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.     

Mixing tree species associated with arbuscular or ectotrophic mycorrhizae reveals dual mycorrhization and interactive effects on the fungal partners

1. Recent studies found that the majority of shrub and tree species are associated with both arbuscular mycorrhizal (AM) and ectomycorrhizal (EM) fungi. However, our knowledge on how different mycorrhizal types interact with each other is still limited. We asked whether the combination of hosts with a preferred association with either AM or EM fungi increases the host tree roots’ mycorrhization rate and affects AM and EM fungal richness and community composition.
2. We established a tree diversity experiment, where five tree species of each of the two mycorrhiza types were planted in monocultures, two‐species and four‐species mixtures. We applied morphological assessment to estimate mycorrhization rates and next‐generation molecular sequencing to quantify mycobiont richness.
3. Both the morphological and molecular assessment revealed dual‐mycorrhizal colonization in 79% and 100% of the samples, respectively. OTU community composition strongly differed between AM and EM trees. While host tree species richness did not affect mycorrhization rates, we observed significant effects of mixing AM‐ and EM‐associated hosts in AM mycorrhization rate. Glomeromycota richness was larger in monotypic AM tree combinations than in AM‐EM mixtures, pointing to a dilution or suppression effect of AM by EM trees. We found a strong match between morphological quantification of AM mycorrhization rate and Glomeromycota richness.
4. Synthesis. We provide evidence that the combination of hosts differing in their preferred mycorrhiza association affects the host''s fungal community composition, thus revealing important biotic interactions among trees and their associated fungi.

     

Neighboring trees affect ectomycorrhizal fungal community composition in a woodland-forest ecotone

Ectomycorrhizal fungi (EMF) are frequently species rich and functionally diverse; yet, our knowledge of the environmental factors that influence local EMF diversity and species composition remains poor. In particular, little is known about the influence of neighboring plants on EMF community structure. We tested the hypothesis that the EMF of plants with heterospecific neighbors would differ in species richness and community composition from the EMF of plants with conspecific neighbors. We conducted our study at the ecotone between pinyon (Pinus edulis)-juniper (Juniperus monosperma) woodland and ponderosa pine (Pinus ponderosa) forest in northern Arizona, USA where the dominant trees formed associations with either EMF (P. edulis and P. ponderosa) or arbuscular mycorrhizal fungi (AMF; J. monosperma). We also compared the EMF communities of pinyon and ponderosa pines where their rhizospheres overlapped. The EMF community composition, but not species richness of pinyon pines was significantly influenced by neighboring AM juniper, but not by neighboring EM ponderosa pine. Ponderosa pine EMF communities were different in species composition when growing in association with pinyon pine than when growing in association with a conspecific. The EMF communities of pinyon and ponderosa pines were similar where their rhizospheres overlapped consisting of primarily the same species in similar relative abundance. Our findings suggest that neighboring tree species identity shaped EMF community structure, but that these effects were specific to host-neighbor combinations. The overlap in community composition between pinyon pine and ponderosa pine suggests that these tree species may serve as reservoirs of EMF inoculum for one another.     

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.     

Arbuscular mycorrhizal dynamics in a chronosequence of Caragana korshinskii plantations

Arbuscular mycorrhizal (AM) fungi in a chronosequence of 5–42-year-old Caragana korshinskii plantations in the semi-arid Loess Plateau region of northwestern China were investigated. AM fungi colonization, spore diversity and PCR-denatured gradient gel electrophoresis-based AM fungal SSU rRNA gene sequences were analyzed. AM fungi colonization [measured as the percent of root length (%RLC), vesicular (%VC) and arbuscular (%AC) colonization] and spore density were significantly correlated with sampling month, but not with plant age, except for %RLC. The percent of vesicular colonization was negatively correlated with soil total nitrogen and organic carbon, and spore density was negatively correlated with soil moisture and available phosphorus. Ten distinguishable AM fungal spore morphotypes, nine Glomus and one Scutellospora species, were found. Nine AM fungal Glomus phylotypes were identified by sequencing, but at each sampling time only four to six AM fungal phylotypes were detected. The AM fungal community was significantly seasonal, whereas the AM fungal species richness did not increase with plantation age. A significant change in AM fungal colonization and community composition over an annual cycle was observed in this study, and our results suggest that the changes of AM are the product of the interaction between host phenology, soil characteristics and habitat. Understanding these interactions is essential if habitat restoration is to be effective.     

干旱胁迫下AM真菌对油蒿叶片保护系统的影响

基利用盆栽试验在正常水分和干旱胁迫条件下研究了灭菌土接种AM真菌摩西球囊霉(Glomus mosseae)和土著AM真菌对油(蒿Artemisia ordosica)生长及叶片保护系统的影响。结果表明,干旱胁迫显著抑制了土著AM真菌对油蒿的侵染,但对G.mosseae的侵染影响较小。正常水分和干旱胁迫条件下,接种AM真菌显著增加了油蒿生物量和干重以及根系含磷量;提高了叶绿素、可溶性糖、可溶性蛋白含量并降低了脯氨酸和丙二醛含量;显著增强了过氧化氢酶(CAT)和过氧化物酶(POD)活性,增强了油蒿对干旱的防御能力。     

土著菌根真菌和混生植物对羊草生长和磷营养的影响

植物种间相互作用直接影响植物生长、根系可塑性及养分吸收,而与植物共生的丛枝菌根真菌可以改变植物个体和种间养分资源的分配,具有协调种间竞争的潜力.以我国北方草甸草原建群种羊草(Leymus chinensis)和混生植物紫花苜蓿(Medicago sativa)及独行菜(Lepidium spetalum)为供试植物,通过模拟盆栽试验,研究了土著菌根真菌和混生植物对羊草生长、根系形态及磷营养的影响.试验结果表明,土著菌根真菌能够与羊草及紫花苜蓿形成良好共生,而独行菜根内基本未形成菌根共生结构.土著菌根真菌显著降低了羊草及独行菜的生物量,但促进了紫花苜蓿的生长;混种紫花苜蓿显著促进了羊草的生长,而混种独行菜则显著抑制了羊草的生长.土著菌根真菌对羊草根系形态的影响表现出与植株生物量类似的趋势,但不同混生植物对羊草根系生长均无显著影响.土著菌根真菌和混生植物对羊草植株磷含量均无显著影响.与混生植物相比,羊草具有较高的比根长和磷吸收能力,这也解释了其负向菌根依赖性.研究证实了菌根真菌和植物种间相互作用均是影响草原优势植物生长和根系发育的重要因素,深入研究其交互作用对于科学管理草地生态系统,维持植物群落的稳定性和生态系统生产力具有重要意义.     

Three-way interactions among ectomycorrhizal mutualists, scale insects, and resistant and susceptible pinyon pines

Herbivores and mycorrhizal fungi are important associates of most plants, but little is known about how these organisms interact. In a 9-yr experiment, we examined how the pinyon needle scale (Matsucoccus acalyptus) affects and is affected by the ectomycorrhizal mutualists found on the roots of scale-resistant and -susceptible pinyon pines (Pinus edulis). Three major results emerged. First, removal experiments demonstrated that scales negatively affected ectomycorrhiza. Second, although ectomycorrhiza could either positively or negatively influence scale performance by improving plant vigor or increasing plant investment in antiherbivore defenses, we found no ectomycorrhizal effect on scale mortality when we experimentally enhanced levels of ectomycorrhiza. This represented the first test of whether ectomycorrhiza promote plant resistance and contrasted with studies showing that arbuscular mycorrhiza negatively affected herbivores. Third, pinyon resistance to scales mediated the asymmetrical interaction between fungal mutualists and scale herbivores. High scale densities suppressed ectomycorrhizal colonization, but only on trees susceptible to scales. Similarities between mycorrhiza-herbivore interactions and competitive interactions among herbivores suggest broader generalities in the way aboveground herbivores interact with belowground plant associates. However, because mycorrhiza are mutualists, mycorrhiza-herbivore interactions do not fit within traditional competition paradigms. The widespread occurrence and importance of both herbivores and mycorrhiza argue for incorporating their interactions into ecological theory.     

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.     

干旱胁迫下覆膜和接种微生物对玉米生长与水分利用效率的影响

针对西北干旱半干旱地区土壤贫瘠与水分缺乏的问题,利用微生物与作物形成互惠互利的共生关系,本研究设置两个水分梯度:干旱胁迫(供试土壤最大持水量的35%)和正常水分(供试土壤最大持水量的75%),两个覆膜方式:无覆膜(NM)和覆膜(FM),4个接种微生物水平:单接AM真菌(AM)、单接解磷细菌(PSB)、联合接种AM真菌与解磷细菌(AM+PSB)以及对照(CK),研究不同水分和覆膜条件下4个接种微生物对玉米生长特性、地上养分吸收与水分利用效率的影响.结果表明: 与正常水分处理相比,干旱胁迫能够显著提高接种AM真菌处理的侵染率,但正常水分处理下土壤根外菌丝密度、总球囊霉素(T-GRSP)与易提取球囊霉素(EE-GRSP)含量明显提高.干旱胁迫下,单接AM真菌处理的促生作用和菌根效应表现最好,能够提高玉米生物量、水分利用效率和土壤有机碳含量,促进土壤N、P、K的吸收与运输,从而增加玉米地上部分N、P、K吸收量;而正常水分下,联合接种AM+PSB处理表现要好于单接AM和PSB处理,且其与覆膜的互作效果最好.相关分析结果表明,玉米生物量、叶片SPAD值和地上部分N、P、K吸收量均与土壤根外菌丝密度呈显著正相关,玉米水分利用效率与其呈显著负相关.     

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.     

Environmental and genetic effects on the formation of ectomycorrhizal and arbuscular mycorrhizal associations in cottonwoods

Although both environment and genetics have been shown to affect the mycorrhizal colonization of host plants, the impacts of these factors on hosts that can be dually colonized by both ectomycorrhizal (EM) and arbuscular mycorrhizal (AM) fungi are less understood. We examined the influence of environment and host crosstype on the EM and AM colonization of cottonwoods (Populus angustifolia and natural hybrids) by comparing levels of colonization of trees growing in common gardens that differed in elevation and soil type. We also conducted a supplemental watering experiment to determine the influence of soil moisture on AM and EM colonization. Three patterns emerged. First, garden location had a significant impact on mycorrhizal colonization, such that EM colonization was 30% higher and AM colonization was 85% lower in the higher elevation garden than the lower elevation garden. Second, crosstype affected total (EM + AM) colonization, but did not affect EM or AM colonization. Similarly, a significant garden × crosstype interaction was found for total colonization, but not for EM or AM colonization. Third, experimental watering resulted in 33% higher EM colonization and 45% lower AM colonization, demonstrating that soil moisture was a major driver of the mycorrhizal differences observed between the gardens. We conclude that environment, particularly soil moisture, has a larger influence on colonization by AM versus EM fungi than host genetics, and suggest that environmental stress may be a major determinant of mycorrhizal colonization in dually colonized host plants.     

Exotic cheatgrass and loss of soil biota decrease the performance of a native grass

Soil disturbances can alter microbial communities including arbuscular mycorrhizal (AM) fungi, which may in turn, affect plant community structure and the abundance of exotic species. We hypothesized that altered soil microbial populations owing to disturbance would contribute to invasion by cheatgrass (Bromus tectorum), an exotic annual grass, at the expense of the native perennial grass, squirreltail (Elymus elymoides). Using a greenhouse experiment, we compared the responses of conspecific and heterospecific pairs of cheatgrass and squirreltail inoculated with soil (including live AM spores and other organisms) collected from fuel treatments with high, intermediate and no disturbance (pile burns, mastication, and intact woodlands) and a sterile control. Cheatgrass growth was unaffected by type of soil inoculum, whereas squirreltail growth, reproduction and nutrient uptake were higher in plants inoculated with soil from mastication and undisturbed treatments compared to pile burns and sterile controls. Squirreltail shoot biomass was positively correlated with AM colonization when inoculated with mastication and undisturbed soils, but not when inoculated with pile burn soils. In contrast, cheatgrass shoot biomass was negatively correlated with AM colonization, but this effect was less pronounced with pile burn inoculum. Cheatgrass had higher foliar N and P when grown with squirreltail compared to a conspecific, while squirreltail had lower foliar P, AM colonization and flower production when grown with cheatgrass. These results indicate that changes in AM communities resulting from high disturbance may favor exotic plant species that do not depend on mycorrhizal fungi, over native species that depend on particular taxa of AM fungi for growth and reproduction.     

Effect of drought and season on arbuscular mycorrhizal fungi in a subtropical secondary forest

Arbuscular mycorrhizal (AM) fungi in both soil and roots were examined in May (summer) and December (winter) under a 4-y drought experiment in a Chinese subtropical secondary forest. Drought significantly decreased AM fungal extra-radical hyphal density, spore density, and root colonization rate in both seasons. These AM parameters were significantly higher in summer than in winter in the control treatment, but only AM fungal extra-radical hyphal density exhibited the same seasonal trend in the drought treatment. In total, 45 AM fungal operational taxonomic units (OTUs) were obtained at a 97% sequence similarity level using Illumina sequencing of 18S rDNA. Drought and season had no significant effects on AM fungal OTU richness in soil and roots. AM fungal community composition in soil and roots was significantly affected by season but not by drought. This finding enhances our understanding of the response of AM fungi to global climate change in subtropical forest ecosystems.     

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.     

毛乌素沙地克隆植物沙鞭生长对AM真菌生态分布的影响

从毛乌素沙地沙鞭群落间隔空地沙鞭根状茎延伸方向设置样地,2007年5月、7月和10月在0-10、10-20、20-30、30-40和40-50 cm共5个土层分别采集土壤样品,研究了AM真菌时空分布及沙鞭侵入间隔空地前后对AM真菌分布的影响。结果表明,沙鞭能与AM真菌形成良好共生关系,菌根结构为中间型(I-型),AM真菌定殖率和孢子密度有明显的时空分布规律,并与土壤因子密切相关。随采样时间后延AM真菌定殖率先增后降,最大值出现在7月份;孢子密度先降后增,最大值出现在10月份。土壤采样深度对AM真菌定殖和孢子密度有显著影响,AM真菌最高定殖率和最大孢子密度均出现在0-20 cm浅土层。孢子密度仅与泡囊定殖率显著正相关。土壤速效N与菌丝和总定殖率极显著负相关;土壤温度与泡囊、丛枝、菌丝和总定殖率显著和极显著正相关,与孢子密度显著负相关;土壤湿度与丛枝、菌丝和总定殖率显著正相关,与孢子密度显著负相关。克隆植物沙鞭的生长对土壤AM真菌活动和分布有很大影响,沙鞭入侵前后样地中AM真菌孢子密度和定殖率变化显著。     

Shrub range expansion alters diversity and distribution of soil fungal communities across an alpine elevation gradient

Global climate and land use change are altering plant and soil microbial communities worldwide, particularly in arctic and alpine biomes where warming is accelerated. The widespread expansion of woody shrubs into historically herbaceous alpine plant zones is likely to interact with climate to affect soil microbial community structure and function; however, our understanding of alpine soil ecology remains limited. This study aimed to (i) determine whether the diversity and community composition of soil fungi vary across elevation gradients and to (ii) assess the impact of woody shrub expansion on these patterns. In the White Mountains of California, sagebrush (Artemisia rothrockii) shrubs have been expanding upwards into alpine areas since 1960. In this study, we combined observational field data with a manipulative shrub removal experiment along an elevation transect of alpine shrub expansion. We utilized next‐generation sequencing of the ITS1 region for fungi and joint distribution modelling to tease apart effects of the environment and intracommunity interactions on soil fungi. We found that soil fungal diversity declines and community composition changes with increasing elevation. Both abiotic factors (primarily soil moisture and soil organic C) and woody sagebrush range expansion had significant effects on these patterns. However, fungal diversity and relative abundance had high spatial variation, overwhelming the predictive power of vegetation type, elevation and abiotic soil conditions at the landscape scale. Finally, we observed positive and negative associations among fungal taxa which may be important in structuring community responses to global change.     

Growth response of Atriplex nummularia to inoculation with arbuscular mycorrhizal fungi at different salinity levels

Chenopods are generally regarded as non-host plants for mycorrhizal fungi and are believed not to benefit from colonization by mycorrhizal fungi. Perennial Atriplex nummularia Lindl., growing under field conditions, showed a relatively high level of colonization by mycorrhizal fungi (10–30% of root length colonized) in spring and summer. Accordingly, two glasshouse experiments were designed to assess the effects of inoculation with mycorrhizal fungi (with a single species or a mixture of different species) on growth, nutrient uptake, and rhizosphere bacterial community composition of A. nummularia at high and low salinity levels (2.2 and 12 dSm^–1). Only low and patchy colonization by mycorrhizal fungi (1–2 of root length colonized) was detected in inoculated plants under glasshouse conditions which was unaffected by salinity. Despite the low colonization, inoculation increased plant growth and affected nutrient uptake at both salinity levels. The effects were higher at an early stage of plant development (6weeks) than at a later stage (9–10 weeks). Salinity affected the bacterial community composition in the rhizosphere as examined by ribosomal intergenic spacer amplification (RISA) of 16S rDNA, digitization of the band patterns and multivariate analysis. The effects of inoculation with mycorrhizal fungi on growth of A. nummularia may be attributed to (i) direct effects of mycorrhizal fungi on plant nutrient uptake and/or (ii) indirect effects via mycorrhizal-induced changes in the bacterial community composition.     

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.     

Early Root Herbivory Impairs Arbuscular Mycorrhizal Fungal Colonization and Shifts Defence Allocation in Establishing Plantago lanceolata

Research into plant-mediated indirect interactions between arbuscular mycorrhizal (AM) fungi and insect herbivores has focussed on those between plant shoots and above-ground herbivores, despite the fact that only below-ground herbivores share the same part of the host plant as AM fungi. Using Plantago lanceolata L., we aimed to characterise how early root herbivory by the vine weevil (Otiorhynchus sulcatus F.) affected subsequent colonization by AM fungi (Glomus spp.) and determine how the two affected plant growth and defensive chemistry. We exposed four week old P. lanceolata to root herbivory and AM fungi using a 2×2 factorial design (and quantified subsequent effects on plant biomass and iridoid glycosides (IGs) concentrations. Otiorhynchus sulcatus reduced root growth by c. 64%, whereas plant growth was unaffected by AM fungi. Root herbivory reduced extent of AM fungal colonization (by c. 61%). O. sulcatus did not influence overall IG concentrations, but caused qualitative shifts in root and shoot IGs, specifically increasing the proportion of the more toxic catalpol. These changes may reflect defensive allocation in the plant against further attack. This study demonstrates that very early root herbivory during plant development can shape future patterns of AM fungal colonization and influence defensive allocation in the plant.