Crop residue and Collembola interact to determine the growth of mycorrhizal pea plants

The effects of collembolan grazing on arbuscular mycorrhizal (AM) fungi and plant growth were studied in a controlled experiment utilizing a mix of AM fungi and the dominant collembolan species (Isotoma sp.) indigenous to the experimental soil. Collembolan (+/– Col) effects were examined in the presence and absence of crop residue (+/– Litter) incorporated into the experimental soil. Significant interactions between collembolans and crop residue occurred for mycorrhizal colonization of roots and plant growth. In the absence of crop residue, collembolans reduced root length colonized by AM fungi, total plant dry mass and seed pod yield. However, in the presence of crop residue, collembolans had no effect on root colonization by AM fungi, and increased total plant mass and pod yield. Crop residue increased root colonization by AM fungi, numbers of bacteria and saprophytic fungi (colony forming units), small- (<5 m) and large- (>5 m) diameter hyphal lengths in soil, and the final population of collembolans in soil. Collembolans reduced both small- and large-diameter hyphae in soil and the number of saprophytic fungi (colony forming units, p =0.052). Feeding preference experiments conducted in vitro showed that Isotoma sp. preferred to graze on mycorrhizal roots over nonmycorrhizal roots when given no other food choice. However, when crop residue was added as a food choice, Isotoma sp. showed a clear feeding preference for crop residue. We conclude that collembolan grazing on mycorrhizae can be detrimental to plant growth when other fungal food sources are limited, but grazing on mycorrhizal fungi does not occur when ample organic matter and associated saprophytic fungi are present in soils.     

Can the arbuscular mycorrhizal fungus Glomus intraradices actively mobilize P from rock phosphates?

Plants colonized by arbuscular mycorrhizal (AM) fungi have been shown to respond positively to the application of insoluble forms of inorganic phosphorus (P) such as rock phosphates (RPs). The mechanism(s) underlying such responses remain(s) unknown and although it has been hypothesized, there is no experimental support for the production of chelating agents by AM fungal hyphae. Here we investigate whether AM fungi can solubilize P from RPs and transfer it to plant roots. Using root-organ cultures of Daucus carrota L. inoculated or not with Glomus intraradices Schenk & Smith and containing P from different RP sources, we predicted that: (1) roots inoculated with G. intraradices would take up more P than those uninoculated; that (2) the amount of P taken up by roots through G. intraradices would be positively correlated with the RP reactivity; and that (3) G. intraradices would have access to RP through localized alterations of pH and/or by the production of organic acid anions that may act as chelating agents. The RP reactivity was positively correlated with P uptake. However, mycorrhizal roots grew initially slower and did not respond differently to any P treatment than those uninoculated. There was no evidence of localized changes in pH in proximity of G. intraradices hyphae, indicating that responses to RP by mycorrhizal plants observed in previous studies do not appear to result from the release of H⁺ ions alone or in combination with organic acid anions.     

Nitrogen capture by grapevine roots and arbuscular mycorrhizal fungi from legume cover-crop residues under low rates of mineral fertilization

The influence of mineral fertilization on root uptake and arbuscular mycorrhizal fungi-mediated ¹⁵N capture from labeled legume (Medicago polymorpha) residue was examined in winegrapes (Vitis vinifera) in the greenhouse, to evaluate compatibility of fertilization with incorporation of cover-crop residue in winegrape production. Plants grown in marginal vineyard soil were either fertilized with 0.25× Hoagland’s solution or not. This low fertilization rate represents the deficit management approach typical of winegrape production. Access to residue in a separate compartment was controlled to allow mycorrhizal roots (roots + hyphae), hyphae (hyphae-intact), or neither (hyphae-rotated) to proliferate in the residue by means of mesh core treatments. Leaves were weekly analyzed for ¹⁵N. On day 42, plants were analyzed for ¹⁵N and biomass; roots were examined for intraradical colonization; and soils were analyzed for ¹⁵N, inorganic N, Olsen-P, X-K, and extraradical colonization. As expected, extraradical colonization of soil outside the cores was unaffected by mesh core treatment, while that inside the cores varied significantly. ¹⁵N atom% excess was highest in leaves of roots + hyphae. In comparison, leaf ¹⁵N atom% excess in hyphae-intact was consistently intermediate between roots + hyphae and hyphae-rotated, the latter of which remained unchanged over time. Fertilization stimulated host and fungal growth, based on higher biomass and intraradical colonization of fertilized plants. Fertilization did not affect hyphal or root proliferation in residue but did lower %N derived from residue in leaves and stems by 50%. Our results suggest that even low fertilization rates decrease grapevine N uptake from legume crop residue by both extraradical hyphae and roots.     

Influence of phenamiphos on the vesicular-arbuscular mycorrhizal symbiosis in Leucaena leucocephala

Summary Concentrations of phenamiphos ranging from 0 to 40 rg/g soil were established in a typical Oxisol (Tropeptic Eutrustox), inoculated or uninoculated with Glomus aggregatum. The effect of the nematicide on the development of vesicular-arbuscular mycorrhizal (VAM) symbiosis was evaluated in the greenhouse using Leucaena leucocephala as an indicator host plant. Treatment of soil with phenamiphos did not have a significant influence on the development of mycorrhizal activity measured in terms of subleaflet phosphorus concentrations. Similarly, the nematicide did not have an adverse effect on the level of mycorrhizal colonization or on the P content of shoots, as determined at the time of harvest. However, shoot dry weight was adversely influenced by phenamiphos when the chemical was applied to the uninoculated soil at 20 g/g soil or higher, and when it was applied to the inoculated soil at 40 g/g soil. It is concluded that phenamiphos is not likely to influence the growth of Leucaena or its symbiotic association with VAM fungi if the concentrations applied do not exceed levels known to suppress nematodes.Hawaii Institute of Tropical Agriculture and Human Resources Journal series No. 3146     

Intraradical hyphae phosphatase of the arbuscular mycorrhizal fungus, Gigaspora margarita

An alkaline phosphatase in the intraradical hyphae of arbuscular mycorrhizal fungi was found to be closely related to an improvement of plant growth. To detect the phosphatase activity in a crude extract of mycorrhizal roots, phosphatase isozymes in mycorrhizal and non-mycorrhizal onion roots were compared with those in Gigaspora margarita by electrophoresis. A mycorrhiza-specific band was found when the phosphatase was stained under alkaline conditions. To clarify the origin of this phosphatase, the phosphatase extracted from intraradical hyphae was also compared with the phosphatase from mycorrhizal roots by electrophoresis. The intraradical hyphae was isolated from mycorrhizal roots by enzyme digestion followed by Percoll gradient centrifugation. The soluble protein was extracted from the hyphae by ultra-sonication after treatment with chitinase. A phosphatase in the hyphal soluble protein showed a similar, but slightly higher, relative mobility on the gel, compared with the mycorrhiza-specific phosphatase from roots. By adding the hyphal extract to the root extract, the relative mobility of the mycorrhiza-specific phosphatase was slightly changed and became identical to that of the phosphatase in the hyphae. This indicated that the specific band of phosphatase found in the crude extract from mycorrhizal roots was of intraradical hyphal origin. Received: 16 April 1997     

Influence of arbuscular mycorrhizal fungi and salinity on growth,biomass, and mineral nutrition of<Emphasis Type="Italic"> Acacia auriculiformis</Emphasis>

The effect of salinity on the efficacy of two arbuscular mycorrhizal fungi, Glomus fasciculatum and G. macrocarpum, alone and in combination was investigated on growth, development and nutrition of Acacia auriculiformis. Plants were grown under different salinity levels imposed by 0.3, 0.5 and 1.0 S m^-1 solutions of 1 M NaCl. Both mycorrhizal fungi protected the host plant against the detrimental effect of salinity. The extent of AM response on growth as well as root colonization varied with fungal species, and with the level of salinity. Maximum root colonization and spore production was observed with combined inoculation, which resulted in greater plant growth at all salinity levels. AM fungal inoculated plants showed significantly higher root and shoot weights. Greater nutrient acquisition, changes in root morphology, and electrical conductivity of soil in response to AM colonization was observed, and may be possible mechanisms to protect plants from salt stress.     

Density-dependent grazing on the extraradical hyphal network of the arbuscular mycorrhizal fungus, Glomus intraradices, by the collembolan, Folsomia candida

The effect of microarthropod density on the relative palatibility of selected microfungi and on the integrity of extraradical arbuscular-mycorrhizal (AM) hyphal networks was examined. A series of microcosm experiments were conducted using two conidial fungi (Alternaria alternata and Trichoderma harzianum), one AM fungus (Glomus intraradices) and the microarthropod Folsomia candida (Collembola). We did not detect a significant effect of athropod density on food preference. Food preferences for conidial over mycorrhizal hyphae were consistent across a wide range of collembolan densities. Collembolan density did, however, have a significant effect on the numbers of extraradical AM hyphae which were severed from the plant root in vitro. Even in the presence of a preferred food source, numbers of severed AM hyphal networks increased with increasing collembolan density. Also, the fecundity of Folsomia candida increased in the presence of preferred food sources. We conclude that microarthropod population densities are influenced by the fungal species available and that the potential of microarthropods to impair the efficacy of AM symbioses is density-dependent. Received: 16 May 1997     

A mycorrhizal fungus grows on biochar and captures phosphorus from its surfaces

Biochar application to soils has potential to simultaneously improve soil fertility and store carbon to aid climate change mitigation. While many studies have shown positive effects on plant yields, much less is known about the synergies between biochar and plant growth promoting microbes, such as mycorrhizal fungi. We present the first evidence that arbuscular mycorrhizal (AM) fungi can use biochar as a physical growth matrix and nutrient source. We used monoxenic cultures of the AM fungus Rhizophagus irregularis in symbiosis with carrot roots. Using scanning electron microscopy we observed that AM fungal hyphae grow on and into two contrasting types of biochar particles, strongly attaching to inner and outer surfaces. Loading a nutrient-poor biochar surface with nutrients stimulated hyphal colonization. We labeled biochar surfaces with ³³P radiotracer and found that hyphal contact to the biochar surfaces permitted uptake of ³³P and its subsequent translocation to the associated host roots. Direct access of fungal hyphae to biochar surfaces resulted in six times more ³³P translocation to the host roots than in systems where a mesh prevented hyphal contact with the biochar.We conclude that AM fungal hyphae access microsites within biochar, that are too small for most plant roots to enter (<10 μm), and can hence mediate plant phosphorus uptake from the biochar. Thus, combined management of biochar and AM fungi could contribute to sustainable soil and climate management by providing both a carbon-stable nutrient reservoir and a symbiont that facilitates nutrient uptake from it.     

Organic acid exudation by mycorrhizal Andropogon virginicus L. (broomsedge) roots in response to aluminum

Elevated aluminum (Al) availability limits plant growth on acidic soils. Although this element is found naturally in soils, acidic conditions create an environment where Al solubility increases and toxic forms of Al impact plant function. Plant resistance to Al is often attributed to organic acid exudation from plant roots and the chelation of cationic Al in the rhizosphere. The association of arbuscular mycorrhizal (AM) fungi with the roots of plants may alleviate Al toxicity by altering soil Al availability or plant exposure through the binding of Al to fungal structures or through the influence of fungi on exudation from roots. Diverse communities of AM fungi are found in soil ecosystems and research suggests that AM fungi exhibit functional diversity that may influence plant performance under varying edaphic environments. In the present study, we evaluated acidic isolates of six AM species in their responses to Al. Andropogon virginicus (broomsedge), a warm-season grass that commonly grows in a range of stressful environments including acidic soils, was used as a plant host for Acaulospora morrowiae, Glomus claroideum, Glomus clarum, Glomus etunicatum, Paraglomus brasilianum, and Scutellospora heterogama. Fungal spores were germinated and exposed to 0 or 100 μM Al on filter paper in sand culture or were grown and exposed to Al in sand culture in association with A. virginicus. Short- and long-term responses to Al were evaluated using direct measurements of fungal spore germination, hyphal elongation, and measurements of A. virginicus colonization and plant growth as a phytometer of AM function in symbio. Spore germination and hyphal elongation varied among AM species in response to Al, but patterns were not consistent with the influences of these AM species on A. virginicus under Al exposure. Exposure to Al did not influence colonization of roots, although large differences existed in colonization among fungal species. Plants colonized by G. clarum and S. heterogama exhibited the least reduction in growth when exposed to Al, produced the highest concentrations of Al-chelating organic acids, and had the lowest concentrations of free Al in their root zones. This pattern provides evidence that variation among AM fungi in Al resistance conferred to their plant hosts is associated with the exudation of Al-binding organic acids from roots and highlights the role that AM fungal diversity may play in plant performance in acidic soil environments.     

Arbuscular mycorrhizal fungi-mediated nitrogen transfer from vineyard cover crops to grapevines

Cover crops are often planted in between vineyard rows to reduce soil erosion, increase soil fertility, and improve soil structure. Roots of both grapevines and cover crops form mutualistic symbioses with arbuscular mycorrhizal (AM) fungi, and may be interconnected by AM hyphae. To study nutrient transfer from cover crops to grapevines through AM fungal links, we grew grapevines and cover crops in specially designed containers in the greenhouse that restricted their root systems to separate compartments, but allowed AM fungi to colonize both root systems. Leaves of two cover crops, a grass (Bromus hordeaceus) and a legume (Medicago polymorpha), were labeled with 99 atom% ¹⁵N solution for 24 h. Grapevine leaves were analyzed for ¹⁵N content 2, 5, and 10 days after labeling. Our results showed evidence of AM fungi-mediated ¹⁵N transfer from cover crops to grapevines 5 and 10 days after labeling. N transfer was significantly greater from the grass to the grapevine than from the legume to the grapevine. Possible reasons for the differences between the two cover crops include lower ¹⁵N enrichment in legume roots, higher biomass of grass roots, and/or differences in AM fungal community composition. Further studies are needed to investigate N transfer from grapevines to cover crops and to determine net N transfer between the two crops throughout their growing seasons, in order to understand the significance of AM fungi-mediated interplant nutrient transfers in the field.     

Rhodotorulic acid enhances root colonization of tomato plants by arbuscular mycorrhizal (AM) fungi due to its stimulatory effect on the pre-symbiotic stages of the AM fungi

Exudates of Rhodotorula mucilaginosa, a yeast commonly found in the rhizosphere, increased hyphal length of the arbuscular mycorrhizal (AM) fungi Gigaspora rosea and Gigaspora margarita. Rhodotorulic acid (RA), a siderophore compound obtained from R. mucilaginosa exudates, increased hyphal length and branching. Thus, the increase in the number of entry points and the higher AM root colonization of tomato plants in the presence of RA can at least partially be explained by the positive effect of RA on the pre-symbiotic stages of the AM fungi.     

Effectiveness of two arbuscular mycorrhizal fungi on concentrations of essential oil and artemisinin in three accessions of Artemisia annua L.

A field experiment was conducted to study and compare the effectiveness of two arbuscular mycorrhizal fungi (AMF), Glomus macrocarpum (GM) and Glomus fasciculatum (GF) on three accessions of Artemisia annua. The AM inoculation significantly increased the production of herbage, dry weight of shoot, nutrient status (P, Zn and Fe) of shoot, concentration of essential oil and artemisinin in leaves as compared to non-inoculated plants. The extent of growth, nutrient concentration and production of secondary plant metabolites varied with the fungus–plant accession combination. The mycorrhizal dependency of the three accessions was related to the shoot: root ratio. Comparing the two fungal inoculants in regard to increase in essential oil concentration in shoot, the effectiveness of GF was more than that of GM. While in two accessions, GM was more effective in enhancing artemisinin concentration than GF. Increase in concentration of essential oil was found to be positively correlated to P-status of the plant. Conversely, no correlation was found between shoot-P and artemisinin concentration.     

Changes in rhizosphere microbial activity mediated by native or allochthonous AM fungi in the reafforestation of a Mediterranean degraded environment

This study was carried out in a semiarid degraded area to assess the effectiveness of mycorrhizal inoculation with a mixture of native arbuscular mycorrhizal (AM) fungi or an allochthonous AM fungus (Glomus claroideum), on the establishment of Olea europaea subsp. sylvestris L. and Retama sphaerocarpa (L.) Boissier in this area. Associated changes in the soil microbiological properties and aggregate stability related to these AM inocula were also recorded. Eighteen months after planting, G. claroideum had increased available P in the rhizosphere of both shrub species. In general, both inoculation treatments increased water-soluble C and water-soluble and total carbohydrates, G. claroideum being the most effective inoculum, particularly in R. sphaerocarpa. The mixture of native AM fungi was the most effective treatment for increasing the aggregate stability of R. sphaerocarpa soil, while that of O. europaea was increased only by G. claroideum. Increased (dehydrogenase, urease, protease-BAA, acid phosphatase and -glucosidase) enzyme activities, in particular of dehydrogenase and acid phosphatase, were recorded in the rhizosphere of both mycorrhizal shrub species. The mixture of native AM fungi was the most effective treatment for stimulating the growth of O. europaea and R. sphaerocarpa (11.6-fold and 3.3-fold, respectively, greater than control plants). The establishment of mycorrhizal shrub species favoured the reactivation of soil microbial activity, which was linked to an increase in aggregate stability.     

Pre-inoculation with arbuscular mycorrhizal fungi suppresses root knot nematode (<Emphasis Type="Italic">Meloidogyne incognita</Emphasis>) on cucumber (<Emphasis Type="Italic">Cucumis sativus</Emphasis>)

A pot experiment was conducted to evaluate the influence of pre-inoculation of cucumber plants with each of the three arbuscular mycorrhizal (AM) fungi Glomus intraradices, Glomus mosseae, and Glomus versiforme on reproduction of the root knot nematode Meloidogyne incognita. All three AM fungi tested significantly reduced the root galling index, which is the percentage of total roots forming galls. Numbers of galls per root system were significantly reduced only in the G. intraradices + M. incognita treatment. The number of eggs per root system was significantly decreased by AM fungus inoculation, no significant difference among the three AM fungal isolates. AM inoculation substantially decreased the number of females, the number of eggs g⁻¹ root and of the number of eggs per egg mass. The number of egg masses g⁻¹ root was greatly reduced by inoculation with G. mosseae or G. versiforme. By considering plant growth, nutrient uptake, and the suppression of M. incognita together, G. mosseae and G. versiforme were more effective than G. intraradices.     

Inter- and intraspecific feeding selectivity of Folsomia candida (Willem) (Collembola,Isotomidae) on fungi

Summary Selective grazing of fungi by soil microarthropods may affect decomposition rates of litter materials and the structure of microarthropod and fungal communities. We developed laboratory methods to assay feeding selectivity and investigated the preferences of the collembolan Folsomia candida on three fungi: Acremonium sp., Paecilomyces varioti, and Penicillium citrinum. F. candida showed stronger preference for Acremonium sp. than for P. varioti and P. citrinum. Oviposition site selection followed the same pattern. Actively metabolizing hyphae of Acremonium sp. and P. varioti were preferred over senescent hyphae, while spores of P. citrinum were preferred over active hyphae. If microarthropod preference for active hyphae is extensive, microarthropod regulation of decomposition could be more important than their biomass indicates. Furthermore, as the P. citrinum results indicate, mechanisms of microbial dissemination may include selective grazing.Contribution in part to International Symposium on Faunal Influences on Soil Structure, Edmonton, Alberta, Canada (11–13 June 1984), organized by W. B. McGill; and the annual meeting of the Ecological Society of America, Minneapolis, Minnesota (17–21 June 1985)     

Differential production of oxalate by mycorrhizal fungi in arid ecosystems

Oxalate crystals and elements binding to the surfaces of mycorrhizal fungal hyphae were examined using scanning electron microscopy coupled with X-ray analysis of elemental composition. Mycorrhizae from the arid zone vegetation types in southern California were examined including chaparral, riparian oak woodlands, coastal sage, grasslands, and deserts. Only mat-forming ectomycorrhizal hyphae, such as Hysterangium separabile, were found to produce oxalate crystals. None of the arbuscular mycorrhizal fungal hyphae (Glomus spp. and Acaulospora elegans) examined had crystal structures associated with them. The hyphae of Hysterangium separabile without crystals did not show the Ca peaks that were present when the crystals existed nor did the arbuscular mycorrhizal fungal hyphae have the Ca peaks. The elimination of arbuscular mycorrhizae using benomyl did not affect soil P or oxalate. These data indicate that there are some fundamental differences in chemical exudation between mycorrhizal fungi that could affect P uptake and cycling in arid ecosystems. Received: 7 December 1994     

External phosphorus requirement of mycorrhizal and non-mycorrhizal barley and soybean plants

An experiment was conducted under greenhouse conditions to evaluate the effects of vesicular arbuscular mycorrhizal (VAM) fungi on the external P requirements of barley and soybeans. The plants were grown in pots containing a P-deficient soil. A range of 10 P levels was obtained by adding 0, 20, 30, 40, 50, 60, 70, 110, 160, or 310 mg P kg^-1 as NaH₂PO₄ 2H₂O. Half of the pots were inoculated with the VAM fungus Glomus intraradices. The P concentration in the soil solution was determined using an adsorption isotherm and plotted against the relative yield. Barley did not respond to mycorrhizal inoculation and we concluded that P nutrition was not the limiting factor on the growth of this lowmycotrophic plant. In contrast, mycorrhizal inoculation stimulated the growth of soybeans. The external P requirements were 0.110 g ml^-1 for mycorrhizal and 0.148 g ml^-1 for non-mycorrhizal soybeans to obtain 80% of the maximum yield. In terms of P fertilization this corresponds to a saving of 222 kg P₂O₅ ha^-1. The mycorrhizal dependency of the soybean was highly correlated with the P concentration in the soil solution and it is proposed that both values should be displayed together.     

不同氮磷比例营养液对AM真菌生长发育的影响

温室条件下,研究不同氮磷比例营养液对丛枝菌根(Arbuscular.mycorrhizae,AM)真菌(Glomus.mosseae)生长发育的影响。结果表明:在一定磷水平条件下增加氮的水平,有利于菌根真菌的生长发育,但超过一定水平则会抑制菌根真菌的生长发育,说明氮水平也会影响菌根真菌的生长发育,这种影响与氮磷比例有关。综合比较菌根长度、根外菌丝量及孢子数三项指标,在20%浓度Hoagland营养液的基础上,将氮磷比例提高到4∶2有利真菌生长。在AM菌剂生产中,通过营养液中氮磷比例的调控能获得较大数量的侵染根段、菌丝及孢子等繁殖体。     

Changes in soil aggregation and glomalin-related soil protein content as affected by the arbuscular mycorrhizal fungal species Glomus mosseae and Glomus intraradices

Arbuscular mycorrhizal (AM) fungi are key organisms of the soil/plant system, influencing soil fertility and plant nutrition, and contributing to soil aggregation and soil structure stability by the combined action of extraradical hyphae and of an insoluble, hydrophobic proteinaceous substance named glomalin-related soil protein (GRSP). Since the GRSP extraction procedures have recently revealed problems related to co-extracting substances, the relationship between GRSP and AM fungi still remains to be verified. In this work the hypothesis that GRSP concentration is positively correlated with the occurrence of AM fungi was tested by using Medicago sativa plants inoculated with different isolates of Glomus mosseae and Glomus intraradices in a microcosm experiment. Our results show that (i) mycorrhizal establishment produced an increase in GRSP concentration - compared to initial values - in contrast with non-mycorrhizal plants, which did not produce any change; (ii) aggregate stability, evaluated as mean weight diameter (MWD) of macroaggregates of 1-2 mm diameter, was significantly higher in mycorrhizal soils compared to non-mycorrhizal soil; (iii) GRSP concentration and soil aggregate stability were positively correlated with mycorrhizal root volume and weakly correlated with total root volume; (iv) MWD values of soil aggregates were positively correlated with values of total hyphal length and hyphal density of the AM fungi utilized.The different ability of AM fungal isolates to affect GRSP concentration and to form extensive and dense mycelial networks, which may directly affect soil aggregates stability by hyphal enmeshment of soil particles, suggests the possibility of selecting the most efficient isolates to be utilized for soil quality improvement and land restoration programs.     

Diversity of arbuscular mycorrhizal fungi in red tangerine (Citrus reticulata Blanco) rootstock rhizospheric soils from hillside citrus orchards

Root colonization, abundance of spores and hyphae, as well as species diversity of arbuscular mycorrhizal (AM) fungi were analyzed in citrus orchards along an altitudinal gradient. The citrus trees were heavily colonized (50.87–77.45%) by native AM fungi. In citrus orchards located at <600 m above sea level (asl), we recorded more extensive hyphal and arbuscular colonization, and higher spore and hyphal length density. AM fungal colonization, spore density, and hyphal length density were closely correlated with edaphic factors such as available phosphorus, pH, and organic matter. A total of 18 AM fungal species belonging to 3 different orders, Archaeosporales (1 species), Diversisporales (7 species) and Glomerales (10 species), were identified on the basis of spore morphological characteristics. In orchards located at higher altitudes (≥700 m asl), we observed a significant decrease in species richness and Shannon–Wiener index values. However, in all of the surveyed orchards, Glomus aggregatum, Funneliformis mosseae and Rhizophagus intraradices were the dominant species. Isolate frequency and relative abundance of AM fungi exhibited clearly distinct distribution patterns among taxonomic families. Canonical correspondence analysis revealed that the AM fungal community structure was significantly influenced by environmental factors, especially altitude, pH, soil moisture, and available nitrogen. Our data indicated that environmental factors are important in determining AM fungal root colonization, propagule numbers, and species diversity in citrus orchards.