to the temporarily increase in organic matter in soil. The epidemic of ash dieback, however, is expected to kill and remove trees in the medium term and so reduce soil community diversity in the long term. • Hedgerows host significant soil oribatid mite biodiversity. • Trimmed hedgerows and tree standards have distinct species composition. • The presence of trees in trimmed hedgerows increases biodiversity. • The positive short term effects of diseased tree may reverse in the long term. [ABSTRACT FROM AUTHOR]

The great variety of earth's microorganisms and their functions are attributed to the heterogeneity of their habitats, but our understanding of the impact of this heterogeneity on microbes is limited at the microscale. In this study, we tested how a gradient of spatial habitat complexity in the form of fractal mazes influenced the growth, substrate degradation, and interactions of the bacterial strain Pseudomonas putida and the fungal strain Coprinopsis cinerea. These strains responded in opposite ways: complex habitats strongly reduced fungal growth but, in contrast, increased the abundance of bacteria. Fungal hyphae did not reach far into the mazes and forced bacteria to grow in deeper regions. Bacterial substrate degradation strongly increased with habitat complexity, even more than bacterial biomass, up to an optimal depth, while the most remote parts of the mazes showed both decreased biomass and substrate degradation. These results suggest an increase in enzymatic activity in confined spaces, where areas may experience enhanced microbial activity and resource use efficiency. Very remote spaces showing a slower turnover of substrates illustrate a mechanism which may contribute to the long-term storage of organic matter in soils. We demonstrate here that the sole effect of spatial microstructures affects microbial growth and substrate degradation, leading to differences in local microscale spatial availability. These differences might add up to considerable changes in nutrient cycling at the macroscale, such as contributing to soil organic carbon storage. • Bacteria and fungi respond in opposite ways to microscale physical complexity • Fungal growth decreases in complex microhabitats • Bacterial growth and nutrient degradation increase in complex habitats • Nutrients in the deepest regions of the mazes are less subject to degradation Arellano-Caicedo et al. show that bacteria and fungi react differently to complex microenvironments. Fungal growth decreases, whereas bacterial growth and nutrient degradation increase, in response to habitat complexity. These findings show the importance of considering habitat microstructure when studying microbial communities in nature. [ABSTRACT FROM AUTHOR]