Abundance of arbuscular mycorrhizal fungi in relation to soil salinity around Lake Urmia in northern Iran analyzed by use of lipid biomarkers and microscopy

Saline soils around Lake Urmia in northern Iran constitute a stressed environment for plants and microbial communities, including arbuscular mycorrhizal (AM) fungi. Soil and root samples were collected from fields cultivated with the glycophytes Allium cepa L. and Medicago sativa L., and sites dominated by the halophyte Salicornia europaea L. Soil and root samples were analyzed for the AM fungal signature neutral lipid fatty acid (NLFA) 16:1ω5. The roots were also examined microscopically for mycorrhizal colonization. Each plant species was sampled across a salt gradient. Microscopic examination showed no AM fungal structures in the roots of S. europaea. The highest root colonization was recorded for M. sativa. The highest NLFA 16:1ω5 values were found in soil around M. sativa roots and the lowest in soil around S. europaea roots. We found evidence for stimulation of vesicle formation at moderate salinity levels in M. sativa, which is an indication of increased carbon allocation to mycorrhiza. On the other hand, we found a negative correlation between salinity and arbuscule formation in A. cepa, which may indicate a less functional symbiosis in saline soils.     

Effects of two grass species on the composition of soil fungal communities

Many studies have shown effects of plants species on fungal communities, but these are often confounded with soil effects. Thus, the specific role of plant species in structuring rhizospheric and soil fungal communities is poorly described. Our study used microcosms in which plants were grown under artificial conditions to bridge this gap. Two perennial grasses dominating subalpine grasslands, Festuca paniculata and Dactylis glomerata, were grown at two levels of fertilization on standard soil. Fungal communities were determined by 454 pyrosequencing of the internal transcribed spacer 1 region. Among the fungal communities characterized by the primers used, original communities were associated to each plant species and also diverged between rhizosphere and bulk soils within each plant species, though there were no significant fertilization effects. Differences regarded global composition of the fungal communities and abundant molecular operational taxonomic units (MOTUs). Both plant species and location effects were reflected more in the abundance than in the composition of MOTUs. The observed differences in fungal communities coincide with differing strategies of plant root growth, with D. glomerata having greater root mass, length, and area than F. paniculata. Our study, by dissociating soil effects from plant effects, demonstrated that plant species exert a key control on soil fungi. We suggest that such effects may be linked to inter-specific differences in root traits and their consequences on nitrogen uptake.     

Differential effect of arbuscular mycorrhizal fungal communities from ecosystems along management gradient on the growth of forest understorey plant species

Arbuscular mycorrhizal fungi (AMF) are important functional components of ecosystems. Although there is accumulating knowledge about AMF diversity in different ecosystems, the effect of forest management on diversity and functional characteristics of AMF communities has not been addressed. Here, we used soil inoculum representing three different AM fungal communities (from a young forest stand, an old forest stand and an arable field) in a greenhouse experiment to investigate their effect on the growth of three plant species with contrasting local distributions - Geum rivale, Trifolium pratense and Hypericum maculatum. AM fungal communities in plant roots were analysed using the terminal restriction fragment length polymorphism (T-RFLP) method. The effect of natural AMF communities from the old and young forest on the growth of studied plant species was similar. However, the AMF community from the contrasting arable ecosystems increased H. maculatum root and shoot biomass compared with forest inocula and T. pratense root biomass compared to sterile control. According to ordination analysis AMF inocula from old and young forest resulted in similar root AMF communities whilst plants grown with AM fungi from arable field hosted a different AMF community from those grown with old forest inocula. AMF richness in plant roots was not related to the origin of AMF inoculum. G. rivale hosted a significantly different AM fungal community to that of T. pratense and H. maculatum. We conclude that although the composition of AM fungal communities in intensively managed stands differed from that of old stands, the ecosystem can still offer the ‘symbiotic service’ necessary for the restoration of a characteristic old growth understorey plant community.     

Effects of soil application of olive mill wastewaters on the structure and function of the community of arbuscular mycorrhizal fungi

Recycling of olive mill wastewaters (OMW) into agricultural soils is a controversial issue since benefits to soil fertility should counterbalance potential short-term toxicity effects. We investigated the short-term effects of OMW on the soil-plant system, regarding the diversity, structure and root colonization capacity of arbuscular mycorrhizal (AM) fungi and the respective growth response of Vicia faba L, commonly used as green manure in olive-tree plantations. A compartmentalized pot system was used that allowed the establishment of an AM fungal community in one compartment (feeder) and the application of three OMW dose levels in an adjacent second compartment (receiver). At 0, 10, and 30 days after OMW treatment (DAT), V. faba pre-germinated seeds were seeded in the receiver compartment. At harvest, shoot and root dry weights, AM fungal root colonization, soil hyphal length and P availability were recorded in the receiver compartment. In addition, OMW effects on AM fungal diversity in plant roots were studied by DGGE. A transient effect of OMW application was observed; plant growth and AM fungal colonization were initially inhibited, whereas soil hyphal length was stimulated, but in most cases differences were absent when seeding was performed 30 DAT. Similarly, changes induced in the structure of the root AM fungal community were of transient nature. Cloning and sequencing of all the major DGGE bands showed that roots were colonized by Glomus spp. The transient effects of OMW on the structure and function of AM fungi could be attributed to OMW-derived phytoxicity to V. faba plants or to an indirect effect via alteration of soil nutritional status. The high OMW dose significantly increased soil P availability in the presence of AM fungi, suggesting efficient involvement of AM fungi in organic-P minerilization. Overall our results indicate that soil application of OMW would cause transient changes in the AM fungal colonization of V. faba plants, which, would not impair their long-term plant growth promoting ability.     

Impact of 12-year field treatments with organic and inorganic fertilizers on crop productivity and mycorrhizal community structure

The effect of manure and mineral fertilization on the arbuscular mycorrhizal (AM) fungal community structure of sunflower (Helianthus annuus L.) plants was studied. Soils were collected from a field experiment treated for 12 years with equivalent nitrogen (N) doses of inorganic N, dairy manure slurry, or without N fertilization. Fresh roots of tall fescue (Festuca arundinacea Schreb.) grass collected from the field plots without N fertilization and unfumigated field soils were used as native microbial inoculum sources. Sunflower plants were sown in pots containing these soils, and three different means of manipulating the microbial community were set: unfumigated soil with fresh grass roots, fumigated soil with fresh grass roots, or fumigated soil with sterilized grass roots. Assessing the implications with respect to plant productivity and mycorrhizal community structure was investigated. Twelve AM fungal OTUs were identified from root or soil samples as different taxa of Acaulospora, Claroideoglomus, Funneliformis, Rhizophagus, and uncultured Glomus, using PCR-DGGE and sequencing of an 18S rRNA gene fragment. Sunflower plants grown in manure-fertilized soils had a distinct AMF community structure from plants either fertilized with mineral N or unfertilized, with an abundance of Rhizophagus intraradices-like (B2). The results also showed that AM inoculation increased P and N contents in inorganic N-fertilized or unfertilized plants, but not in manure-fertilized plants.     

The influence of tillage on the structure of rhizosphere and root-associated arbuscular mycorrhizal fungal communities

Soil environmental factors affect the structure of arbuscular mycorrhizal (AM) fungal communities present in soil. However, it is not understood to which degree management practices such as tillage lead to dissimilarities between intra- and extraradical AM fungal communities. This study aims to assess the influence of two different soil management practices (conventional tillage and no-till) on the diversity of AMF communities, both in rhizosphere soil and inside corn roots. We hypothesized that under no-till, roots are colonized as they grow through the undisturbed fungal mycelia left from the previous crop whereas under conventional tillage they are colonized by those propagules that survived disturbance and can re-establish in their new relocated and mixed environment. We predicted that the degree of similarity of AM fungal communities inside versus outside the roots would be greater under no-till than under tillage. Using terminal restriction fragment length polymorphism (T-RFLP) and denaturing gradient gel electrophoresis we observed a different AM fungal community present in roots under no-till than under conventional tillage. Moreover, the communities present in the rhizosphere soil were different than in the roots of the corn plants. These results suggest that soil management does alter the diversity of AM fungal communities associated with corn roots and that plants influence the structure of the AMF community colonizing their roots. Sequencing results indicated that the majority of AMF species found in this agricultural soil was Glomus spp. However, further work is required to determine the extent to which AM fungal genotypic alterations by soil management influences competitive relationships.     

Rhizosphere fungal communities are influenced by Senecio jacobaea pyrrolizidine alkaloid content and composition

The contents and the compositions of the pyrrolizidine alkaloid (PA) complex of ragwort (Senecio jacobaea L.) were examined as potential drivers of fungal community structure in the rhizosphere. S. jacobaea plants within the coastal sand dune reserve of Meijendel (the Netherlands) were assayed for concentration and composition of PAs in roots. Rhizosphere soil was collected from pre-flowering plants, which differed up to 8-fold in PA production, and represented both jacobine and senecionine/seneciphylline chemotypes. Bulk soil samples from the same site were also collected for comparative examination. A culture-independent approach, involving direct DNA isolation, PCR of fungal 18S rRNA genes, and denaturing gradient gel electrophoresis (DGGE), was applied to compare the fungal communities of plants with different PA contents, as well as differences between bulk and rhizosphere samples. Cluster analysis of PCR-DGGE profiles revealed no clear evidence for PA-induced selection of specific fungal communities. However, canonical variance analysis showed that fungal communities associated with high-PA jacobine chemotypes could be discriminated from low PA samples and from the senecionine/seneciphylline chemotypes. The diversity of DGGE banding patterns, both in terms of band number and evenness, showed a trend toward lower diversity in the rhizosphere of high-PA plants as compared to low-PA plants and bulk soil. These results indicate that PA chemotypes of S. jacobaea differ in their influence on soil-borne fungal communities, with jacobine-containing plants exerting a greater selection in the rhizosphere than plants containing senecionine/seneciphylline.     

Development of microbial community during primary succession in areas degraded by mining activities

Together with plants, soil microbial communities play an essential role in the development of stable ecosystems on degraded lands, such as postmining spoil heaps. Our study addressed concurrent development of the vegetation and soil fungal and bacterial communities in the course of primary succession in a brown coal mine spoil deposit area in the Czech Republic across a chronosequence spanning 54 years. During succession, the plant communities changed from sparse plants over grassland and shrubland into a forest, becoming substantially more diverse with time. Microbial biomass increased until the 21st year of ecosystem development and later decreased. Although there was a close association between fungi and vegetation, with fungi mirroring the differences in plant community assemblages, the development of the bacterial community was different. The early succession community in the barren nonvegetated soil largely differed from that in the older sites, especially in its high abundance of autotrophic and free‐living N₂‐fixing bacteria. Later in succession, bacterial community changes were minor and reflected the chemical parameters of the soil, including pH, which also showed a minor change with time. Our results show that complex forest ecosystems developed over 54 years on the originally barren soil of the temperate zone and indicate an important role of bacteria in the initial stage of soil development. Although the arrival of vegetation affects substantially fungal as well as bacterial communities, it is mainly fungi that respond to the ongoing development of vegetation.     

Impact of fresh root material and mature crop residues of oilseed rape (Brassica napus) on microbial communities associated with subsequent oilseed rape

Glasshouse bioassays were conducted to assess the impact of different inputs of oilseed rape plant material on soil and rhizosphere microbial diversity associated with subsequently grown oilseed rape (Brassica napus) plants. The first bioassay focussed on the effect of oilseed rape rhizodeposits and fresh detached root material on microbial communities, in a rapid-cycling experiment in which oilseed rape plants were grown successively in pots of field soil for 4 weeks at a time, with six cycles of repeated vegetative planting in the same pot. Molecular analyses of the microbial communities after each cycle showed that the obligate parasite Olpidium brassicae infected the roots of oilseed rape within 4 weeks after the first planting (irrespective of the influence of rhizodeposits alone or in the presence of fresh detached root material), and consistently dominated the rhizosphere fungal community, ranging in relative abundance from 43 to 88 % when oilseed rape was grown more than once in the same soil. Fresh detached root material also led to a reduction in diversity within the soil fungal community, due to the increased relative abundance of O. brassicae. In addition, rhizosphere bacterial communities were found to have a reduced diversity over time when fresh root material was retained in the soil. In the second glasshouse experiment, the effect of incorporating mature, field-derived oilseed rape crop residues (shoots and root material) on microbial communities associated with subsequently grown oilseed rape was investigated. As before, molecular analyses revealed that O. brassicae dominated the rhizosphere fungal community, despite not being prevalent in either the residue material or soil fungal communities.     

Mycorrhizal inoculation affects arbuscular mycorrhizal diversity in watermelon roots,but leads to improved colonization and plant response under water stress only

Horticultural crops in the Mediterranean basin have to cope with severe drought conditions. The effect of inoculating watermelon plants grown under limited water availability conditions with AM fungi on the fruit yield, water use efficiency (WUE), root-N and -P content was examined. We focused on the impact of watering level and inoculation with allochthonous AM fungi on the diversity and presence of AM fungi in the watermelon roots using molecular techniques. An open field experiment was conducted and plants were grown with (M) and without AM fungal inoculum (NM), subjected to water stress (NW) and no stress conditions (W). Suboptimal water application (NW) resulted in significant reduction of fruit yield, root-N and -P content. Inoculation of plants grown under water stress resulted in a significant increase of WUE (19%), fruit yield (19%), root-N (27%) and -P (40%). However, only root-P responded to AM inoculation under non water stress conditions demonstrating 23% increase in M plants. DNA extracted from root samples was subjected to PCR–DGGE analysis. The native mycorrhizal population colonized watermelon roots, as indicated by DGGE bands in NM treatments. Some members of this colonizer community appear sensitive to the introduction of allochthonous inocula and to water stress conditions. Cloning and sequencing of AM fungi revealed that watermelon roots were colonized by Glomus and Paraglomus species. A TaqMan real-time PCR assay was also carried out targeting the 18S rRNA gene for the quantification of AM nucleic acids. The 18S rRNA copy numbers of AM fungi were significantly increased in M plants compared to NM plants under water stress. On the contrary, under non stress conditions M and NM plants did not show significant differences, indicating that inoculation with AM fungi was related to the response of plants to water stress conditions. Principle coordinate analysis of the DGGE banding patterns showed that the diversity of AM fungal colonizers was strongly affected (i) by inoculation and (ii) by water stress in the inoculated plants. Inoculation affected fungal presence under water limitation conditions only. The latter was in line with the significant beneficial effect of inoculation on both WUE and yield only under water limitation.     

Studies on soil fumigation—IV: Effects on fungi

The effect of soil fumigation with a chloropicrin-methyl bromide mixture(1:1) at 440 kg·ha^?1 on the fungal flora of a wheat-field has been investigated. Recolonization of fumigated soil and the occurrence of fungi on roots of wheat growing in fumigated and untreated soil were also followed. Very few fungi survived in fumigated soil that had been covered with polythene sheeting, but in uncovered fumigated soil some fungi survived especially at or near the soil surface.Study of recolonization of covered fumigated soil showed that some fungi, notably species of Chaetomium and Mortierella, appeared to survive fumigation and then increase in number, but many of the fungi recolonizing surface soil (2.5 cm), Alternaria, Stemphylium, Mucor, Cladosporium, Epicoccum), appeared to have come from the air. Recolonization of surface soil was not uniform; high counts were often due to the spores of one or a few fungi and samples collected a few cm apart might show different fungi in high number. In subsurface soil (5–22.5 cm) recolonization was much slower and even 117 days after fumigation the number of colonies and species of fungi was low compared with untreated soil.The common fungi on roots of plants grown in uncovered fumigated or untreated soil were very similar though initially there were fewer fungi on roots from fumigated soil. The main differences recorded were that Chaetomium species were more frequent on roots from fumigated soil and that. in general, Cylindrocarpon destruetans, Embellisia chlamydospora, species of Pythium and Rhizoctonia, and Gaeumannomyces graminis were more common on roots in untreated soil.     

The application of an organic amendment modifies the arbuscular mycorrhizal fungal communities colonizing native seedlings grown in a heavy-metal-polluted soil

A mesocosm experiment was conducted to investigate whether communities of arbuscular mycorrhizal (AM) fungi associated with roots of native (Piptatherum miliaceum, Retama sphaerocarpa, Psoralea bituminosa, Coronilla juncea, and Anthyllis cytisoides) and for comparison (Lolium perenne) seedlings in a heavy-metal-contaminated, semiarid soil were affected by the application of composted sugar beet waste. We also investigated whether there were relation between AMF diversity and metal concentration (Al, Cd, Cu, Fe, Mn, Pb and Zn) and total P in shoot as well as some soil parameters (total organic carbon and total N) when the SB waste was added to the soil. We analyzed a portion of approximately 795 base pairs of the small-subunit (SSU) rRNA gene by nested PCR, cloning, sequencing, and phylogenetic analyses. Twelve different AMF sequence types were distinguished: seven of these belonged to Glomus group A, one to Glomus group B, one to Diversispora, one to Archaeospora, and two to Paraglomus. The AM fungal populations colonizing roots in a heavy-metal-polluted soil were quite dependent on the host plant, the highest diversity values being obtained in authochtonous plants recognized as metallophytes, such as P. bituminosa, and in an allochtonous, invasive species (L. perenne). No significant correlation was found between AMF diversity and plant metal concentration and soil parameters. Excepting P. bituminosa, when sugar beet waste was added to soil, the populations of AM fungi in roots increased and the shoot metal concentrations decreased in all host plant species studied. Therefore, the addition of sugar beet waste can be considered a good strategy for the remediation and/or phytostabilization of mine tailing sites.     

Arbuscular mycorrhizal fungi affect the growth,nutrient uptake and water status of maize (Zea mays L.) grown in two types of coal mine spoils under drought stress

Drought stress greatly affects the growth and development of plants in coal mine spoils located in the Inner Mongolia grassland ecosystem. Arbuscular mycorrhizal fungi (AMF) can increase plant tolerance to drought. However, little is known regarding the contribution of AMF to plants that are grown in different types of coal mine spoils under drought stress. To evaluate the mycorrhizal effects on the drought tolerance of maize (Zea mays L.) grown in weathered (S1) and spontaneously combusted (S2) coal mine spoils, a greenhouse pot experiment was conducted to investigate the effects of inoculation with Rhizophagus intraradices on the growth, nutrient uptake, carbon:nitrogen:phosphorus (C:N:P) stoichiometry and water status of maize under well-watered, moderate and severe drought stress conditions. The results indicated that drought stress increased mycorrhizal colonization and decreased plant dry weights, nutrient contents, leaf moisture percentage of fresh weight (LMP), water use efficiency (WUE) and rehydration rate. A high level of AMF colonization ranging from 65 to 90% was observed, and the mean root colonization rates in S1 were lower than those in S2. In both substrates, inoculation with R. intraradices significantly improved the plant growth, P contents, LMP and WUE and decreased the C:P and N:P ratios of plants under drought stress. In addition, maize grown in S1 and S2 exhibited different wilting properties in response to AMF inoculation, and plant rehydration after drought stress occurred faster in mycorrhizal plants. The results suggested that inoculation with R. intraradices played a more positive role in improving the drought stress resistance of plants grown in S2 than those grown in S1. AMF inoculation has a beneficial effect on plant tolerance to drought and effectively facilitates the development of plants in different coal mine spoils.     

Woodland trees modulate soil resources and conserve fungal diversity in fragmented landscapes

Resource islands around woody plants are thought to define the structure and function of many semiarid and arid ecosystems, but their role in patterning of soil microbial communities remains largely unexamined in dry environments. This study examined soil resource distribution and associated fungal communities in two Allocasuarina luehmannii (buloke) remnants of semiarid north-western Victoria, Australia. These savannah-like woodlands are listed as endangered due to extensive clearing for agriculture. We used the DNA-based profiling technique T-RFLP and ordination-based statistical methods to compare fungal community compositions in surface soils from two remnants (located 1.6 km apart) and three sampling positions (beneath individual buloke canopies; grassy inter-canopy areas; and adjoining cleared paddocks). Resource island formation beneath buloke trees was clearly evident in soil physicochemical properties (e.g. threefold concentrations of total carbon and nitrogen in canopy versus non-canopy soils). This heterogeneity of resources was moderately correlated with soil fungal community compositions, which were distinct for each sampling position. We argue that fungal composition patterns reflected multiple roles of fungi in dryland ecosystems, namely: responses of saprotrophic fungi to tree organic matter inputs; specificity of ectomycorrhizal fungi to tree rooting zones; and fungal involvement in biological soil crusts that variably covered non-canopy soils. Our data did not indicate that buloke canopy areas were particular hotspots of soil fungal diversity, but that they increased landscape-level diversity by supporting a distinct suite of fungi. In addition, we provide evidence of phylogenetic differentiation of soil fungal communities between our two remnants, which adds to growing evidence of fungal genetic structure at localised scales. These findings highlight the importance of remnant trees in conserving both soil resources and microbial genetic diversity. In addition, evidence of differentiation of soil fungal phylogenetics between nearby but isolated remnants suggests that conserving soil fungal diversity requires conservation of host habitats over their entire (remaining) range, and indicates previously unseen consequences of tree loss from extensively cleared landscapes.     

Growth and Metal Accumulation of Mycorrhizal Sorghum Exposed to Elevated Copper and Zinc

Arbuscular mycorrhizal fungi (AMF) alter heavy metal acquisition by higher plants and may alter plant response to soil-contaminating heavy metals. Two communities comprised of Glomus intraradices and G. spurcum were investigated for their influence on copper (Cu) and zinc (Zn) resistance of Sorghum bicolor. One community was isolated from a Cu- and Zn-contaminated soil (AMF-C) and one consisted of isolates from non-contaminated soil (AMF-NC). Non-mycorrhizal (NM) sorghum plants were also included. The two community ecotypes differed in their capacity to protect sorghum from Cu and Zn toxicity and exhibited differential metal uptake into hyphae and altered heavy metal uptake by roots and translocation to plant shoots. AMF-C reduced Cu acquisition under elevated Cu conditions, but increased Cu uptake and translocation by sorghum under normal Cu conditions, patterns not exhibited by AMF-NC or NM plants. Hyphae of both fungal ecotypes accumulated high concentrations of Cu under Cu exposure. AMF-C exhibited elevated hyphal Zn accumulation and stimulated Zn uptake and translocation in sorghum plants compared to AMF-NC and NM plants. Differences in metal resistance between fungal treatments and between mycorrhizal and non-mycorrhizal plants were not related to differences in nutrient relations. The enhanced Cu resistance of sorghum and altered patterns of Cu and Zn translocation to shoots facilitated by AMF isolated from the metal-contaminated soil highlight the potential for metal-adapted AMF to increase the phytoremediation potential of mycotrophic plants on metal-contaminated environments.     

In situ dynamics of soil fungal communities under different genotypes of potato, including a genetically modified cultivar

Fungi are key to the functioning of soil ecosystems, and exhibit a range of interactions with plants. Given their close associations with plants, and importance in ecosystem functioning, soil-borne fungi have been proposed as potential biological indicators of disturbance and useful agents in monitoring strategies, including those following the introduction of genetically modified (GM) crops. Here we report on the impact of potato crop varieties, including a cultivar that was genetically modified for its starch quality, on the community composition of the main phyla of fungi in soils, i.e. Ascomycota, Basidiomycota and Glomeromycota in rhizosphere and bulk soil. Samples were collected at two field sites before sowing, at three growth stages during crop development and after the harvest of the plants, and the effects of field site, plant growth stage and plant cultivar (genotype) on fungal community composition assessed using three phylum-specific T-RFLP profiling strategies and multivariate statistical analysis (NMDS ordinations with ANOSIM test). In addition, fungal biomass, arbuscular mycorrhizal colonization of roots and activities of extracellular fungal enzymes (laccases, Mn-peroxidases and cellulases) involved in degradation of lignocelluloses-rich organic matter were determined. Fungal community compositions, densities and activities were observed to differ significantly between the rhizosphere and bulk soil. The most important factors determining fungal community composition and functioning were plant growth stage for the rhizosphere communities and location and soil properties for the bulk soil communities. The basidiomycetes were the most numerous fungal group in the bulk soils and in the rhizosphere of young plants, with a shift toward greater ascomycete numbers in the rhizosphere at later growth stages. There were no detectable differences between the GM cultivar and its parental cultivar in terms of influence on fungal community structure of function. Fungal community structure and functioning of both GM- and parental cultivars fell within the range of other cultivars at most sampling moments.     

Phylogenetic structure of arbuscular mycorrhizal community shifts in response to increasing soil fertility

Understanding the underlying mechanisms driving responses of belowground communities to increasing soil fertility will facilitate predictions of ecosystem responses to anthropogenic eutrophication of terrestrial systems. We studied the impact of fertilization of an alpine meadow on arbuscular mycorrhizal (AM) fungi, a group of root-associated microorganisms that are important in maintaining sustainable ecosystems. Species and phylogenetic composition of AM fungal communities in soils were compared across a soil fertility gradient generated by 8 years of combined nitrogen and phosphorus fertilization. Phylogenetic patterns were used to infer the ecological processes structuring the fungal communities. We identified 37 AM fungal virtual taxa, mostly in the genus Glomus. High fertilizer treatments caused a dramatic loss of Glomus species, but a significant increase in genus richness and a shift towards dominance of the lineage of Diversispora. AM fungal communities were phylogenetically clustered in unfertilized soil, random in the low fertilizer treatment and over-dispersed in the high fertilizer treatments, suggesting that the primary ecological process structuring communities shifted from environmental filtering (selection by host plants and fungal niches) to a stochastic process and finally to competitive exclusion across the fertilization gradient. Our findings elucidate the community shifts associated with increased soil fertility, and suggest that high fertilizer inputs may change the dominant ecological processes responsible for the assembly of AM fungal communities towards increased competition as photosynthate from host plants becomes an increasingly limited resource.     

Impact of plant species grown as monocultures on sporulation and root colonization by native arbuscular mycorrhizal fungi in potato

Rhizosphere soils from 12 different plant species grown as monocultures at a field site of biodiversity and ecological processes in terrestrial herbaceous ecosystems (BIODEPTH) in northern Sweden were used as inoculum on potato to investigate mycorrhizal traits. Potato roots showed significantly higher mycorrhizal colonization when inoculated with soil samples from Festuca ovina and Leucanthemum vulgare compared to soil samples from other plants. The soil samples of F. ovina, L. vulgare, Phalaris arundinacea and Trifolium pratense rhizospheres were chosen for arbuscular mycorrhizal fungi identification based on spore morphology and large subunit (LSU) ribosomal DNA sequences amplified from single spores and roots. Spore morphological identification showed that Glomus mosseae and Glomus intraradices were found in F. ovina and L. vulgare soils at the site as well as in our potato trap experiment. Also, Glomus geosporum spores were present in all four plants’ soils in the potato trap experiment. LSU rDNA sequences were obtained from AM fungal spores from the collection site or potato trap experiment and colonized potato roots inoculated with L. vulgare soil. Sequences showed highest similarity to G. mosseae. Our results suggest that the host F. ovina and L. vulgare could be considered in crop rotation to enhance AM fungal inocula for potatoes.     

Possible role of ectomycorrhizal fungi in cycling of aluminium in podzols

Budget studies in boreal podzols indicate a considerable upward transport of aluminium (Al) from the mineral soil into the organic horizon. In this paper we studied if ectomycorrhizal (EcM) fungi can be involved in this upward transport via their extramatrical hyphae. We tested the use of gallium (Ga) as proxy for Al. Transport of Al through EcM hyphae was studied in vitro in a two-compartment Petri dish system. Two of the five fungal isolates tested transported Al. Using Ga instead of Al revealed the same trend in these systems, confirming the use of Ga as proxy for Al. Upward transport of Ga was studied in an artificial podzol. Pinus sylvestris seedlings were colonised by a natural community of EcM fungi from fresh forest soil. Gallium addition to the mineral soil led to increased Ga content in roots in the organic horizon and in the shoot. Upward Ga allocation was significantly higher when roots were excluded from the mineral soil by a mesh, only allowing fungal mycelium to grow through. We conclude that at least some EcM fungi transport Al and that Ga, and probably also Al, can be transported upwards by EcM fungal hyphae in a podzol system. These findings support the hypothesis that EcM fungi play a role in upward transport of Al in podzols.     

Influence of grass species and soil type on rhizosphere microbial community structure in grassland soils

A number of studies have reported species specific selection of microbial communities in the rhizosphere by plants. It is hypothesised that plants influence microbial community structure in the rhizosphere through rhizodeposition. We examined to what extent the structure of bacterial and fungal communities in the rhizosphere of grasses is determined by the plant species and different soil types. Three grass species were planted in soil from one site, to identify plant-specific influences on rhizosphere microbial communities. To quantify the soil-specific effects on rhizosphere microbial community structure, we planted one grass species (Lolium perenne L.) into soils from three contrasting sites. Rhizosphere, non-rhizosphere (bulk) and control (non-planted) soil samples were collected at regular intervals, to examine the temporal changes in soil microbial communities. Rhizosphere soil samples were collected from both root bases and root tips, to investigate root associated spatial influences. Both fungal and bacterial communities were analysed by terminal restriction fragment length polymorphism (TRFLP). Both bacterial and fungal communities were influenced by the plant growth but there was no evidence for plant species selection of the soil microbial communities in the rhizosphere of the different grass species. For both fungal and bacterial communities, the major determinant of community structure in rhizospheres was soil type. This observation was confirmed by cloning and sequencing analysis of bacterial communities. In control soils, bacterial composition was dominated by Firmicutes and Actinobacteria but in the rhizosphere samples, the majority of bacteria belonged to Proteobacteria and Acidobacteria. Bacterial community compositions of rhizosphere soils from different plants were similar, indicating only a weak influence of plant species on rhizosphere microbial community structure.