Effect of arbuscular mycorrhizal fungi, organic fertilizer and soil sterilization on maize growth ☆

The effects of inoculation with arbuscular mycorrhizal (AM) fungi, organic fertilizer (F) applications, and soil sterilization on maize growth were evaluated in a pot experiment. The experiment was in a completely randomized factorial design (2 × 4 × 2) with six replicates for each treatment. There were two soil treatments (sterilized soil, SS and unsterilized soil, US), four organic fertilizer treatments (0.0, 0.5, 1.0 and 2.0 g kg^-1 soil), and two AM fungi treatments (inoculation with Glomus mosseae, +AM and uninoculated control, －AM). Inoculated plants generally had greater AM colonization, plant height, dry weight and phosphorus (P) uptake than uninoculated controls, and these parameters were significantly increased as the organic fertilizer application increased up to 0.5 g kg^-1 but decreased or had no significant effect compared to the uninoculated plants at the highest fertilizer rate (2.0 g kg^-1). Plant growth, P uptake and AM colonization of root system were significantly higher in sterilized soil compared to the unsterilized control. Our results indicated that the inoculation of AM fungi in field soil with optimal organic fertilizer application greatly improved maize growth and nutrient uptake, and the effect was greater under sterilized soil condition.     

Isolation and sequence analysis of a β-tubulin gene from arbuscular mycorrhizal fungi

A full-length β-tubulin gene has been cloned and sequenced from Gigaspora gigantea and Glomus clarum, two arbuscular mycorrhizal fungi (AMF) species in the phylum Glomeromyota. The gene in both species is organized into five exons and four introns. Both genes are 94.9% similar and encode a 447 amino acid protein. In comparison with other fungal groups, the amino acid sequence is most similar to that of fungi in the Chytridiomycota. The codon usage of the gene in both AMF species is broad and biased in favor of an A or a T in the third position. The four introns varied in length from 87 to 168 bp for G. gigantea and from 90 to 136 bp for G. clarum. Of all fungi in which full-length sequences have been published, only AMF do not have an intron before codon 174. The introns positioned at codons 174 and 257 in AMF match the position of different introns in β-tubulin genes of some Zygomycete, Basidiomycete, and Ascomycete fungi. The 5′ and 3′ splice site consensus sequences are similar to those found in introns of most fungi. Sequence analysis from single-strand conformation polymorphism analysis confirmed the presence of two β-tubulin gene copies in G. clarum, but only one copy was evident in G. gigantea based on Southern hybridization analysis.     

Do arbuscular mycorrhizal fungi recolonize revegetated grasslands?

Fifteen native and common exotic herbaceous species from four functional groups (C4 grass, C3 grass, chamaephyte and hemicryptophyte) occurring within remnant and revegetated grassland and grassy woodlands were sampled for evidence of structures associated with functioning arbuscular mycorrhizal fungi (AMF) from across a broad geographical range of central and south‐western Victoria, Australia. Revegetated communities had been established on ex‐agricultural land by direct seeding. They included sites that had been kept fallow with herbicide for up to 3 years prior to seeding and those from which topsoil had been removed (scalped) to a depth of 100 mm prior to seeding. Structures associated with AMF (external and internal aseptate hyphae, arbuscules and vesicles) were observed in root samples from all native and exotic species, regardless of site history (remnant or revegetated; fallowed or scalped). These findings indicate that AMF are ubiquitous in the herbaceous flora of this region (native and exotic), even in situations where sites had been intensively disturbed prior to revegetation treatment. However, while there was evidence of AMF in all revegetated communities, only sites which had been scalped prior to direct seeding supported species‐rich native herbaceous communities.     

Do arbuscular mycorrhizal fungi play a role in the ability of rare plant species to colonize abandoned fields?

While some plant species colonize abandoned agricultural fields and dry grasslands with similar frequency (generalists), others are absent or underrepresented in abandoned fields (specialists). We tested if inoculation with dry grassland or abandoned field soil could improve specialist performance in an abandoned field and compared the effects of inoculation in the stage of sown seeds and transplanted seedlings. Arbuscular mycorrhizal fungi from abandoned field had higher root colonization potential. This could explain the higher performance of the sown specialists inoculated with the abandoned field inoculum compared to those inoculated with dry grassland inoculum. This difference disappeared when specialists were transplanted instead of sown. The results do not provide any support for higher performance of specialists inoculated with dry grassland inoculum. Transplantation, however, seems to be an efficient way to introduce specialists into the abandoned fields.     

Does responsiveness to arbuscular mycorrhizal fungi depend on plant invasive status?

Differences in the direction and degree to which invasive alien and native plants are influenced by mycorrhizal associations could indicate a general mechanism of plant invasion, but whether or not such differences exist is unclear. Here, we tested whether mycorrhizal responsiveness varies by plant invasive status while controlling for phylogenetic relatedness among plants with two large grassland datasets. Mycorrhizal responsiveness was measured for 68 taxa from the Northern Plains, and data for 95 taxa from the Central Plains were included. Nineteen percent of taxa from the Northern Plains had greater total biomass with mycorrhizas while 61% of taxa from the Central Plains responded positively. For the Northern Plains taxa, measurable effects often depended on the response variable (i.e., total biomass, shoot biomass, and root mass ratio) suggesting varied resource allocation strategies when roots are colonized by arbuscular mycorrhizal fungi. In both datasets, invasive status was nonrandomly distributed on the phylogeny. Invasive taxa were mainly from two clades, that is, Poaceae and Asteraceae families. In contrast, mycorrhizal responsiveness was randomly distributed over the phylogeny for taxa from the Northern Plains, but nonrandomly distributed for taxa from the Central Plains. After controlling for phylogenetic similarity, we found no evidence that invasive taxa responded differently to mycorrhizas than other taxa. Although it is possible that mycorrhizal responsiveness contributes to invasiveness in particular species, we find no evidence that invasiveness in general is associated with the degree of mycorrhizal responsiveness. However, mycorrhizal responsiveness among species grown under common conditions was highly variable, and more work is needed to determine the causes of this variation.     

Differential RNA accumulation of two β-tubulin genes in arbuscular mycorrhizal fungi

RNA was isolated from spores of different arbuscular mycorrhizal (AM) fungi and used for RT-PCR with degenerate primers for beta-tubulin genes. PCR products were cloned and the sequence of several clones was analysed for each fragment. Comparison of sequences identified two loci for beta-tubulin genes with different GC content and codon usage. Btub1 sequences were most similar to beta-tubulin genes from the Oomycota, while Btub2 sequences showed highest similarity to sequences from the Zygomycota. RT-PCR experiments were carried out to monitor RNA accumulation patterns of Btub1 and Btub2 in asymbiotic germinating spores and in symbiotic extraradical hyphae of three different AM fungi. This indicated that Btub1 is constitutively expressed in Gigaspora rosea, but down-regulated during symbiosis in Glomus mosseae and Glomus intraradices. In contrast, Btub2 showed constitutive expression in the two Glomus species, but down-regulation in G. rosea. Further analysis of different fungi indicated that Btub2 primers could be used to specifically monitor RNA accumulation of AM fungi in environmental samples.     

Are there benefits of simultaneous root colonization by different arbuscular mycorrhizal fungi?

Arbuscular mycorrhizal fungal (AMF) communities were established in pots using fungal isolates from a single field in Switzerland. It was tested whether multispecies mixtures provided more phosphorus and supported greater plant growth than single AMF species. Two host plants, medic (Medicago truncatula) and leek (Allium porrum), were inoculated with three AMF species (Glomus mosseae, G. claroideum and G. intraradices), either separately or in mixtures. The composition of the AMF communities in the roots was assessed using real-time PCR to determine the copy number of large ribosomal subunit genes. Fungal communities in the roots were usually dominated by one AMF species (G. mosseae). The composition of the communities depended on both plant identity and the time of harvest. Leek colonized by a mixture of G. claroideum and G. intraradices acquired more P than with either of the two AMF separately. Direct evidence is provided for functional complementarity among species within the AMF community colonizing a single root system. Competition among the species poses a major challenge in interpreting experiments with mixed inoculations, but this is greatly facilitated by use of real-time PCR.     

The role of in vitro cultivation on asymbiotic trait variation in a single species of arbuscular mycorrhizal fungus

Cultivating arbuscular mycorrhizal (AM) fungi in vitro is an efficient way to produce material for industry and research. However, such artificial growing conditions may impose selective pressure on fungi grown in vitro over many generations. We hypothesized that isolates subjected to long term propagation in vitro may develop increasingly ruderal traits. We proposed a predictive framework for the effect of in vitro cultivation on asymbiotic AM fungal traits. Using photomicrography and image processing, we analyzed morphology and growth traits for 14 isolates representing an in vitro cultivation gradient from 0 to >80 generations in vitro. We investigated the range of trait variation among asymbiotic growth of arbuscular mycorrhizal (AM) fungus isolates (Rhizoglomus irregulare). Spore dormancy was strongly associated with in vitro cultivation. We observed extremely high levels of inter-isolate variation for most fungal traits, but this was not related to time in vitro. Our results indicate that intra-specific diversity may have a strong ecological role in AM fungal communities.     

Is resource allocation and grain yield of rice altered by inoculation with arbuscular mycorrhizal fungi?

Aims Our study quantified the combined effects of fertilization and inoculation with arbuscular mycorrhizal fungi (AMF) on grain yield and allocation of biomass and nutrients in field-grown rice (Oryza sativa L.).Methods A two-factor experiment was conducted at a field site in northeast of China (in Shuangcheng, Heilongjiang Province, Songhua River basin): six nitrogen–phosphorus–potassium fertilizer levels were provided (0, 20, 40, 60, 80 and 100% of the local norm of fertilizer supply), with or without inoculation with Glomus mosseae. At maturity, we quantified the percentage of root length colonization by AMF, grain yield, shoot:root ratios, shoot N and P contents and nutrients allocated to panicles, leaves and stems.Important findings As expected, inoculation resulted in greatly increased AMF colonization, which in turn led to higher shoot:root ratios and greater shoot N contents. Shoot:root ratios of inoculated rice increased with increasing fertilization while there was a significant interaction between fertilization and inoculation on shoot:root ratio. Additionally, AMF inoculation increased panicle:shoot ratios, panicle N:shoot N ratios and panicle P:shoot P ratios, especially in plants grown at low fertilizer levels. Importantly, inoculated rice exhibited higher grain yield, with the maximum improvement (near 62%) at the lower fertilizer end. Our results showed that (i) AMF-inoculated plants conform to the functional equilibrium theory, albeit to a reduced extent compared to non-inoculated plants and (ii) AMF inoculation resulted in greater allocation of shoot biomass to panicles and increased grain yield by stimulating N and P redistribution to panicles.     

What is the role of arbuscular mycorrhizal fungi in plant-to-ecosystem responses to Elevated atmospheric CO2?

 We advocate the concept of an arbuscular mycorrhiza (AM) as a temporally and spatially complex symbiosis representing a suite of hosts and fungi, as against the more traditional "dual organism" view. We use the hierarchical framework presented in Fig. 1 as a basis for organizing many unanswered questions, and several questions that have not been asked, concerning the role of AM in responses to elevated atmospheric CO₂. We include the following levels: plant host, plant population, plant community, functional group and ecosystem. Measurements of the contributions of AM fungi at the various levels require the use of different response variables. For example, hyphal nutrient translocation rates or percent AM root infection may be important measures at the individual plant level, but hyphal biomass or glomalin production and turnover are more relevant at the ecosystem level. There is a discrepancy between our knowledge of the multifaceted role of AM fungi in plant and ecosystem ecology and most of the current research aimed at elucidating the importance of this symbiosis in global-change scenarios. Our framework for more integrated and multifactorial research on mycorrhizal involvement in regulating CO₂ responses may also serve to enhance communication between researchers working at different scales on large global-change ecosystem projects. Accepted: 12 February 1999     

Effects of single and mixed inoculation with two arbuscular mycorrhizal fungi in two different levels of phosphorus supply on β-carotene concentrations in sweet potato (Ipomoea batatas L.) tubers

Aims

This study aimed to determine the effect of arbuscular mycorrhizal (AM) fungi and phosphorus (P) supply levels on β-carotene concentrations in sweet potato (Ipomoea batatas L.) tubers.

Methods

Two commercial AM fungal isolates of Glomus intraradices (IFP Glintra) and Glomus mosseae (IFP Glm) which differ in their life cycles were used. Sweet potato plants were grown in a horizontal split-root system that consisted of two root compartments. A root-free fungal compartment that allowed the quantification of mycelial development was inserted into each root compartment. The two root compartments were inoculated either with the same or with different AM isolates, or remained free of mycorrhizal propagules. Each fungal treatment was carried out in two P supply levels.

Results

In the low P supply level, mycorrhizal colonization significantly increased β-carotene concentrations in sweet potato tubers compared with the non-mycorrhizal plants. Glomus intraradices appeared to be more efficient in increasing β-carotene concentrations than G. mosseae. Dual inoculation of the root system with the two mycorrhizal fungi did not result in a higher increase in tuber β-carotene concentrations than inoculation with the single isolates. Improved P nutrition led to higher plant tuber biomass but was not associated with increased β-carotene concentrations.

Conclusions

The results indicate a remarkable potential of mycorrhizal fungi to improve β-carotene concentrations in sweet potato tubers in low P fertilized soils. These results also suggest that β-carotene metabolism in sweet potato tubers might be specifically activated by root mycorrhizal colonization.     

Impact of growth and uptake patterns of arbuscular mycorrhizal fungi on plant phosphorus uptake—a modelling study

In this paper we present a mathematical model for estimating external mycelium growth of arbuscular mycorrhizal fungi and its effect on root uptake of phosphate (P). The model describes P transport in soil and P uptake by both root and fungi on the single root scale. We investigate differences in soil P depletion and overall P influx into a mycorrhizal root by assuming that different spatial regions of mycelia are active in P uptake. When all external hyphae contribute to P uptake, overall uptake is dominated by the fungus and the most effective growth pattern appears to be the one using a high level of anastomosis. The same is true when only the proportion of external hyphae assumed to be active contributes to uptake. When uptake is restricted to the tips, hyphal contribution to overall P uptake is less dominant; the most effective growth pattern appears to be the one characterised by nonlinear branching where branching stops at a given maximal hyphal tip density. Comparison to measured P depletion in the literature suggests that the scenario where active hyphae are contributing to P uptake is likely to fit the data best. These quantitative predictions promote our understanding of the mycorrhizal symbiosis and its role in plant P nutrition.     

When do arbuscular mycorrhizal fungi protect plant roots from pathogens?

Arbuscular mycorrhizal (AM) fungi are mainly thought to facilitate phosphorus uptake in plants, but they can also perform several other functions that are equally beneficial. Our recent study sheds light on the factors determining one such function, enhanced plant protection from root pathogens. Root infection by the fungal pathogen Fusarium oxysporum was determined by both plant susceptibility and the ability of an AM fungal partner to suppress the pathogen. The non-susceptible plant species (Allium cepa) had limited F. oxysporum infection even without AM fungi. In contrast, the susceptible plant species (Setaria glauca) was heavily infected and only AM fungi in the family Glomeraceae limited pathogen abundance. Plant susceptibility to pathogens was likely determined by contrasting root architectures between plants, with the simple rooted plant (A. cepa) presenting fewer sites for infection. AM fungal colonization, however, was not limited in the same way in part because plants with fewer, simple roots are more mycorrhizal dependent. Protection only by Glomus species also indicates that whatever the mechanism(s) of this function, it responds to AM fungal families differently. While poor at pathogen protection, AM fungal species in the family Gigasporaceae most benefited the growth of the simple rooted plant species. Our research indicates that plant trait differences, such as root architecture can determine how important each mycorrhizal function is to plant growth but the ability to provide these functions differs among AM fungi.Key words: arbuscular mycorrhizal fungi, Fusarium oxysporum, root architecture, pathogen protection, multi-functionalityArbuscular mycorrhizas (AM) represent the oldest and most widespread symbiosis with land plants.¹ Most mycorrhizal research has focused on the ability of AM fungi to facilitate nutrient uptake, particularly phosphorus.² Although researchers recognize that AM fungi are multi-functional,³ it is not clear what factors determine which function an AM fungus performs or its relative importance to the plant.⁴ Newsham et al. (1995)³ hypothesized that AM function is based on root architecture: plants with simple rooting systems are dependent on mycorrhizas for nutrient uptake, while those with complex root systems are less dependent on mycorrhizas for nutrient uptake, but are more susceptible to root pathogens because of increased numbers of infections sites.³ These two functions, phosphorus uptake and enhanced pathogen protection from mycorrhizas also depend on the identity of the fungus. Arbuscular mycorrhizal fungi in the family Gigasporaceae are more effective at enhancing plant phosphorus, while AM fungi in the Glomeraceae better protect plants from root pathogens.⁵Our results support both plant and fungal control of a common pathogen, Fusarium oxysporum, and the interaction between these two factors ultimately determined the level of pathogen infection and plant mycorrhizal benefit. We inoculated two plant species that have contrasting root architectures with one of six AM fungal species from two families (or no AM fungi). After five months of growth, plants were inoculated with F. oxysporum, grown for another month and then harvested. All plant seeds and fungi were collected in a local old field community.⁶ Allium cepa (garden onion) was not susceptible to F. oxysporum likely because it has only a few adventitious roots below the main bulb that do not present many sites for infection. In contrast, Setaria glauca (yellow foxtail) was heavily infected by F. oxysporum and has fine roots with increased numbers of branching points and lateral meristems where fungi can colonize.⁷ For the susceptible plant (S. glauca), AM fungal species from the family Glomeraceae were effective at reducing pathogen abundance while species from the Gigasporaceae were not. Forming a symbiosis with a Glomus species resulted in S. glauca plants that were as large as control plants. AM fungal species from the family Gigaspoaceae were more beneficial to growth of the simple rooted A. cepa, which had fewer roots to take up soil nutrients.Reduced rooting structures may limit pathogen infection sites, but AM fungal colonization was not limited in the same way and may actually alter plant root architecture. While the simple rooted A. cepa had limited pathogen susceptibility, it had twice the AM fungal colonization of the complex rooted S. glauca. Because the simple rooted plant has a greater dependence on mycorrhizas,⁸ it likely transmits chemical signals to rapidly initiate mycorrhizal formation,⁹ but then may have less control on the spread of AM fungi within the root. In contrast, S. glauca is more susceptible to fungal pathogens and may be less mycorrhizal dependent in nature.¹⁰ As a result, S. glauca may treat all colonizing root fungi as potential parasites. Colonization by AM fungi from the Glomeraceae was also much greater than those in the Gigasporaceae due to differences in fungal life history strategy between these families.^11,12 AM fungal colonization can reduce root branching in plants and alter plant allocation to roots, thereby increasing mycorrhizal dependence for nutrients^10,13 and potentially reducing pathogen infection sites. Mycorrhizal induced changes to plant root architecture may therefore reinforce current mycorrhizal associations and alter future fungal colonization attempts.¹⁴ An important next step is to test if AM fungal families (or species) alter plant root architecture in different ways and the degree to which these effects depend on colonization timing and the plant host.Our study did not isolate the particular mechanism by which AM fungi control pathogens, but this mechanism clearly differentiates between AM fungal families. AM fungi can control pathogens through several mechanisms including direct competition for colonization sites, indirect initiation of plant defensive responses or altering other rhizosphere biota.¹⁵ Although these AM fungal families differ in the intensity of root colonization,¹¹ percentage of root length colonized by an AM fungus is a poor predictor of pathogen limitation compared to family identity,^12,16 suggesting that direct competition for space is unlikely. AM fungi share many cell surface molecules with pathogenic fungi like Fusarium.¹⁷ These molecules can act as signals that initiate plant production of defensive compounds such as phytoalexins, phenolics and other compounds.¹⁸ While AM fungi appear to evade these defenses, only AM fungal species in the family Glomeraceae would have elicited plant responses which altered future infection by F. oxysporum. AM fungi in the Gigasporaceae may differ more from F. oxysporum in their chemical signals or not colonize roots sufficiently to induce a sustained, system-wide plant response. In addition, many rhizosphere related microbes are antagonistic to pathogenic fungi¹⁵ and may differ in their response to the different AM fungal families.¹⁹ Because rhizosphere microbes also differ among plant species, plant pathogen protection may be influenced by multiple ecological interactions that determine the specific cases when mycorrhizal pathogen protection occurs. To distinguish between these mechanisms, future experiments could test whether biochemical similarity or ecological similarity (especially with other soil biota) between an AM fungus and fungal pathogen can predict mycorrhizal induced pathogen protection.Plant and fungal identity clearly affect AM fungal function and benefit, but to accurately use AM fungi in agriculture and restoration^20,21 we must clearly understand how functional mechanisms differ. Different mycorrhizal functions may be based on common plant traits like root architecture, but ecology, colonization timing and environment may alter the specific function AM fungi provide and its importance to plants. While it may be useful to establish greenhouse rules about which fungal species perform specific mycorrhizal functions, predicting their role in more complex systems relies on understanding if other factors will enhance or negate these effects. Most AM fungal species vary in their ability to perform each function and these can be locally adapted to limiting soil nutrients.²² In plants, there is also a range to which specific mycorrhizal functions may benefit plant fitness, and these responses are based on both plant traits (which change throughout a plant''s life cycle) and the local environment.^23,24 Given this variation, it is critical to understand if AM fungi can respond to cues from the plant or the environment to identify what factors limit plant growth and whether a the most effective AM fungus shows a greater response.     

Do arbuscular mycorrhizal fungi affect the allometric partition of host plant biomass to shoots and roots? A meta-analysis of studies from 1990 to 2010

Arbuscular mycorrhizas (AM) are ubiquitous root symbioses with often pervasive effects on the plant host, one of which may be above- and belowground biomass allocation. A meta-analysis was conducted on 516 trials that were described in 90 available articles to examine whether AM colonization could result in a modification of partitioning of plant biomass in shoots and roots. It was hypothesized that alleviating plant nutrient limitations could result in a decrease of root to shoot (R/S) ratio in AM plants or, alternatively, the direction of shifts in the R/S ratio would be determined by the changes in total dry biomass. In our analysis, we considered four types of stresses: drought stress, single heavy metal stress, multiple heavy metal stress, and other potential abiotic plant stress factors. When disregarding any factors that could regulate effects, including stress status and mode of propagation, the overall AM effect was a significant modification of biomass towards shoot growth. However, the responses of stressed and clonally propagated plants differed from those of seed-grown unstressed plants. Our meta-analysis detected a considerable decline in the R/S ratio when plants were grown from seeds in the absence of abiotic stresses. Moreover, we demonstrate that additional regulators of the AM-mediated impact on R/S ratio were presence of competition from other plants, plant growth outcome of the symbiosis, growth substrate volume, experimental duration, and the identities of both plant and AM fungus. Our results indicate that a prediction of AM effects on R/S allocation becomes more accurate when considering regulators, most notably propagation mode and stress. We discuss possible mechanisms through which stress and other regulators may operate.     

Effects of heavy metal (Pb) on arbuscular mycorrhizal fungi in vitro

The responses of Ri-TDNA-transformed roots and arbuscular mycorrhizal fungi established on Ri-TDNA-transformed roots to lead-amended media was investigated in vitro. At exposure to increasing concentrations of lead (2–10 mg/l[ppm]), three Ri-TDNA-transformed root clones viz., Swa, Swb and Swc, exhibited profuse growth. At exposure to increasing concentrations of lead (0.1–5 ppm), the dual cultures of Ri-TDNA-transformed roots and arbuscular mycorrhizal fungi., Glomus lamellosum/Swa, Glomus intraradices/Swb and Glomus proliferum/Swc, exhibited tolerance to 5 ppm of lead. When subjected to one physiological stress, either exposure to Pb or inoculation with AM fungi, Ri-TDNA-transformed root clones exuded more phenols in the growth medium than retained in the roots. When subjected to dual physiological stress, mycorrhizal Ri-TDNA-transformed roots growing on Pb-enriched medium, the total phenol content increased in the roots and exudation into the medium decreased.     

The carbon origin of arbuscular mycorrhizal fungi estimated from δ13C values of individual spores

 The origin of carbon in the spores of arbuscular mycorrhizal (AM) fungi was quantified based on their obligate symbiosis with C₃ and C₄ plants showing clearly different δ¹³C values. The δ¹³C values of individual spores of the AM fungus Gigaspora margarita were analyzed. In monoculture pots of a C₃ or a C₄ plant species, spore δ¹³C values were ca. 3.5‰ lower than those of host roots. In coculture pots of a C₃ and a C₄ plant species, spore δ¹³C values varied between those of the roots of C₃ and C₄ plants, and increased linearly from the C₃ to the proximity of the C₄ plant (P<0.01). This reflects the higher δ¹³C values in C₄ plants than in C₃ plants. Thus the carbon origin of G. margarita spores changed with growth state and combination of host plants. In the presence of fresh plant residue instead of living host plants, spore δ¹³C values did not vary with distance from the residue. This finding supports the current view that AM fungi are obligate symbionts. Accepted: 12 February 1999     

Multiple factors influence the role of arbuscular mycorrhizal fungi in soil aggregation—a meta-analysis

Background and aims

Soil aggregation is a crucial aspect of ecosystem functioning in terrestrial ecosystems. Arbuscular mycorrhizal fungi (AMF) play a key role in soil aggregate formation and stabilization. Here we quantitatively analyzed the importance of experimental settings as well as biotic and abiotic factors for the effectiveness of AMF to stabilize soil macroaggregates.

Methods

We gathered 35 studies on AMF and soil aggregation and tested 13 predictor variables for their relevance with a boosted regression tree analysis and performed a meta-analysis, fitting individual random effects models for each variable.

Results and conclusions

The overall mean effect of inoculation with AMF on soil aggregation was positive and predictor variable means were all in the range of beneficial effects. Pot studies and studies with sterilized sandy soil, near neutral soil pH, a pot size smaller than 2.5 kg and a duration between 2.2 and 5 months were more likely to result in stronger effects of AMF on soil aggregation than experiments in the field, with non-sterilized or fine textured soil or an acidic pH. This is the first study to quantitatively show that the effect of AMF inoculation on soil aggregation is positive and context dependent. Our findings can help to improve the use of this important ecosystem process, e.g. for inoculum application in restoration sites.     

Is there a role for arbuscular mycorrhizal fungi in production agriculture?

This review presents the point of view that arbuscular mycorrhizal fungi (AMF) do not play a vital role in the nutrition and growth of plants in many production-orientated agricultural systems. Highly available soil P often limits AM colonisation and causes the C-costs to the host to outweigh any benefits from colonisation. Even when P availability is low and AM colonisation levels are high, as may occur in organic and biodynamic agricultural systems, AMF may not always contribute to plant growth for reasons not yet understood. AM fungal activity may also be greatly limited by soil fumigation, non-responsive plant varieties, or rotations based primarily on non-mycorrhizal crops or crops of low AM dependency. Thus, profitability may sometimes be enhanced by management practices, such as tillage and P-fertilisation, which limit AM colonisation. Manipulation of agricultural systems to favour AMF must occur only if there is clear evidence that AMF make a positive contribution to yield or are vital for maintenance of ecosystem health and sustainability. A crucial role for AMF in soil structural stability or in enhancing micronutrient concentrations in produce may be sufficient evidence and may eventually compel consideration of AMF responsiveness when breeding new crop varieties.