Competition and substrate colonization strategies of three polyxenically grown arbuscular mycorrhizal fungi

Intra- and extraradical colonization competition and hyphal interactions among arbuscular mycorrhizal fungi (AMF) Glomus intraradices, Glomus proliferum and Gigaspora margarita were investigated in two in vitro experimental systems. AMF were polyxenically cultured with a Ri T-DNA transformed carrot root organ culture (ROC) in either big Petri plates containing three culture compartments and a common hyphal compartment (i.e. an independent host root for each AMF) or two by two in the culture compartment of regular bicompartmented Petri dishes (i.e. a common host root and a common hyphal compartment). Maps of the extraradical mycelial development of the three AMF were obtained. Two distinct substrate colonization strategies (Glomus-type and Gigaspora-type) were identified, reflecting intrinsic differences among AMF genera/families. Our data reveal a general lack of antagonism between the isolates when extraradical hyphae explore and exploit the substrate outside the root influence zone; however certain growth restrictions were imposed by Gi. margarita extraradical mycelium when developing near the host root and by G. proliferum intraradical hyphae. This work highlights once more the appropriateness of AM in vitro culture systems to perform in vivo studies on the biology of this symbiosis and opens new avenues to the formulation of in vitro AMF inoculants.     

Ecological roles of arbuscular mycorrhizal fungi in two wild legume plants

Two wild legume plants,Glycine soja andCassia mimosoides var.nomame, and a cultivated plant, soybean (Glycine max), were employed for a study of triple symbiosis with an inoculum ofScutellispora heterogama harvested from natural soils and an inoculum of their own rhizobial cells. The dry weight, colonization of arbuscular mycorrhizal fungus, nodule formation and N₂-fixation activity were estimated as the parameters of triple symbiosis. The two wild legume plants showed greater growth with colonization of arbuscular mycorrhizae than with nodulation, whereas the cultivated legume showed more nodulation than colonization of arbuscular mycorrhizae. Moreover,S. heterogama appeared to stimulate the triple symbiosis for the wild legume plants. The results suggested that spores ofS. heterogama are important in disturbed soils in Korea.     

Biodiversity of arbuscular mycorrhizal fungi in roots and soils of two salt marshes

The occurrence of arbuscular mycorrhizal fungi (AMF) was assessed by both morphological and molecular criteria in two salt marshes: (i) a NaCl site of the island Terschelling, Atlantic Coast, the Netherlands and (ii) a K₂CO₃ marsh at Schreyahn, Northern Germany. The overall biodiversity of AMF, based on sequence analysis, was comparably low in roots at both sites. However, the morphological spore analyses from soil samples of both sites exhibited a higher AMF biodiversity. Glomus geosporum was the only fungus of the Glomerales that was detected both as spores in soil samples and in roots of the AMF-colonized salt plants Aster tripolium and Puccinellia sp. at both saline sites and on all sampling dates (one exception). In roots, sequences of Glomus intraradices prevailed, but this fungus could not be identified unambiguously from DNA of soil spores. Likewise, Glomus sp. uncultured, only deposited as sequence in the database, was widely detected by DNA sequencing in root samples. All attempts to obtain the corresponding sequences from spores isolated from soil samples failed consistently. A small sized Archaeospora sp. was detected, either/or by morphological and molecular analyses, in roots or soil spores, in dead AMF spores or orobatid mites. The study noted inconsistencies between morphological characterization and identification by DNA sequencing of the 5.8S rDNA-ITS2 region or part of the 18S rDNA gene. The distribution of AMF unlikely followed the salt gradient at both sites, in contrast to the zone formation of plant species. Zygotes of the alga Vaucheria erythrospora (Xanthophyceae) were retrieved and should not be misidentified with AMF spores.     

长期定位施肥土壤中AM真菌对寄主植物的侵染状况

分别于2003年9月和2004年5月在莱阳农学院长期(26年)定位施肥试验田采集玉米和小麦根系,以测定不同施肥处理对丛枝菌根(AM)真菌侵染状况的影响。结果表明长期定位施氮(N)肥显著降低了AM真菌对寄主植物的侵染率(MCP)、丛枝着生率(ACP)、单位根长泡囊数(NV)和侵入点数(NE)。高N处理的小麦MCP低于低N处理,高N处理的玉米ACP低于低N处理。长期定位施有机肥,尤其是高有机肥处理显著降低了MCP、ACP和NE。有机肥和N肥配施也降低了MCP、ACP、NV和NE,以高有机肥和高N肥配施处理的降低效应最大。除小麦根系NV外,长期定位NPK配合施用降低了玉米和小麦的MCP、ACP、NV和NE。其中,以NP处理的玉米ACP、小麦的MCP和ACP最低。另外,低有机肥与N肥配施和NPK配施处理条件下玉米MCP高于小麦。结论认为不同施肥体制对作物菌根生长发育及其结构具有不同影响,而且有机肥和N肥在影响AM真菌侵染方面存在互作。     

黄顶菊与3种本地植物竞争对丛枝菌根真菌种类和多样性的影响

【目的】外来植物黄顶菊对生态环境和农业经济造成了严重危害,了解黄顶菊与3种不同本地植物种植生长对丛生菌根(AM)真菌群落结构和多样性造成的影响,可以从土壤微生物角度进一步解释黄顶菊的入侵机制。【方法】通过同质园小区试验模拟黄顶菊入侵的生态进程,以黄顶菊和3种本地植物狗尾草、藜、黄香草木樨为研究对象,采用AM真菌的形态学鉴定方法,研究黄顶菊与3种本地植物不同种植方式对AM真菌群落结构和多样性的影响。【结果】(1)黄顶菊根际土壤聚集的AM真菌种类与其伴生本地植物种类有关:黄顶菊与狗尾草混种处理中优势种为网状球囊霉和根内球囊霉,而黄顶菊分别与藜、草木樨混种处理中优势种均为网状球囊霉、根内球囊霉和缩球囊霉;(2)黄顶菊分别与狗尾草和黄香草木樨混种处理中AM真菌种类既高于本地单种处理,也高于黄顶菊单种处理,说明随着黄顶菊的入侵和地上植物多样性的改变,AM真菌种类也发生改变;(3)与3种本地植物单种相比,黄顶菊各混种处理和黄顶菊单种处理中黄顶菊根际土壤根内球囊霉的重要值均增加,表明黄顶菊入侵有利于根内球囊霉的生长和发育。【结论】黄顶菊入侵改变了根际土壤AM真菌的群落结构和多样性,AM真菌的改变既与本地植物种类有关,也与入侵程度有关。     

Survival strategies of arbuscular mycorrhizal fungi in Cu-polluted environments

This review provides an overview of the mechanisms evolved by arbuscular mycorrhizal (AM) fungi to survive in Cu-contaminated environments. These mechanisms include avoidance strategies to restrict entry of toxic levels of Cu into their cytoplasm, intracellular complexation of the metal in the cytosol and compartmentalization strategies. Through the activity of specific metal transporters, the excess of Cu is translocated to subcellular compartments, mainly vacuoles, where it would cause less damage. At the level of the fungal colony, AM fungi have also evolved compartmentalization strategies based on the accumulation of Cu into specific fungal structures, such as extraradical spores and intraradical vesicles. In addition to the avoidance and compartmentalization strategies, AM fungi have also mechanisms to combat the Cu-generated oxidative stress or to repair the damage induced.     

Community assembly, species richness and nestedness of arbuscular mycorrhizal fungi in agricultural soils

Understanding how communities assemble is a central goal of ecology. This is particularly relevant for communities of arbuscular mycorrhizal fungi (AMF), because the community composition of these beneficial plant symbionts influences important ecosystem processes. Moreover, AMF may be used as sensitive indicators of ecological soil quality if they respond to environmental variation in a predictable way. Here, we use a molecular profiling technique (T-RFLP of 25S rRNA gene fragments) to test which factors determine AM fungal community composition in 40 agricultural soils in the Netherlands. In particular, we test whether species richness, dominance structure and community nestedness are influenced by management type (in pairs of organically and conventionally farmed fields), and we examine the contribution of crop species (maize vs. potato), soil type (sand vs. clay-textured soils) and habitat (plant root vs. bulk soil) on AMF community characteristics. AMF richness varied from 1 to 11 taxa per field. Communities from species-poor fields were found to be subsets of those in richer fields, indicating nestedness and a progressive 'loss' from the species pool. AMF taxa richness and occurrence in soil and plant roots were highly correlated, and richness was related to management intensity (phosphate availability and grass-cropping history together explained 32% and 50% of richness in roots and soils). Soil type together with soil chemical parameters explained only 17% of variance in AMF community structure. We synthesize these results by discussing the potential contribution of a 'bottleneck effect' on AMF communities through increased stochastic effects under environmental stress.     

丛枝菌根真菌(Glomus intraradices)对铜污染土壤上玉米生长的影响

盆栽试验研究了不同土壤施Cu水平(0、50、200 mg/kg)下,接种不同来源的两个丛枝菌根真菌Glomus intraradices菌株对玉米生长、Cu、P以及微量元素Fe、Mn、Zn吸收的影响.结果表明:接种菌根真菌显著提高了玉米的生物量,增加了玉米植株P浓度和吸收量;随着施Cu水平提高,各处理根系Cu浓度显著增加.各施Cu水平下玉米根系Cu浓度远远高于地上部分Cu浓度,同一施Cu水平下接种处理根系Cu浓度要显著高于对照;尤其在200 mg/kg施Cu水平下,接种处理根系Cu浓度大约是地上部分的45～58倍,对照根系Cu浓度大约是地上部分的12倍.总体上,试验条件下两个菌株对玉米的接种效应没有明显差异.试验表明丛枝菌根对重金属Cu有较强的固持作用,这可能是菌根减轻宿主植物Cu毒害的一个重要机制.     

丛枝菌根真菌物种多样性研究进展

丛枝菌根真菌(Arbuscular mycorrhizal fungi,AMF)在不同生态系统均发挥至关重要的作用,研究其多样性能够为AMF物种资源的保护和利用提供科学依据。AMF不能被离体纯培养以及自身的高变异性等因素严重阻碍了对其进行深入研究,随着研究方法的不断改进,尤其是新一代测序技术的运用,极大加速了人们对AMF物种多样性的认识。本文主要从AMF分类系统、不同宿主植物和不同生境中的AMF物种多样性及AMF物种多样性研究方法(包括形态鉴定、Sanger测序和高通量测序)方面介绍AMF物种多样性研究进展,并且探讨AMF物种多样性研究中存在的主要问题,认为在今后AMF物种多样性研究中不仅要注重运用新的研究手段,还应该着重解决AMF不能离体纯培养的问题。     

Molecular biodiversity of arbuscular mycorrhizal fungi in trace metal-polluted soils

We assessed the indigenous arbuscular mycorrhizal fungi (AMF) community structure from the roots and associated soil of Plantago major (plantain) plants growing on sites polluted with trace metals (TM) and on unpolluted sites. Uncontaminated and TM-contaminated sites containing As, Cd, Cu, Pb, Sn and Zn were selected based on a survey of the TM concentration in soils of community gardens in the City of Montréal. Total genomic DNA was extracted directly from these samples. PCR followed by denaturing gradient gel electrophoresis (PCR-DGGE), augmented by cloning and sequencing, as well as direct sequencing techniques, was all used to investigate AMF community structure. We found a decreased diversity of native AMF (assessed by the number of AMF ribotypes) in soils and plant roots harvested from TM-polluted soils compared with unpolluted soils. We also found that community structure was modified by TM contamination. Various species of Glomus, Scutellospora aurigloba and S. calospora were the most abundant ribotypes detected in unpolluted soil; ribotypes of G. etunicatum, G. irregulare/G. intraradices and G. viscosum were found in both polluted and unpolluted soils, while ribotypes of G. mosseae and Glomus spp. (B9 and B13) were dominant in TM-polluted soils. The predominance of G. mosseae in metal-polluted sites suggests the tolerance of this species to TM stress, as well as its potential use for phytoremediation. These data are relevant for our understanding of how AMF microbial communities respond to natural environments that contain a broad variety of toxic inorganic compounds and will substantially expand our knowledge of AMF ecology and biodiversity.     

丛枝菌根真菌物种多样性研究进展

丛枝菌根(arbuscular mycorrhiza, AM)真菌是生态系统中生物多样性的重要组分之一,具有十分丰富的物种多样性、遗传多样性和功能多样性.该真菌分类地位不断提高已上升至门,下设1个纲、4个目、13个科,19个属,现已报道214种.丛枝菌根对保持生态平衡、稳定和提高生态系统可持续生产力具有重要作用.本文分析了世界范围内丛枝菌根真菌物种多样性研究现状、不同生态系统中影响丛枝菌根真菌物种多样性的关键因子及其调控途径;认为分子生物学技术是今后丛枝菌根真菌物种多样性研究的主要方法.     

Dispersal of arbuscular mycorrhizal fungi and plants during succession

Arbuscular mycorrhizal (AM) fungi are important root symbionts that enhance plant nutrient uptake and tolerance to pathogens and drought. While the role of plant dispersal in shaping successional vegetation is well studied, there is very little information about the dispersal abilities of AM fungi. We conducted a trap-box experiment in a recently abandoned quarry at 10 different distances from the quarry edge (i.e. the potential propagule source) over eleven months to assess the short term, within-year, arrival of plant and AM fungal assemblages and hence their dispersal abilities. Using DNA based techniques we identified AM fungal taxa and analyzed their phylogenetic diversity. Plant diversity was determined by transporting trap soil to a greenhouse and identifying emerging seedlings. We recorded 30 AM fungal taxa. These contained a high proportion of ruderal AM fungi (30% of taxa, 79% of sequences) but the richness and abundance of AM fungi were not related to the distance from the presumed propagule source. The number of sequences of AM fungi decreased over time. Twenty seven plant species (30% of them ruderal) were recorded from the soil seed traps. Plant diversity decreased with distance from the propagule source and increased over time. Our data show that AM fungi with ruderal traits can be fast colonizers of early successional habitats.     

Differential growth response to arbuscular mycorrhizal fungi and plant density in two wild plants belonging to contrasting functional types

The effect of arbuscular mycorrhizal fungi (AMF) on plant growth was examined in two wild plant species belonging to contrasting functional types: an annual forb (Bidens pilosa, Asteraceae) and a deciduous shrub (Acacia caven, Fabaceae) at three contrasting plant densities (one, two, and three individuals per pot). AMF had a slightly negative effect on B. pilosa when the species grew in isolation while they positively affected A. caven. Positive effects of AMF on shoot mass of A. caven decreased at higher plant densities, while shoot mass of individuals of B. pilosa showed less marked differences between plant densities. When considering total biomass per pot, AMF positively affected A. caven growth while negatively affecting B. pilosa, at all three plant densities. Root/shoot ratio per pot was negatively affected by AMF but not plant density in both species. These findings highlight the importance of including plants belonging to different life forms and/or traits in research regarding the interaction between AMF and intraspecific plant competition.     

Presence of arbuscular mycorrhizal fungi in South Florida native plants

The roots of 27 species of South Florida plants in 15 families (including one cycad, six palms, one Smilax, and 19 dicotyledons) native to pine rockland and tropical hardwood hammock communities were examined for arbuscular mycorrhizal fungi (AMF). These plants grow in the biologically diverse but endangered Greater Everglades habitat. Roots from field-grown and potted plants were cleared and stained. All 27 species had AMF and include 14 species having an endangered or threatened status. The Paris-type colonization occurred in two species in the families Annonaceae and Smilacaceae. The Arum-type occurred in 22 species in the families Anacardiaceae, Arecaceae (Palmae), Boraginaceae, Cactaceae (questionable), Euphorbiaceae, Fabaceae, Lauraceae, Melastomataceae, Polygalaceae, Rubiaceae, Simaroubaceae, Ulmaceae, and Zamiaceae. Three species in the families Fabaceae, Lauraceae, and Simaroubaceae had a mix of Paris- and Arum-types. The results have implications for the restoration of these endangered plant communities in the Everglades.     

Four new species of arbuscular mycorrhizal fungi (Glomeromycota) associated with endemic plants from ultramafic soils of New Caledonia

Four new species of arbuscular mycorrhizal (AM) fungi (Glomeromycota) were isolated from the rhizosphere of endemic metallophytic plants in ultramafic soils in New Caledonia (South Pacific) and propagated on Sorghum vulgare. Acaulospora saccata and A. fragilissima are placed in the Acaulosporaceae, Scutellospora ovalis in the Gigasporaceae, and Rhizophagus neocaledonicus in the Glomeraceae. The novelty of these species is supported by morphological characters of spores and phylogenetic analyses of sequences of the rDNA region, comprising partial small subunit rRNA gene, the internal transcribed spacers, 5.8S rRNA gene, and the partial large subunit rRNA gene. New Caledonia is known for its high degree of endemism in plants, which is due to its geographic position and geological history. This is the first taxonomic study exploring local Glomeromycota of this island, which may help to address the question of possible AMF endemism in future studies.     

Calcium opens the dialogue between plants and arbuscular mycorrhizal fungi

Calcium ion is considered a ubiquitous second messenger in all eukaryotic cells. Analysis of intracellular Ca²⁺ concentration dynamics has demonstrated its signalling role in plant cells in response to a wide array of environmental cues. The implication of Ca²⁺ in the early steps of the arbuscular mycorrhizal symbiosis has been frequently claimed, mainly by analogy with what firmly demonstrated in the rhizobium-legume symbiosis. We recently documented transient Ca²⁺ changes in plant cells challenged with diffusible molecules released by arbuscular mycorrhizal fungi. Ca²⁺ measurements by the recombinant aequorin method provided new insights into the molecular communications between plants and these beneficial fungi.Key words: legume symbioses, arbuscular mycorrhiza, calcium signalling, fungal signal, plant cell cultures, aequorinIn the rhizosphere plants meet a wide array of microorganisms. In favorable interactions, such as arbuscular mycorrhizal (AM) and nitrogen fixing symbioses, a dialogue is progressively established between the two interacting organisms to make the appropriate partner choice. These two-way communications rely on the interchange of signals released by both potential symbionts. After perception of the signalling molecules, a signal transduction pathway is induced, leading to the activation of the proper genetic and developmental program in both partners.Variations in intracellular free Ca²⁺ concentration occur as one of the initial steps in signalling pathways activated in plants when they encounter pathogens,¹ fungal biocontrol agents² and nitrogen-fixing bacteria.³ Molecules secreted by microorganisms, after binding to specific receptors, trigger in plant cells transient changes in cytosolic Ca²⁺ level, due to the influx of the ion from the extracellular environment and/or the release from internal Ca²⁺ storage compartments.^4,5 Ca²⁺ messages delivered to plant cells are at least partly deciphered on the basis of their spatial and temporal features. The occurrence of different Ca²⁺ signatures guarantees the specificity of the ensuing physiological responses.In the legume-rhizobium symbiosis a definite pattern of Ca²⁺ oscillations has been reported to occur in response to the rhizobial signalling molecule, the Nod factor, in the nucleus and perinuclear cytoplasm of the root hair.⁶ The Ca²⁺ spike number has been recently demonstrated to regulate nodulation gene expression.⁷Legumes are able to engage in a dual symbiotic interaction, with rhizobia and AM fungi. Components of the Ca²⁺-mediated signalling pathway are shared by the two symbioses.⁸ In the mycorrhizal signal transduction pathway the involvement of Ca²⁺ has long been speculated, based on the observed similarities with symbiotic nitrogen fixation.³To evaluate the possible participation of Ca²⁺ in the early steps of the AM symbiosis, we have used a simplified experimental system given by plant cell suspension cultures stably expressing the bioluminescent Ca²⁺-sensitive reporter aequorin.⁹ The use of cultured cells circumvents the problem posed by multilayered organs: in aequorin-transformed seedlings, possible Ca²⁺ changes occurring in rhizodermal cells—the first place where the AM fungal signals are perceived and transduced—can be misrecorded due to luminescence calibration over all root cell layers, resulting in an underestimation of the Ca²⁺ signal in the responsive cells. An experimental design based on challenging host plant cells with the culture medium of different AM fungi (Gigaspora margarita, Glomus mosseae and intraradices) provided the first firm evidence that Ca²⁺ is involved as intracellular messenger during mycorrhizal signalling, at least in a pre-contact stage. Cytosolic Ca²⁺ changes, characterized by specific kinetic parameters, were triggered by diffusates obtained from AM resting and germinating spores,⁹ and extraradical mycelium.¹⁰ Cultured plant cells demonstrated to be competent to perceive the diffusible signal released by AM fungi and to decode the message in a Ca²⁺-dependent pathway. Based on these experiments, it seems that AM fungi announce their presence to the plant through the constitutive release of a chemical signal, even before experiencing the proximity of the plant or its AM symbiotic signals. The notion that the secreted fungal molecules herald, through Ca²⁺, a beneficial message which can be acknowledged only by competent receivers, is supported by: (1) the lack of defense response induction and the upregulation of some genes essential for the AM symbiosis initiation in host plant cells; (2) the unresponsiveness of cultured cells from the nonhost plant Arabidopsis thaliana.Ca²⁺-mediated perception of both AM fungal and rhizobial signals by plant cells unifies the signalling pathways activated in the two symbioses. However, the actual occurrence of Ca²⁺ spiking in AM symbiosis remains to be ascertained, due to limitations of the recombinant aequorin method, when applied to an asynchronous cell population. Contribution of internal Ca²⁺ stores, in particular the nucleus, to the observed Ca²⁺ changes will be a future research goal to be achieved through a pharmacological approach and/or targeting of Ca²⁺ indicators to intracellular compartments.The identification of the plant-derived mycorrhizal signal as strigolactones¹¹ and their inducing activity on AM fungi¹² have represented a major breakthrough in the AM symbiosis research field. Elucidation of the chemical nature of the AM fungal factor, which plays several effects on host plants,^9,13–15 is eagerly awaited.Understanding how AM fungi and rhizobia select compatible plant hosts, thus activating the appropriate symbiotic program, is another facet to be considered in the future to get a complete overview of early signaling events in legume symbioses. Analysis of Ca²⁺ signalling implication in the microbial partner would require the delivery of reliable and sensitive Ca²⁺ probes (such as aequorinor GFP-based¹⁶) for Ca²⁺ measurements in living microorganisms. The recombinant aequorin method has been successfully applied to monitor dynamic changes in intracellular Ca²⁺ levels in the bacteria Anabaena sp.,¹⁷ E. coli,¹⁸ and recently by us in rhizobial strains.¹⁹ Unfortunately, AM fungi have proved not to be amenable to stable transformation, being coenocytic, multinucleate and heterokaryotic,^20,21 and only transient transformants have been obtained so far.^22,23 Further development of the transformation technologies may provide in the future a valuable tool to analyse, from the fungal side, signal perception and transduction during arbuscular mycorrhiza establishment.     

Influence of two legume species on hyphal production and activity of two arbuscular mycorrhizal fungi

 Two arbuscular mycorrhizal (AM) fungi (Glomus mosseae and G. intraradices) were compared for abundance of intraradical and soil-borne hyphae in association with Astragalus sinicum, a small-seeded, and Glycine max, a large-seeded legume. A. sinicum was more responsive than G. max to mycorrhizal formation, especially at early growth stages. Biomass allocation was greater in roots than shoots for mycorrhizal A. sinicum, while the opposite was true for G. max. Hyphal development in root and soil compartments was estimated by trypan blue staining and after staining for succinate dehydrogenase (SDH) or alkaline phosphatase (ALP) activity. Total fungal abundance increased steadily in roots and soil with time to a maximum 8 weeks after planting. SDH- and ALP-active AM hyphae increased in roots during plant growth but decreased in soil at later harvests. Mycorrhizal root mass in A. sinicum and G. max increased about 14-fold and 2.5-fold, respectively, but total length of soil hyphae produced per plant differed little, so that the pattern of AM soil to root abundance of the two fungi varied considerably with the host plant. Accepted: 23 July 1997     

我国北方农田土壤中AM真菌的多样性

AM真菌是农业生态系中一类重要的土壤微生物,它在农田土壤中的发生和分布受多种环境因素的影响。为深入了解我国北方农田土壤中AM真菌的多样性规律,于2000年在河北、山东的农田土壤中采集有代表性的土样127个。通过进一步扩繁、纯化,从中分离出AM真菌5属22种,鉴定了20个种,包括一个国内新记录种沾屑球囊霉(Glomus spurcum)。分析AM真菌的多样性特点及其影响因素发现,农田土壤中以球囊霉属(Glomus)的频度最高,其次为无梗囊霉属(Acaulospora);优势种类为幼套球囊霉(Glomus etunicatum)和摩西球囊霉(Glomus mosseae).土壤速效磷含量、pH状况主要对孢子密度产生影响,对种群分布影响不大。宿主植物类型对AM真菌的侵染状况和多样性影响较大;比较玉米(Zea mays)、甘薯(Ipomoea batatas)根区AM真菌的种群组成后发现两者有所不同,但优势种一致.     

Establishment and effectiveness of inoculated arbuscular mycorrhizal fungi in agricultural soils

Arbuscular mycorrhizal fungi (AMF) are promoted as biofertilizers for sustainable agriculture. So far, most researchers have investigated the effects of AMF on plant growth under highly controlled conditions with sterilized soil, soil substrates or soils with low available P or low inoculum potential. However, it is still poorly documented whether inoculated AMF can successfully establish in field soils with native AMF communities and enhance plant growth. We inoculated grassland microcosms planted with a grass–clover mixture (Lolium multiflorum and Trifolium pratense) with the arbuscular mycorrhizal fungus Rhizoglomus irregulare. The microcosms were filled with eight different unsterilized field soils that varied greatly in soil type and chemical characteristics and indigenous AMF communities. We tested whether inoculation with AMF enhanced plant biomass and R. irregulare abundance using a species specific qPCR. Inoculation increased the abundance of R. irregulare in all soils, irrespective of soil P availability, the initial abundance of R. irregulare or the abundance of native AM fungal communities. AMF inoculation had no effect on the grass but significantly enhanced clover yield in five out of eight field soils. The results demonstrate that AMF inoculation can be successful, even when soil P availability is high and native AMF communities are abundant.