Influence of liming,inoculum level and inoculum placement on root colonization of subterranean clover

Arbuscular mycorrhizal (AM) fungi differ in their response to soil pH. Thus, change in soil pH may influence the relative abundance of mycorrhizal fungi inside roots. Root colonization by two AM fungi was studied in relation to addition of lime (CaCO3), quantity of inoculum and inoculum placement. Addition of CaCO3 to an acid soil decreased the colonization of roots by Acaulospora laevis but increased colonization by Glomus invermaium when both fungi were present. In acid soil (pH 4.7), almost all roots were colonized by A. laevis, while G. invermaium was dominant when soil pH was increased to pH 7.3. This occurred regardless of whether the inoculum was banded or mixed throughout the soil. There was no effect of CaCO3 on the relative abundance of fungi inside roots at intermediate rates of CaCO3 application (pH 5.3-6.3) when both fungi were inoculated together. In this experiment, both fungi colonized roots at all levels of CaCO3 when inoculated alone, except for A. laevis at the highest level of CaCO3. We conclude that soil pH affects the competitive ability of these two AM fungi during mycorrhiza formation primarily by affecting hyphae growth in soil and thus the relative abundance of hyphae at the root surface and subsequently inside the root.     

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.     

Slash Pile Burning Effects on Soil Biotic and Chemical Properties and Plant Establishment: Recommendations for Amelioration

Ponderosa pine forest restoration consists of thinning trees and reintroducing prescribed fire to reduce unnaturally high tree densities and fuel loads to restore ecosystem structure and function. A current issue in ponderosa pine restoration is what to do with the large quantity of slash that is created from thinning dense forest stands. Slash piling burning is currently the preferred method of slash removal because it allows land managers to burn large quantities of slash in a more controlled environment in comparison with broadcast burning slash. However burning slash piles is known to have adverse effects such as soil sterilization and exotic species establishment. This study investigated the effects of slash pile burning on soil biotic and chemical variables and early herbaceous succession on burned slash pile areas. Slash piles were created following tree thinning in two adjacent approximately 20‐ha ponderosa pine (Pinus ponderosa) restoration treatments in the Coconino National Forest near Flagstaff, Arizona. We selected 30 burned slash pile areas and sampled across a gradient of the burned piles for arbuscular mycorrhizal (AM) propagule densities, the soil seed bank, and soil chemical properties. In addition, we established five 1‐m² plots in each burned pile to quantify the effect of living soil (AM inoculum) and seeding amendments on early herbaceous succession in burned slash pile areas. The five treatments consisted of a control (no treatment), living soil (AM inoculum) amendment, sterilized soil (no AM inoculum) amendment, seed amendment, and a seed/soil (AM inoculum) amendment. Slash pile burning nearly eliminated populations of viable seeds and AM propagules and altered soil chemical properties. Amending scars with native seeds increased the cover of native forbs and grasses. Furthermore adding both seed and living soil more than doubled total native plant cover and decreased ruderal and exotic plant cover. These results indicate that seed/soil amendments that increase native forbs and grasses may enhance the rate of succession in burned slash pile areas by allowing these species to outcompete exotic and ruderal species also establishing at the site through natural regeneration.     

High effectiveness of exotic arbuscular mycorrhizal fungi is reflected in improved rhizobial symbiosis and trehalose turnover in Cajanus cajan genotypes grown under salinity stress

Arbuscular mycorrhizal fungi (AMF) improve functioning of legume-Rhizobium symbiosis under salinity. However, plant responses to mycorrhization vary depending on the plant and fungal species. The current study aimed to compare the effectiveness of a native inoculum from saline soil and two exotic isolates, Funneliformis mosseae and Rhizophagus irregularis on two Cajanus cajan (pigeonpea) genotypes (Paras, Pusa 2002) subjected to NaCl stress. Salinity depleted nodulation and nutrient status in both genotypes with higher negative effects in Paras. Although all AM fungi improved growth, R. irregularis performed better by promoting higher biomass accumulation, nodulation, N₂ fixation and N, P uptake which correlated with higher AM colonization. R. irregularis inoculated plants also accumulated higher trehalose in nodules due to decreased trehalase and increased trehalose-6-P synthase, trehalose-6-phosphatase activities. The results suggest that higher stability of R. irregularis-pigeonpea symbiosis under salt stress makes it an effective ameliorator for overcoming salt stress in pigeonpea.     

Field response of wheat to arbuscular mycorrhizal fungi and drought stress

Mycorrhizal plants often have greater tolerance to drought than nonmycorrhizal plants. This study was conducted to determine the effects of arbuscular mycorrhizal (AM) fungi inoculation on growth, grain yield and mineral acquisition of two winter wheat (Triticum aestivum L.) cultivars grown in the field under well-watered and water-stressed conditions. Wheat seeds were planted in furrows after treatment with or without the AM fungi Glomus mosseae or G. etunicatum. Roots were sampled at four growth stages (leaf, tillering, heading and grain-filling) to quantify AM fungi. There was negligible AM fungi colonization during winter months following seeding (leaf sampling in February), when soil temperature was low. During the spring, AM fungi colonization increased gradually. Mycorrhizal colonization was higher in well-watered plants colonized with AM fungi isolates than water-stressed plants. Plants inoculated with G. etunicatum generally had higher colonization than plants colonized with G. mosseae under both soil moisture conditions. Biomass and grain yields were higher in mycorrhizal than nonmycorrhizal plots irrespective of soil moisture, and G. etunicatum inoculated plants generally had higher biomass and grain yields than those colonized by G. mosseae under either soil moisture condition. The mycorrhizal plants had higher shoot P and Fe concentrations than nonmycorrhizal plants at all samplings regardless of soil moisture conditions. The improved growth, yield and nutrient uptake in wheat plants reported here demonstrate the potential of mycorrhizal inoculation to reduce the effects of drought stress on wheat grown under field conditions in semiarid areas of the world.     

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.     

Influence of commercial inoculation with Glomus intraradices on the structure and functioning of an AM fungal community from an agricultural site

The use of commercial arbuscular mycorrhizal (AM) inoculants is growing. However, we know little about how resident AM communities respond to inoculations under different soil management conditions. The objective of this study was to simulate the application of a commercial AM fungal inoculant of Glomus intraradices to soil to determine whether the structure and functioning of that soil’s resident AM community would be affected. The effects of inoculation were investigated over time under disturbed or undisturbed soil conditions. We predicted that the introduction of an infective AM fungus, such as G. intraradices, would have greater consequences in disturbed soil. Using a combination of molecular (terminal restriction length polymorphism analysis based on the large subunit of the rRNA gene) and classical methods (AM fungal root colonization and P nutrition) we found that, contrary to our prediction, adding inoculant to soil containing a resident AM fungal community does not necessarily have an impact on the structure of that community either under disturbed or undisturbed conditions. However, we found evidence of positive effects of inoculation on plant nutrition under disturbed conditions, suggesting that the inoculant interacted, directly or indirectly, with the resident AM fungi. The inoculant significantly improved the P content of the host but only in presence of the resident AM fungal community. In contrast to inoculation, soil disturbance had a significant negative impact on species richness of AM fungi and influenced the AM fungal community composition as well as its functioning. Thus, we conclude that soil disturbance may under certain conditions have greater consequences for the structure of resident AM fungal communities in agricultural soils than commercial AM fungal inoculations with G. intraradices.     

Comparisons of Mycorrhizal Responsiveness with Field Soil and Commercial Inoculum for Six Native Montane Species and Bromus tectorum

Reestablishing native perennial plants and reducing invasive species are pivotal for many ecological restoration projects. The interactions among plant species, arbuscular mycorrhizal fungi, and soil P availability may be critical determinants of the success of native and non‐native plants in restoration and species invasions. Here we assessed mycorrhizal responsiveness for three late‐successional and three early‐successional plant species native to Rocky Mountain National Park and for the non‐native Downy brome, cheatgrass (Bromus tectorum L.) using field soil and commercial inoculum. Factorial greenhouse experiments were conducted to compare biomass of plant species with and without field soil and commercial inoculum treatments along a phosphorus (P) gradient, which ranged from ambient field levels to 12% of field levels, using dilutions of native soils. The two field soil inoculum treatments resulted in significant biomass differences for all species studied. Late‐successional species responded positively to field inoculum, whereas early‐successional species responded negatively. The two commercial inocula had low colonization rates (14 of 166 inoculated plants). The commercial inocula substrates had significant treatment effects on five of seven species included in the study in the apparent absence of mycorrhizal symbiosis. Soil P levels influenced mycorrhizal responsiveness in only one species, Smooth blue aster (Aster laevis L.). Our results show that, at least for the species studied here, locally collected field inoculum is the best choice for reestablishment of late‐successional native plant species.     

Disturbance influences the outcome of plant–soil biota interactions in the invasive Acacia longifolia and in native species

Interrelated causes of plant invasion have been gaining increasing recognition. However, research on this subject has mainly focused around conceptual models. Here we explore whether plant–soil biota feedbacks and disturbance, two major factors capable of facilitating invasive plants in introduced ranges, interact to preferentially benefit exotics compared to native plants. We investigated the influence of fire disturbance on plant–soil biota interactions for the invasive Acacia longifolia and two dominant natives (Cytisus striatus and Pinus pinaster) in Portuguese dune systems. In the first experiment, we grew exotic and native plants in soil inoculated with soil biota from unburned or recently burned soils collected in an area with small invasion intensity by A. longifolia. Soil biota effects on the exotic legume A. longifolia changed from neutral to positive after fire, whereas the opposite outcome was observed in the native legume C. striatus, and a change from negative to neutral effects after fire occurred in the native P. pinaster. Fire reduced mycorrhizal colonization in all species and rhizobial colonization in C. striatus but not in A. longifolia. In the second experiment, we grew the exotic and native plants with conspecific and heterospecific soil biota from undisturbed soils (area with low invasion intensity by A. longifolia), and from post‐fire soils (area affected by a fire ～12 years ago and currently heavily invaded by A. longifolia). The exotic benefited more from post‐fire than from undisturbed soil biota, particularly from those associated with natives. Natives did not experience detrimental effects with invasive‐associated soil biota. Our results show that fire disturbance affected the functional interactions between soil biota and plants that may benefit more the exotic than some native species. Disturbance may open a window of opportunity that promotes invader success by altering soil enemy and mutualistic impacts.     

Effects of inoculation with native arbuscular mycorrhizal fungi on clonal growth of Potentilla reptans and Fragaria moschata (Rosaceae)

Many clonal plants live in symbiosis with ubiquitous arbuscular mycorrhizal (AM) fungi, however, little is known about their interaction with respect to clonal reproduction and resource acquisition. The effects of arbuscular mycorrhiza on the growth and intraclonal integration between ramets of two stoloniferous species were studied experimentally in a nutritionally homogenous soil environment. Two species coexisting at the same field site, Potentilla reptans and Fragaria moschata, were selected as model plants for the study. Pairs of their ramets were grown in neighbouring pots with each ramet rooted separately. Four inoculation treatments were established: (1) both mother and daughter ramets remained non-inoculated, (2) both ramets were inoculated with a mixture of three native AM fungi from the site of plant origin, (3) only mother or (4) daughter ramet was inoculated. The stolons connecting the ramets were either left intact or were disrupted. Despite the consistent increase in phosphorus concentrations in inoculated plants, a negative growth response of both plant species to inoculation with AM fungi was observed and inoculated ramets produced fewer stolons and fewer offspring ramets and had lower total shoot dry weights as compared to non-inoculated ones. A difference in the extent of the negative mycorrhizal growth response was recorded between mother and daughter ramets of P. reptans, with daughter ramets being more susceptible. Due to AM effect on ramet performance, and thereby on the source-sink relationship, inoculation also significantly influenced biomass allocation within clonal fragments. Physiological integration between mother and daughter ramets was observed when their root systems were heterogeneous in terms of AM colonization. These results hence indicate the potential of mycorrhizal fungi to impact clonal growth traits of stoloniferous plant species, with possible consequences for their population dynamics.     

The foliar endophytic fungal community composition in Cirsium arvense is affected by mycorrhizal colonization and soil nutrient content

Foliar fungal endophytes are ubiquitous, but understudied symbionts of most plant species; relatively little is known about the factors affecting their occurrence, diversity and abundance. We tested the effects of soil nutrient content and arbuscular mycorrhizal (AM) colonization on the occurrence of foliar endophytic fungi in Cirsium arvense in two field studies. In the first study, we assessed relationships between soil moisture, organic matter, carbon and nitrogen content and plant water, nitrogen and carbon content and AM colonization and the occurrence of foliar endophytic fungal species. In the second study, we manipulated soil nutrient content and AM colonization of potted seedlings and identified differences in endophytic fungal species composition of the leaves and stems. The results reveal that endophytes can occur either more or less frequently, depending on soil nutrient and plant water content and AM colonization. We propose that these patterns were the result of differences in fungal growth responses to nutrient availability in the leaves, which can be affected by resources obtained from the soil or symbiotic fungi in the roots.     

The dark side of the mycorrhiza

Plant association with arbuscular mycorrhizal (AM) fungi is usually regarded as mutualistic. However, this positive effect could disappear if the benefit of the fungal-plant association changes with colonization density. In order to test the conditionality of this interaction, we evaluated plant performance and tolerance to defoliation across five levels of commercial AM fungal inoculum concentrations. Additionally, we evaluated if plant performance and tolerance were similarly affected by a whole soil community collected under a native congener. Along the gradient of inoculation, plant performance exhibited a peak at intermediate inoculum concentration, indicating the presence of an optimum level of AM fungal concentration that maximized AM fungal benefit. Root colonization by fungal hyphae increased linearly across the experimental inoculation gradient. Paralleling root colonization, plant tolerance to defoliation decreased linearly along the inoculum gradient. Plant performance was similar under the whole soil and commercial treatments. Our results show a negative correlation between tolerance to defoliation and AM fungal inoculum concentration, indicating that AM fungi colonization could constrain the evolution of plant tolerance to herbivory.Key words: compensation, defences, ecological interactions, herbivory, multitrophic interactions, mycorrhizal fungi, toleranceArbuscular mycorrhizal (AM) fungi occur in all ecosystems of the world and associate with the roots of about 70% of all vascular plants.¹ This association is typically regarded as mutualistic, because there is a bidirectional transfer of nutrients between the host plant and its fungal partners. Carbon compounds are passed from the plant to the fungus and, in return, there is a transfer of mineral nutrients, principally nitrate and phosphate.² However, this association also entails costs. The amount of carbon allocated to AM fungi is estimated to range from 4% to 20% of a plant''s total carbon budget.² Throughout the literature, there are examples of the conditionality of this relationship exemplified by a continuum of the effects of AM fungal colonization on hosts from positive, through null to negative.^3–5 Moreover, it has been suggested that the benefit of a plant associating with fungal symbionts depends not only on the identity of AM fungi⁴ and plant genotypes⁶ but also on hyphal colonization density in roots.⁷ In a recent greenhouse study, we examined components of the conditionality of plant interactions with soil biota.⁸ We were interested in knowing how the performance and tolerance to defoliation of the annual plant Datura stramonium varied along a concentration gradient of commercial AM fungal inoculum containing four Glomus species (Mycorrhizal Applications, Grants Pass, OR USA).We found a curvilinear relationship between AM fungal inoculum concentration and plant performance, as predicted by previous models.⁷ The quadratic decelerating function between inoculum concentration and plant performance indicates an optimum level of AM fungal concentration (1/24^th total pot volume) that maximizes AM fungal benefit (Fig. 1A). This result suggests that, in D. stramonium, positive associations between AM fungi and plant fitness may not be proportional and, that at high colonization densities, mycorrhizae may have detrimental effects, perhaps by competing with plants for nutrients, or by interfering with other essential interactions.^4,5 We also found, from root examination, that hyphal colonization of roots increased linearly with AM fungi inoculum concentration. Moreover, we found that tolerance to herbivory decreased linearly with increasing AM fungal inoculum concentration (r² = −0.40; F_1,27 = 5.89; p = 0.0222; Fig. 1B), suggesting that, in our system, at high densities, mycorrhizae may become parasitic and may compete for resources (e.g., carbon) with the induced host plant response to leaf damage.Open in a separate windowFigure 1Effect of a gradient in AM fungi inoculum concentration on D. stramonium performance. (A) Non-linear relation between seed production and inoculum concentration. In general, plants achieved their maximal performance at an inoculum concentration of 1/24^th total pot volume. (B) Tolerance to defoliation decreased linearly with inoculum concentration. Tolerance was calculated as the difference in standardized seed production between related damaged and undamaged genetically related plants corresponding to six genetic full-sib families.In order to know whether the effects we found in the greenhouse using commercial inoculum could be expected in the field, we addressed whether or not D. stramonium performance and tolerance were similarly influenced by whole soil field communities; including AM fungi, pathogens, root herbivores, etc. Unfortunately, D. stramonium is not native to the area where this research was undertaken, so we collected soil immediately below plants of a native congener Datura wrightii, a perennial herb that grows at the Putah Creek Reserve (UC, Davis). Pots were inoculated at a 1/12^th total pot volume with this live soil and plants were grown concurrently with those in the previous experiment. We compared plant performance and tolerance under the live soil treatment and the last level of the commercial AMF inoculum gradient (both inoculated at a 1/12^th total pot volume). Results indicated no differences in foliar area (F_1,94 = 1.18; p = 0.2782), root mass (F_1,94 = 0.99; p = 0.3222), flowering day (χ² = 0.31; p = 0.5804) and fitness (χ² = 0.03; p = 0.8691). Moreover, root colonization levels were (F_1,94 = 0.75; p = 0.3877) in both 1/12^th volume vs. live soil, as well as in the 0 AMF and sterilized soil (F_1,94 = 2.56; p = 0.1130). Despite these similarities, plant tolerance did differ significantly between AMF and live soil treatments (F_1,94 = 5.49; p = 0.0411), tolerance being greater under the live soil treatment (0.3755 ± 0.0311 tolerance) relative to the 1/12^th AM fungal treatment (−0.5744 ± 0.2714 tolerance). This result suggests that the expression of plant tolerance may also depend on the identity of AMF colonizing roots or the number and identities of soil bacteria. We did not know which microbial species were in the soils we collected.We show that, when inoculated over a gradient of abundance, Glomus AM fungal colonization consistently decreased tolerance to herbivory. The presence of mycorrhizae could therefore decrease the adaptive value of traits increasing tolerance. We also show here that though live soil inoculum had similar effects in magnitude and direction to those of commercial AMF incoculum on growth and fitness, live soil biota collected under a congener of D. stramonium increased tolerance to herbivory at the same levels of root colonization. Overall, the results of this study indicate that the interaction between soil biotic components and the response of D. stramonium to leaf damage is highly conditional; and can depend on amounts of root colonization, as well as perhaps identities of AM fungi and bacteria. In both cases, soil biota affected the impact of damage to leaves aboveground. AM fungi may mediate the efficacy of tolerance as a defense, and this effect may be especially important in light of herbivore adaptation, when tolerance may be favored over resistance as a plant defense strategy.¹⁰     

Field microplot performance of the peach-almond hybrid GF-677 after inoculation with arbuscular mycorrhizal fungi in a replant soil infested with root-knot nematodes

The effects of arbuscular mycorrhizae (AM) on the development and nutrition of the peach almond hybrid GF-677 rootstock in a replant soil heavily infested with Meloidogyne javanica were evaluated in field microplot conditions for two growing seasons. There was a significant beneficial effect of mycorrhizal inoculation on plant growth and nutrition in previously pasteurized replant soil. In natural replant soil, early inoculation with a mixed AM inoculum of Glomus intraradices, Glomus mosseae and Glomus etunicatum did not affect growth parameters. Whilst inoculation with these AM fungi led to suppression of root-knot nematode reproduction, natural mycorrhizal colonization of the replant soil with native AM fungi did not. Accepted: 6 December 2000     

Availability and function of arbuscular mycorrhizal and ectomycorrhizal fungi during revegetation of dewatered reservoirs left after dam removal

Revegetation following dam removal projects may depend on recovery of arbuscular mycorrhizal (AM) and ectomycorrhizal (EM) fungal communities, which perform valuable ecosystem functions. This study assessed the availability and function of AM and EM fungi for plants colonizing dewatered reservoirs following a dam removal project on the Elwha River, Olympic Peninsula, Washington, United States. Availability was assessed via AM fungal spore density in soils and EM root tip colonization of Salix sitchensis (Sitka willow) in an observational field study. The effect of mycorrhizal fungi from 4 sources (reservoir soils, commercial inoculum, and 2 mature plant community soils) on growth and nutrient status of S. sitchensis was quantified in a greenhouse study. AM fungal spores and EM root tips were present in all field samples. In the greenhouse, plants receiving reservoir soil inoculum had only incipient mantle formation, while plants receiving inoculum from mature plant communities had fully formed EM root tips. EM formation corresponded with alleviation of phosphorus stress in plants (lower shoot nitrogen:phosphorus). Thus, revegetating plants have access to AM and EM fungi following dam removal, and EM formation may be especially important for plant P uptake in reservoir soils. However, availability of mycorrhizal fungi declines with distance from established plant communities. Furthermore, EM fungal communities in recently dewatered reservoirs may not be as effective at forming beneficial mycorrhizae as those from mature plant communities. Whole soil inoculum from mature plant communities may be important for the success of revegetating plants and recovery of mycorrhizal fungal communities.     

干旱条件下AM真菌对植物生长和土壤水稳定性团聚体的影响

采用分室培养系统,模拟正常水分和干旱胁迫两种环境条件,探讨不同丛枝菌根真菌(arbuscular mycorrhizal fungi,AMF)对紫花苜蓿(Medicago sativa L.)生长和土壤水稳性团聚体的影响.试验条件下,Glomus intraradices对苜蓿根系的侵染率均显著高于Acaulospora scrobiculata和Diversispora spurcum接种处理.正常水分条件下,供试AM真菌均能显著提高植株生物量及磷浓度.干旱胁迫显著抑制了植株生长和菌根共生体发育,总体上菌根共生体对植株生长没有明显影响,接种D.spurcum甚至趋于降低植株生物量;同时,仅有G.intraradices显著提高了植株磷浓度.AM真菌主要影响到＞2mm的水稳性团聚体数量,以G.intraradices作用效果最为显著.在菌丝室中,G.intraradices显著提高了总球囊霉素含量.研究表明AM真菌对土壤大团聚体形成具有积极作用,而菌根效应因土壤水分条件和不同菌种而异,干旱胁迫下仅有G.intraradices对土壤结构和植物生长表现出显著积极作用.在应用菌根技术治理退化土壤时,需要选用抗逆性强共生效率高的菌株,对于不同AM真菌抗逆性差异的生物学与遗传学基础尚需进一步研究.     

Inoculation with arbuscular mycorrhizal fungi: the status quo in Japan and the future prospects

Inoculation of arbuscular mycorrhizal (AM) fungi has potential benefits in not only sustainable crop production but also environmental conservation. However, the difficulty of inoculum production due to the obligate biotrophic nature of AM fungi has been the biggest obstacle to putting inoculation into practice. Nevertheless, several companies have sought to produce inoculum of AM fungi. Firstly in this review, the present status of inoculum production and its use in Japan is described. Secondly, although the effectiveness of inoculation is primarily limited by environmental and biological factors, some possible ways to improve inoculation performance are discussed. Approaches include use of chemicals to increase spore germination and colonization and soil application of charcoal to provide a microhabitat for AM fungi to colonize and survive.     

Symbiosis of Arbuscular Mycorrhizal Fungi and Robinia pseudoacacia L. Improves Root Tensile Strength and Soil Aggregate Stability

Robinia pseudoacacia L. (black locust) is a widely planted tree species on Loess Plateau for revegetation. Due to its symbiosis forming capability with arbuscular mycorrhizal (AM) fungi, we explored the influence of arbuscular mycorrhizal fungi on plant biomass, root morphology, root tensile strength and soil aggregate stability in a pot experiment. We inoculated R. pseudoacacia with/without AM fungus (Rhizophagus irregularis or Glomus versiforme), and measured root colonization, plant growth, root morphological characters, root tensile force and tensile strength, and parameters for soil aggregate stability at twelve weeks after inoculation. AM fungi colonized more than 70% plant root, significantly improved plant growth. Meanwhile, AM fungi elevated root morphological parameters, root tensile force, root tensile strength, Glomalin-related soil protein (GRSP) content in soil, and parameters for soil aggregate stability such as water stable aggregate (WSA), mean weight diameter (MWD) and geometric mean diameter (GMD). Root length was highly correlated with WSA, MWD and GMD, while hyphae length was highly correlated with GRSP content. The improved R. pseudoacacia growth, root tensile strength and soil aggregate stability indicated that AM fungi could accelerate soil fixation and stabilization with R. pseudoacacia, and its function in revegetation on Loess Plateau deserves more attention.     

Relationship of seed source and arbuscular mycorrhizal fungi inoculum type to growth and colonization of big bluestem (Andropogon gerardii)

Big bluestem (Andropogon gerardii) is a key grass of tallgrass prairies and is commonly included in restoration projects. In many cases, it has been found to benefit significantly from arbuscular mycorrhizal (AM) fungi, however results have varied under non-sterile soil conditions. This study investigated the effects of two types of AM fungi inoculum (commercial and prairie) on growth and root colonization of big bluestem from five different seed sources grown in non-sterile soils. Seed sources were collected from five remnant prairies in the Tallgrass Prairie Peninsula located in the Midwestern United States. Growth of big bluestem and root length colonized by AM fungi was highly variable among seed source treatments. Overall, percentage of root length colonized by AM fungi was positively correlated with the total dry weight of plants, and plants that received inoculum generally grew better than those that did not receive inoculum. Inoculum treatment affected both big bluestem growth and percentage root length colonized and there was an interaction between seed source and inoculum treatment relative to colonization. Root colonization responses were not significantly different between the prairie and commercial inoculum types, although there was a significant response between plants that received additional inoculum as opposed to no additional inoculum. Seed sources from Ohio and Illinois had the highest biomasses and greatest percentage of root length colonized while plants from Wisconsin and Missouri grew poorly and had low root colonization. These results demonstrate the importance of considering both seed source and inoculum type before the incorporation of AM inoculum to prairie restoration projects.     

Evidence for mutualist limitation: the impacts of conspecific density on the mycorrhizal inoculum potential of woodland soils

The ability of seedlings to establish can depend on the availability of appropriate mycorrhizal fungal inoculum. The possibility that mycorrhizal mutualists limit the distribution of seedlings may depend on the prevalence of the plant hosts that form the same type of mycorrhizal association as the target seedling species and thus provide inoculum. We tested this hypothesis by measuring ectomycorrhizal (EM) fine root distribution and conducting an EM inoculum potential bioassay along a gradient of EM host density in a pinyon–juniper woodland where pinyon is the only EM fungal host while juniper and other plant species are hosts for arbuscular mycorrhizal (AM) fungi. We found that pinyon fine roots were significantly less abundant than juniper roots both in areas dominated aboveground by juniper and in areas where pinyon and juniper were co-dominant. Pinyon seedlings establishing in pinyon–juniper zones are thus more likely to encounter AM than EM fungi. Our bioassay confirmed this result. Pinyon seedlings were six times less likely to be colonized by EM fungi when grown in soil from juniper-dominated zones than in soil from either pinyon–juniper or pinyon zones. Levels of EM colonization were also reduced in seedlings grown in juniper-zone soil. Preliminary analyses indicate that EM community composition varied among sites. These results are important because recent droughts have caused massive mortality of mature pinyons resulting in a shift towards juniper-dominated stands. Lack of EM inoculum in these stands could reduce the ability of pinyon seedlings to re-colonize sites of high pinyon mortality, leading to long-term vegetation shifts.     

Arbuscular mycorrhizal soil infectivity and spores distribution across plantations of tropical,subtropical and exotic tree species: a case study from the forest reserve of Bandia,Senegal

Several fast‐growing and multipurpose trees such as exotic and valuable native species have been widely used in West Africa to reverse the tendency of massive degradation of plant cover and restore soil productivity. Although benefic effects have been reported on soil stabilization, a lack of information about their impact on soil symbiotic microorganisms still remains. This investigation has been carried out in field trees of 28 years old in a forest reserve at Bandia. To determine the mycorrhizal inoculum potential (MIP) of soils, a mycorrhizal bioassay was conducted using seedlings of Zea mays L. Spores concentration, arbuscular mycorrhizal (AM) fungi morphotypes and mycorrhizal colonization of field plants were examined. Results showed that fungal communities were dominated in all samples by the genus Glomus. Nevertheless, the others genera Gigaspora and Scutellospora occurred preferentially out of the plantations. The number and richness of spores as well as the MIP of soils were decreased in the tree plantations. Accordingly, the amount of annual herbaceous plants kept out of the tree plantations was much greater than those under the tree plantations. The colonization was higher in field root systems of herb plants in comparison with that of the tree plants. Comparisons allowed us to conclude that vegetation type modifies the AM fungal communities, and the results suggest further adoption of management practices that could improve or sustain the development of herbaceous layers and thus promote the AM fungal communities.