Effect of arbuscular mycorrhizal fungi inoculation on root traits and root volatile organic compound emissions of Sorghum bicolor

Volatile organic compounds (VOCs) emitted by plant roots have important functions that can influence the rhizospheric environment. The aim of this study was to examine the effects of arbuscular mycorrhizal (AM) fungi on the profile of root VOCs. Sorghum (Sorghum bicolor) plants were grown in pots inoculated with either Glomus mosseae or Glomus intraradices, which formed mycorrhiza with the roots. Control plants were grown in pots inoculated with sterile inoculum and did not form mycorrhiza. Forty-four VOCs were determined using headspace solid-phase microextraction (HS-SPME) and gas chromatography–mass spectrometry (GC-MS). Alkanes were the most abundant type of VOCs emitted by both mycorrhizal and non-mycorrhizal plants. Both the quantity and type of volatiles were dramatically altered by the presence of AM fungi, and these changes had species specificity. Compared with non-mycorrhizal plants, mycorrhizal plants emitted more alcohols, alkenes, ethers and acids but fewer linear-alkanes. The AM fungi also influenced the morphological traits of the host roots. The total root length and specific root length of mycorrhizal plants were significantly greater than those of non-mycorrhizal plants; however, both the incidence and length of root-hair were dramatically decreased. Our findings confirm that AM fungi can alter the profile of VOCs emitted by roots as well as the root morphology of sorghum plants, indicating that AM fungi have the potential to help plants adapt to and alter soil environments.     

水分胁迫下AM真菌对沙打旺生长和抗旱性的影响

利用盆栽试验研究了水分胁迫条件下接种AM真菌对优良牧草和固沙植物沙打旺(Astragalus adsurgens Pall.)生长和抗旱性的影响。在土壤相对含水量为70%、50%和30%条件下,分别接种摩西球囊霉(Glomus mosseae)和沙打旺根际土著菌,不接种处理作为对照。结果表明,水分胁迫显著降低了沙打旺植株(无论接种AM真菌与否)的株高、分枝数、地上部干重和地下部干重,并显著提高了土著AM真菌的侵染率,对摩西球囊霉的侵染率无显著影响。接种AM真菌可以促进沙打旺生长和提高植株抗旱性,但促进效应因土壤含水量和菌种不同而存在差异。不同水分条件下,接种AM真菌显著提高了植株菌根侵染率、根系活力、地下部全N含量和叶片CAT活性。土壤相对含水量为30%和50%时,接种株地上部全N、叶片叶绿素、可溶性蛋白、脯氨酸含量和POD活性显著高于未接种株;接种AM真菌显著降低了叶片MDA含量;接种土著AM真菌的植株株高、分枝数、地上部和地下部干重显著高于未接种株。土壤相对含水量为30%时,接种AM真菌显著增加了地上部全P含量和叶片相对含水量;接种摩西球囊霉的植株株高、分枝数、地上部和地下部干重显著高于未接种株。水分胁迫40d,接种AM真菌显著提高了叶片可溶性糖含量。水分胁迫80d,接种株叶片SOD活性显著增加。菌根依赖性随水分胁迫程度增加而提高。沙打旺根际土著菌接种效果优于摩西球囊霉。水分胁迫和AM真菌的交互作用对分枝数、菌根侵染率、叶片SOD、CAT和POD活性、叶绿素、脯氨酸、可溶性蛋白、地上部全N和全P、地下部全N和根系活力有极显著影响,对叶片丙二醛和地下部全P有显著影响。AM真菌促进根系对土壤水分和矿质营养的吸收,改善植物生理代谢活动,从而提高沙打旺抗旱性,促进其生长。试验结果为筛选优良抗旱菌种,充分利用AM真菌资源促进荒漠植物生长和植被恢复提供了依据。     

Hydrolytic enzymes and ability of arbuscular mycorrhizal fungi to colonize roots

The production of hydrolytic enzymes from external mycelia associated with roots and colonized soybean roots (Glycine max L.) inoculated with different arbuscular-mycorrhizal (AM) fungi of the genus GLOMUS:, and the possible relationship between these activities and the capacity of the AM fungi to colonize plant roots was studied. There were differences in root colonization and plant growth between the GLOMUS: strains, and also between two isolates of G. mosseae. Hydrolytic activities in the root and external mycelia associated with roots differed in the AM fungi tested. Correlations were only found between the endoxyloglucanase activity of the external mycelia associated with roots of the AM fungi tested and the percentage root colonization or plant growth. However, hydrolytic activities of roots colonized by the different endophytes correlated with those of external mycelia. The hydrolytic activities were not qualitatively different because the endoxyloglucanase from AM colonized roots and the external mycelia did not show a high degree of polymorphism in the different species of fungus tested. The possible role of the hydrolytic activity of external hyphae of AM fungi was discussed as a factor affecting fungal ability to colonize the root and influence plant growth.     

Systemic inhibition of arbuscular mycorrhiza development by root exudates of cucumber plants colonized by<Emphasis Type="Italic"> Glomus mosseae</Emphasis>

The arbuscular mycorrhizal (AM) non-host plants mustard, sugar beet, lupin and the AM host plant cucumber were used as test plants. Cucumber plants were grown either in the absence of the AM fungus (AMF) Glomus mosseae or in a split-root system, with one side mycorrhizal and one side non-mycorrhizal. Root exudates of the AM non-host plants, the non-mycorrhizal cucumber plants and the mycorrhizal and the non-mycorrhizal side of the split-root system of mycorrhizal cucumber plants were collected and applied to cucumber plants inoculated with the AMF. Root exudates of non-mycorrhizal cucumber plants showed a significant stimulatory effect on root colonization, whereas root exudates from the mycorrhizal and the non-mycorrhizal sides of a split-root system of a mycorrhizal cucumber plant did not show this stimulatory effect and were even slightly inhibitory. Root exudates of the two AM non-host plants mustard and sugar beet significantly reduced root colonization in cucumber plants, whereas no such effect was observed when root exudates of the AM non-host plant lupin were applied.     

Cadmium immobilization in the rhizosphere of arbuscular mycorrhizal plants by the fungal extraradical mycelium

The aim of the study was to assess how the extraradical mycelium (ERM) of arbuscular mycorrhizal (AM) fungi contributes to Cd immobilization in the rhizosphere. Substrates prepared by cultivation of AM and non-mycorrhizal tobacco (Nicotiana tabacum L.) in quartz sand in two experiments were amended with Cd in a range of concentrations and Cd immobilization was assessed as Cd toxicity using root growth tests. Split-root plants, inoculated at one part of the root system, and hyphal compartments colonized by ERM only were used to separate the effects of ERM from plant-mediated effects of mycorrhiza and from the effects of roots. AM decreased Cd toxicity in the substrates obtained by 12 weeks of cultivation (Experiment 1), while the effect was less clear after 8 weeks (Experiment 2). No indication was found for an involvement of plant-mediated effects; in contrast, the effect of ERM could be clearly demonstrated. Lower Cd toxicity in the substrates colonized by ERM was related to ERM-induced alkalinization, but not directly to ERM density. It is concluded that the ERM of AM fungi may enhance Cd immobilisation in soil not only due to its high Cd sorption capacity but also by its activity.     

Nitrogen fertilization alters the functioning of arbuscular mycorrhizas at two semiarid grasslands

The effects of nitrogen (N) fertilization on arbuscular mycorrhizas were studied at two semiarid grasslands with different soil properties and N-enrichment history (Shortgrass Steppe in Colorado, and Sevilleta National Wildlife Refuge in New Mexico). These sites are part of the National Science Foundation's Long-Term Ecological Research Network. The experimental plots at Shortgrass Steppe were fertilized with ammonium nitrate (NH₄NO₃) from 1971 to 1975, and have not received additional N since then. The experimental plots at Sevilleta were also fertilized with NH₄NO₃, but were established in 1995, 2 years before the soils were used for this study. Greenhouse experiments were conducted to compare the growth response of local grasses to arbuscular mycorrhizal (AM) fungi from fertilized (FERT) and unfertilized (UNFERT) field soils, at each site. Two species per site were chosen, Bouteloua gracilis and Elymus elymoides from Shortgrass Steppe, and B. gracilis and B. eriopoda from Sevilleta. Plants were grown for 3 months at HIGH N and LOW N levels, with FERT or UNFERT soil inoculum and in a non-mycorrhizal condition. Fertilization with N altered the functioning of AM fungi at both sites. Grasses inoculated with AM fungi from UNFERT soils had the most tillers, greatest biomass and highest relative growth rates. There were no significant differences in the growth response of plants inoculated with AM fungi from FERT soils and the non-mycorrhizal controls. These results were consistent across sites and species except for the plants grown at LOW N in Sevilleta soils. These plants were deficient in N and phosphorus (P) and did not show growth enhancement in response to AM inoculation with either FERT or UNFERT soils. Percent root length colonized by AM fungi was not directly related to plant performance. However, enrichment with N consistently decreased root colonization by AM fungi in the grasses grown in soils from Shortgrass Steppe with high P availability (18.4 mg kg^–1), but not in the grasses grown in Sevilleta soils with low P availability (6.6 mg kg^–1). Our study supports the hypotheses that (1) fertilization with N alters the balance between costs and benefits in mycorrhizal symbioses and (2) AM fungal communities from N fertilized soils are less beneficial mutualists than those from unfertilized soils.     

Arbuscular mycorrhizas in cycads of southern India

Root and soil samples of three potted or ground-grown cycads ( Cycas circinalis, C. revoluta, Zamiasp.) were collected between November 1999 and June 2000 and surveyed for arbuscular mycorrhizal (AM) colonization and spore populations. AM fungi were associated with all root systems and rhizosphere samples examined. Root colonization was of a typical Arum type and AM colonization levels differed significantly between species and between potted and ground-grown cycads. Mycorrhizal colonization levels were inversely related to root hair number and length. Spores of nine morphotypes belonging to three genera ( Acaulospora, Glomus, Scutellospora) were extracted from soil. The percentage root length colonized by AM fungi was not related to soil factors, but total AM fungal spore numbers in the rhizosphere soil were inversely related to soil nitrogen and phosphorus levels. AM fungal spore numbers in the soil were linearly related to root length colonized. The co-occurrence of septate non-mycorrhizal fungi was recorded for the first time in cycads. These observations and the relationship between plant mycorrhizal status and soil nutrients are discussed.     

Arbuscular mycorrhizal fungi from three genera induce two-phase plant growth responses on a high P-fixing soil

Plant growth enhancing effects of arbuscular mycorrhizal (AM) fungi are suitably quantified by comparisons of mycorrhizal and non-mycorrhizal plant growth responses to added phosphorus (P). The ratio between the amounts of added P required for the same yield of mycorrhizal and non-mycorrhizal plants is termed the relative effectiveness of the mycorrhiza. Variation in this relative effectiveness was examined for subterranean clover grown on a high P-fixing soil. Plants were either left non-mycorrhizal or inoculated with one of three AM fungal species with well-characterised differences in external hyphal spread. With no P added, plants from all treatments produced <10% of their maximum growth achieved at non-limiting P supply. The growth response of non-mycorrhizal plants was markedly sigmoid. Mycorrhizal growth responses were not sigmoid but their shape was two-phased. The first phase was an asymptotic approach to 25–30% of maximum growth, followed by a second asymptotic rise to maximum growth. Growth effects of Glomus invermaium and Acaulospora laevis were quite similar. Plants in these treatments produced up to four times greater shoot dry biomass than non-mycorrhizal plants. Scutellospora calospora was less effective. The relative effectiveness of AM fungi varied with the level of P application. This is expected to apply to all soils on which a sigmoid response is obtained for growth of non-mycorrhizal plants. In a simple approximation the relative effectiveness was calculated to range from 1.46 to 15.57. Shoot P contents were increased by up to 25 times by A. laevis, significantly more than by the other two fungi. The further mycelial spread of this fungus is thought to have contributed to its relatively greater effect on plant P content.     

The differential behavior of arbuscular mycorrhizal fungi in interaction with Astragalus sinicus L. under salt stress

Three arbuscular mycorrhizal (AM) fungi (Glomus mosseae, Glomus claroideum, and Glomus intraradices) were compared for their root colonizing ability and activity in the root of Astragalus sinicus L. under salt-stressed soil conditions. Mycorrhizal formation, activity of fungal succinate dehydrogenase, and alkaline phosphatase, as well as plant biomass, were evaluated after 7 weeks of plant growth. Increasing the concentration of NaCl in soil generally decreased the dry weight of shoots and roots. Inoculation with AM fungi significantly alleviated inhibitory effect of salt stress. G. intraradices was the most efficient AM fungus compared with the other two fungi in terms of root colonization and enzyme activity. Nested PCR revealed that in root system of plants inoculated with a mix of the three AM fungi and grown under salt stress, the majority of mycorrhizal root fragments were colonized by one or two AM fungi, and some roots were colonized by all the three. Compared to inoculation alone, the frequency of G. mosseae in roots increased in the presence of the other two fungal species and highest level of NaCl, suggesting a synergistic interaction between these fungi under salt stress.     

Multi-functionality and biodiversity in arbuscular mycorrhizas

Plant roots in natural ecosystems are typically colonized by a wide range of fungi. Some of these are pathogenic, others appear to be opportunistic and have no apparent impact, while mycorrhizal fungi are generally regarded as mutualistic. Of the various types of mycorrhizal fungi, the arbuscular mycorrhizal (AM) association is by far the most abundant and widespread. While the most widely accepted model of AM function depends upon plants benefiting from the facilitation of phosphorus uptake, recent data from field-based studies in temperate ecosystems indicate that only plant species with poorly branched root systems benefit from AM fungi in this way: species with highly branched root systems may benefit in other ways, such as by being protected against root pathogenic fungi. These two responses apparently represent extremes along a continuum of AM benefit determined by root system architecture.     

Role of the modification in root exudation induced by arbuscular mycorrhizal colonization on the intraradical growth of Phytophthora nicotianae in tomato

We studied the role of modification in root exudation induced by colonization with Glomus intraradices and Glomus mosseae in the growth of Phytophthora nicotianae in tomato roots. Plants were grown in a compartmentalized plant growth system and were either inoculated with the AM fungi or received exudates from mycorrhizal plants, with the corresponding controls. Three weeks after planting, the plants were inoculated or not with P. nicotianae growing from an adjacent compartment. At harvest, P. nicotianae biomass was significantly reduced in roots colonized with G. intraradices or G. mosseae in comparison to non-colonized roots. Conversely, pathogen biomass was similar in non-colonized roots supplied with exudates collected from mycorrhizal or non-mycorrhizal roots, or with water. We cannot rule out that a mycorrhiza-mediated modification in root exudation may take place, but our results did not support that a change in pathogen chemotactic responses to host root exudates may be involved in the inhibition of P. nicotianae.     

Arbuscular mycorrhizal fungi as (agro)ecosystem engineers

Symbiotic interactions have been shown to facilitate shifts in the structure and function of host plant communities. For example, parasitic plants can induce changes in plant diversity through the suppression of competitive community dominants. Arbuscular mycorrhizal (AM) fungi have also be shown to induce shifts in host communities by increasing host plant nutrient uptake and growth while suppressing non-mycorrhizal species. AM fungi can therefore function as ecosystem engineers facilitating shifts in host plant communities though the presumed physiological suppression of non-contributing or non-mycorrhizal plant species. This dichotomy in plant response to AM fungi has been suggested as a tool to suppress weed species (many of which are non-mycorrhizal) in agro-ecosystems where mycorrhizal crop species are cultivated. Rinaudo et al. (2010), this issue, have demonstrated that AM fungi can suppress pernicious non-mycorrhizal weed species including Chenopodium album (fat hen) while benefiting the crop plant Helianthus annuus (sunflower). These findings now suggest a future for harnessing AM fungi as agro-ecosystem engineers representing potential alternatives to costly and environmentally damaging herbicides.     

Arbuscular Mycorrhizas, Microbial Communities, Nutrient Availability, and Soil Aggregates in Organic Tomato Production

Effects of arbuscular mycorrhzal (AM) fungi on plant growth and nutrition are well-known, but their effects on the wider soil biota are less clear. This is in part due to difficulties with establishing appropriate non-mycorrhizal controls in the field. Here we present results of a field experiment using a new approach to overcome this problem. A previously well-characterized mycorrhizal defective tomato mutant (rmc) and its mycorrhizal wildtype progenitor (76R MYC+) were grown at an organic fresh market tomato farm (Yolo County, CA). At the time of planting, root in-growth cores amended with different levels of N and P, were installed between experimental plants to study localized effects of mycorrhizal and non-mycorrhizal tomato roots on soil ecology. Whilst fruit yield and vegetative production of the two genotypes were very similar at harvest, there were large positive effects of colonization of roots by AM fungi on plant nutrient contents, especially P and Zn. The presence of roots colonized by AM fungi also resulted in improved aggregate stability by increasing the fraction of small macroaggregates, but only when N was added. Effects on the wider soil community including nematodes, fungal biomass as indicated by ergosterol, microbial biomass C, and phospholipid fatty acid (PLFA) profiles were less pronounced. Taken together, these data show that AM fungi provide important ecosystem functions in terms of plant nutrition and aggregate stability, but that a change in this one functional group had only a small effect on the wider soil biota. This indicates a high degree of stability in soil communities of this organic farm.     

Growth responses of tropical forage plant species to vesicular-arbuscular mycorrhizae

Summary Growth and mineral uptake of twenty-four tropical forage legumes and grasses were compared under glasshouse conditions in a sterile low P oxisol, one part inoculated and the other not inoculated with mycorrhizal fungi. Shoot and root dry weights and total uptake of P, N, K, Ca, and Mg of all the test plants were significantly increased by mycorrhizal inoculation. Mycorrhizal inoculation, with few exceptions, decreased the root/shoot ratio. Non-mycorrhizal plants contained always lower quantities of mineral elements than mycorrhizal plants. Plant species showed differences in percentage mycorrhizal root length and there was no correlation between percentage mycorrhizal infection and plant growth parameters. A great variation in dependence on mycorrhiza was observed among forage species. Total uptake of all elements by non-mycorrhizal legumes and uptake of P, N and K by non-mycorrhizal grasses correlated inversely with mycorrhizal dependency. Mycorrhizal plants of all species used significantly greater quantities of soil P than the nonmycorrhizal plants. Utilization of soil P by non-mycorrhizal plants was correlated inversely with mycorrhizal dependency.     

Dark septate endophytes colonizing the roots of ‘non-mycorrhizal’ plants in a mine tailing pond and in a relatively undisturbed environment,Southwest China

Dark septate endophytes (DSEs), one of the most common fungal colonizers of roots, are considered to overlap in function with mycorrhizal fungi. However, there is little knowledge on the distribution and identity of DSEs in ‘non-mycorrhizal’ plants. In the current study, colonization and diversity of DSEs colonizing the roots of eight typically ‘non-mycorrhizal’ families were assessed. In total, 120 root samples of 31 plant species were all colonized by DSEs. Intensity of DSE colonization varied greatly among different plant species, with a range of 0.56–47.56%, 8.13% on average. Cladosporium, Cyphellophora and Phialophora were the dominant genera, with a relative abundance of more than 60% over a total of 90 isolates. Our results showed that diverse DSE species colonized the roots of ‘non-mycorrhizal’ plants, especially they were more common in degraded mine tailings than in the undisturbed site, but their integral roles to the functional roots are in need of further experimental demonstration.     

Indigenous and introduced arbuscular mycorrhizal fungi contribute to plant growth in two agricultural soils from south-western Australia

Arbuscular mycorrhizal (AM) fungi occur in all agricultural soils but it is not easy to assess the contribution they make to plant growth under field conditions. Several approaches have been used to investigate this, including the comparison of plant growth in the presence or absence of naturally occurring AM fungi following soil fumigation or application of fungicides. However, treatments such as these may change soil characteristics other than factors directly involving AM fungi and lead to difficulties in identifying the reason for changes in plant growth. In a glasshouse experiment, we assessed the contribution of indigenous AM fungi to growth of subterranean clover in undisturbed cores of soil from two agricultural field sites (a cropped agricultural field at South Carrabin and a low input pasture at Westdale). We used the approach of estimating the benefit of AM fungi by comparing the curvature coefficients ( C) of the Mitscherlich equation for subterranean clover grown in untreated field soil, in field soil into which inoculum of Glomus invermaium was added and in soil fumigated with methyl bromide. It was only possible to estimate the benefit of mycorrhizas using this approach for one soil (Westdale) because it was the only soil for which a Mitscherlich response to the application of a range of P levels was obtained. The mycorrhizal benefit ( C of mycorrhizal vs. non-mycorrhizal plants or C of inoculated vs. uninoculated plants) of the indigenous fungi corresponded with a requirement for phosphate by plants that were colonised by AM fungi already present in the soil equivalent to half that required by non-mycorrhizal plants. This benefit was independent of the plant-available P in the soil. There was no additional benefit of inoculation on plant growth other than that due to increased P uptake. Indigenous AM fungi were present in both soils and colonised a high proportion of roots in both soils. There was a higher diversity of morphotypes of mycorrhizal fungi in roots of plants grown in the Westdale soil than in the South Carrabin soil that had a history of high phosphate fertilizer use in the field. Inoculation with G. invermaium did not increase the level of colonisation of roots by mycorrhizal fungi in either soil, but it replaced approximately 20% of the root length colonised by the indigenous fungi in Westdale soil at all levels of applied P. The proportion of colonised root length replaced by G. invermaium in South Carrabin soil varied with the level of application of P to the soil; it was higher at intermediate levels of recently added soil P.     

The effects of arbuscular mycorrhizal (AM) fungal and garlic mustard introductions on native AM fungal diversity

Introduced, non-native organisms are of global concern, because biological invasions can negatively affect local communities. Arbuscular mycorrhizal (AM) fungal communities have not been well studied in this context. AM fungi are abundant in most soils, forming symbiotic root-associations with many plant species. Commercial AM fungal inocula are increasingly spread worldwide, because of potentially beneficial effects on plant growth. In contrast, some invasive plant species, such as the non-mycorrhizal Alliaria petiolata, can negatively influence AM fungi. In a greenhouse study we examined changes in the structure of a local Canadian AM fungal community in response to inoculation by foreign AM fungi and the manipulated presence/absence of A. petiolata. We expected A. petiolata to have a stronger effect on the local AM fungal community than the addition of foreign AM fungal isolates. Molecular analyses indicated that inoculated foreign AM fungi successfully established and decreased molecular diversity of the local AM fungal community in host roots. A. petiolata did not affect molecular diversity, but reduced AM fungal growth in the greenhouse study and in a in vitro assay. Our findings suggest that both introduced plants and exotic AM fungi can have negative impacts on local AM fungi.     

Impact of soil sterilization and irrigation intervals on P and K acquisition by mycorrhizal onion (<Emphasis Type="Italic">Allium cepa</Emphasis>)

Drought is a world-spread problem seriously influencing crop production. Arbuscular mycorrhizal (AM) association and soil microorganisms can help plant growth under water stress condition by improvement of its nutrient and water uptake. In this experiment, onion plants (Allium cepa L. cv. Red Azar Shahr) were inoculated with three AM fungi species (Glomus versiforme, G. intraradices, G. etunicatum) or left un-inoculated as non-mycorrhizal plants, in a sterile or non-sterile sandy loam soil. Plants were irrigated at 7, 9 or 11-day intervals to keep the soil moisture content to field capacity at the irrigation time. Mycorrhizal root colonization decreased (p < 0.05) with an increase in irrigation interval, and the highest root colonization was achieved at 7-day irrigated onions in symbiosis with G. versiforme. Phosphorus content in plant tissue was significantly increased in mycorrhizal than non-mycorrhizal onions. Plants inoculated with G. versiforme at 9-day interval treatment had the highest leaf P content, while the lowest P was observed in non-mycorrhizal plants at all irrigation intervals. Onions inoculated by G. versiforme or G. etunicatum at 9-day irrigation interval had the highest K content. Results revealed that the inoculation of onion plant with G. versiforme or G. etunicatum and increasing irrigation interval up to 9 days, could improve P and K uptake.     

Endo- and ectomycorrhizas in Quercus agrifolia Nee. (Fagaceae): patterns of root colonization and effects on seedling growth

We documented the patterns of root occupancy by Glomalean and ectomycorrhizal (EM) fungi in Quercus agrifolia, and host plant responses to inoculation with each mycorrhizal type alone or in combination. Glomalean hyphae, coils and vesicles, and EM root tips were recorded. Colonization patterns conformed to a succession from Glomalean and EM fungi in 1-year-old seedlings to predominantly EM in saplings (>11 years old); both mycorrhizal types were rarely detected within the same root segment. Inoculation of Q. agrifolia seedlings with EM or Glomalean fungi (AM) alone or in combination (EM+AM) altered the cost:benefit relationship of mycorrhizas to the host plant. Seedling survival, plant biomass, foliar nitrogen (N), and phosphorus (P) status were greatest in EM- or AM-only inoculated seedlings. Seedlings inoculated with both mycorrhizal types (AM+EM) exhibited the lowest survival rates, biomass, foliar N, and P levels. Roots of these plants were highly colonized by both EM (38% root length colonized) and Glomalean fungi (34%). Because these levels of colonization were similar to those detected in 1-year-old field seedlings, the presence of both mycorrhizal types may be a carbon cost and, in turn, less beneficial to oaks during establishment in the field. However, the shift to EM colonization in older plants suggests that mycorrhizal effects may become positive with time.     

Evidence that soil nutrient stoichiometry controls the competitive abilities of arbuscular mycorrhizal vs. root-borne non-mycorrhizal fungi

A majority of plant species has roots that are colonized by both arbuscular mycorrhizal (AM) and non-mycorrhizal (NM) fungi. The latter group may include plant mutualists, commensals, parasites and pathogens. The co-occurrence of these two broad groups may translate into competition for root volume as well as for plant-derived carbon (C). Here we provide evidence that the relative availability of soil nitrogen (N) and phosphorus (P) (i.e., soil nutrient stoichiometry) controls the competitive balance between these two fungal guilds. A decrease in the soil available N:P ratio resulted in a lower abundance of AM fungi and a corresponding increase in NM fungi. However, when the same fertilization treatments were applied in a soil in which AM fungi were absent, lowering the soil available N:P ratio did not affect NM fungal abundance. Taken collectively, our results suggest that the increase in NM fungal abundance was not a direct response to soil nutrient stoichiometry, but rather a competitive release from AM fungi responding negatively to higher soil P. We briefly discuss the mechanisms that may be responsible for this competitive release.