Dynamics of vesicular-arbuscular mycorrhizae during old field succession

Summary The species composition of vesicular-arbuscular mycorrhizal (VAM) fungal communities changed during secondary succession of abandoned fields based on a field to forest chronosequence. Twenty-five VAM fungal species were identified. Seven species were clearly early successional and five species were clearly late successional. The total number of VAM fungal species did not increase with successional time, but diversity as measured by the Shannon-Wiener index tended to increase, primarily because the community became more even as a single species, Glomus aggregatum, became less dominant in the older sites. Diversity of the VAM fungal community was positively correlated with soil C and N. The density of VAM fungi, as measured by infectivity and total spore count, first increased with time since abandonment and then decreased in the late successional forest sites. Within 12 abandoned fields, VAM fungal density increased with increasing soil pH, H₂O soluble soil C, and root biomass, but was inversely related to extractable soil P and percent cover of non-host plant species. The lower abundance of VAM fungi in the forest sites compared with the field sites agrees with the findings of other workers and corresponds with a shift in the dominant vegetation from herbaceous VAM hosts to woody ectomycorrhizal hosts.     

The Glycine-Glomus-Rhizobium Symbiosis : VII. Photosynthetic Nutrient-Use Efficiency in Nodulated, Mycorrhizal Soybeans

Four consecutive trifoliate leaves of 56-day-old symbiotic or nonsymbiotic soybean plants were evaluated individually for CO₂ exchange rates (CER), leaf area and dry weight, and leaf N, P, and starch concentrations. Plants had been inoculated with the vesicular-arbuscular mycorrhizal (VAM) fungus Glomus mosseae and Rhizobium japonicum, with either of the endophytes alone, or with neither at time of planting. Plants lacking one or both endophytes received N and/or P fertilizers to produce plants of equal total leaf dry weight in all four treatments. Photosynthetic P-use efficiency (CER per unit leaf P) was higher in the leaves of VAM plants than in P-fertilized plants regardless of the N source (N₂ fixation or combined N). Photosynthetic N-use efficiency was also higher in VAM than in non-VAM plants, but it was affected by the N source, with higher CER in the nodulated plants. The greatest differences in CER, starch accumulation and leaf area were found between the nonsymbiotic plants and those with both endophytes. Statistical evaluations of leaf parameters for treatment or nutrient concentration (N and P) effects between the tri-partite and the nonsymbiotic treatments showed significant changes in concentration of P, but not N, with decreasing leaf age. Both endophytes apparently enhance CO₂ fixation at N and/or P concentrations lower than those of the nonsymbiotic plants. The effects of the endophytes on CO₂ fixation were additive.     

Glycine-Glomus-Bradyrhizobium Symbiosis : X. Relationships between Leaf Gas Exchange and Plant and Soil Water Status in Nodulated, Mycorrhizal Soybean under Drought Stress

Soybean (Glycine max [L.] Merr.) plants were colonized by the vesicular-arbuscular mycorrhizal (VAM) fungus Glomus mosseae (Nicol. and Gerd.) Gerd. and Trappe (VAM plants) or fertilized with KH₂PO₄ (nonVAM plants) and grown for 50 days under controlled conditions. Plants were harvested over a 4-day period during which the soil was permitted to dry slowly. The harvest was terminated when leaf gas exchange was no longer measurable due to drought stress. Significantly different effects in shoot water content, but not in shoot water potential, were found in VAM and nonVAM plants in response to drought stress. Leaf conductances of the two treatments showed similar response patterns to changes in soil water and shoot water potential but were significantly different in magnitude and trend relative to shoot water content. The relationships between transpiration, CO₂ exchange and water-use efficiency (WUE) were the same in VAM and nonVAM plants in response to decreasing soil water and shoot water potential. As a function of shoot water content, however, WUE showed different response patterns in VAM and nonVAM plants.     

Effect of salinity on mycorrhizal onion and tomato in soil with and without additional phosphate

Summary Vesicular-arbuscular mycorrhizal fungi (VAM) are known to increase plant growth in saline soils. Previous studies, however, have not distinguished whether this growth response is due to enhanced P uptake or a direct mechanism of increased plant salt tolerance by VAM. In a glasshouse experiment onions (Allium cepa L.) were grown in sterilized, low-P sandy loam soil amended with 0, 0.8, 1.6 mmol P kg^–1 soil with and without mycorrhizal inoculum. Pots were irrigated with saline waters having conductivities of 1.0, 2.8, 4.3, and 5.9 dS m^–1. Onion colonized withGlomus deserticola (Trappe, Bloss, and Menge) increased growth from 394% to 100% over non-inoculated control plants when soil P was low ( 0.2 mmol kg^–1 NaHCO₃-extractable P) at soil saturation extract salinities from 1.1 dS m^–1 to 8.8 dS m^–1. When 0.8 and 1.6 mM P was added no dry weight differences due to VAM were observed, however, K and P concentrations were higher in VAM plants in saline treatments.Glomus fasciculatum (Gerdeman and Trappe) andGlomus mosseae (Nicol. and Gerd.) isolates increased growth of VAM tomato 44% to 193% in non-sterilized, saline soil (10 dS m^–1 saturation extract) despite having little effect on growth in less saline conditions when soil P was low. Higher tomato water potentials, along with improved K nutrition by VAM in onion, indicate mechanisms other than increased P nutrition may be important for VAM plants growing under saline stress. These effects appear to be secondary to the effects of VAM on P uptake.     

Light effects in mycorrhizal soybeans

Soybean (Glycine max. L. Merr.) plants were grown in an experiment with a 3 × 3 factorial design using different levels of light (170, 350, and 700 μE·m⁻²·s⁻¹) and P as factors. Plants were grown in a greenhouse in pot cultures using a soil low in plant-available P under three P regimes: no additional P, P added as KH₂PO₄, or P uptake enhanced by colonization of the host plant with the vesicular-arbuscular mycorrhizal (VAM) fungus Glomus fasciculatum (Thaxt. sensu Gerd.) Gerd. and Trappe. Development of the VAM fungal endophyte and of plants under all three P regimes was depressed by limiting light. However, the growth response of VAM plants to increasing light relative to non-VAM plants in the absence of additional P increased while the response relative to non-VAM plants with additional P decreased slightly. The highly significant interaction between the factors (P < 0.001) of the experiment was due to differences in the magnitude and direction of simple effects of the factors. The implications of these differences in terms of source-sink relationships of the symbionts and the value of different non-VAM controls in interpreting VAM effects are discussed.     

Salinity and flooding stress effects on mycorrhizal and non-mycorrhizal citrus rootstock seedlings

Seedlings of the rootstocks Pineapple sweet orange (SwO), Carrizo citrange (CC), and sour orange (SO) were grown in low phosphorus (P) sandy soil and either inoculated with the vesicular-arbuscular mycorrhizal (VAM) fungus,Glomus intraradices, or were non-mycorrhizal (NM) and fertilized with P. VAM and NM seedings of similar shoot size and adequate P-status were selected for study of salinity and flooding stress. One-third of each of the VAM and NM plants were given 150 mM NaCl for a period of 24 days. One-third of the plants were placed into plastic bags and flooded for 21 days while the remaining third were non-stressed controls. In general, neither stress treatment affected mycorrhizal colonization. Salinity stress reduced the hydraulic conductivity of roots, leaf water potential, stomatal conductance and net assimilation of CO₂ (ACO₂) of mycorrhizal and non-mycorrhizal seedlings to a similar extent. VAM plants of CC and SO accumulated more Cl in leaves than NM plants. Cl was higher in non-mycorrhizal roots of SwO and CC than in mycorrhizal roots. Flooding the root zone for 3 weeks did not produce visible symptoms in the shoot but did influence plant water relations and reduce ACO₂ of all 3 rootstocks. VAM and NM plants of each rootstock were affected similarly by flooding. Comparable reduction in nitrogen and P content of both mycorrhizal and non-mycorrhizal plants suggested that flooding stress was primarily affecting root rather than hyphal nutrient uptake. Florida Agricultural Experimental Station Journal Series No. 7773.     

Field nursery inoculation of Hevea brasiliensis Muell. Arg. seedling rootstock with vesicular-arbuscular mycorrhizal (VAM) fungi

The growth response of Hevea brasiliensis to vesicular-arbuscular mycorrhizal (VAM) fungi inoculation was assessed in two field nursery sites containing indigenous mycorrhizal fungi (IMF). Seedling rootstocks were inoculated with mixed VAM-fungal species in a factorial combination with phosphorus (P) fertilizer application, and planted in randomised blocks on sandy (site 1) and clayey (site 2) soils. Plants were harvested after 26 weeks for measurements of shoot dry weight (DW), stem diameter, height, mycorrhizal root colonization and leaf nutrient contents. At site 1, VAM increased shoot DW, stem diameter and plant height only in treatments without P applied. Increases in shoot DW due to VAM were 70% greater than the uninoculated controls although this was reduced to 5% when P was applied. At site 2, VAM inoculation also increased shoot DW and stem diameter but the magnitude of the increases was smaller. Shoot DW response due to VAM was only 29%. At this second site, applying phosphate to uninoculated plants did not increase shoot yields further. Leaf concentrations of all nutrients were unaffected by VAM at both sites, except for copper (Cu) which was increased by VAM in treatments where P was not applied. However, leaf contents of P, potassium (K), magnesium (Mg) and Cu were increased by VAM at site 1, and of leaf nitrogen (N) and K at site 2. These experiments demonstrate that VAM-fungi could be introduced into field nursery sites to improve growth and P uptake by H. brasiliensis. The relevance of VAM-fungi to H. brasiliensis seedling rootstock development and the influence of IMF in determining field responses is discussed.     

Increased Sporulation of Vesicular-Arbuscular Mycorrhizal Fungi by Manipulation of Nutrient Regimens

Adjustment of pot culture nutrient solutions increased root colonization and sporulation of vesicular-arbuscular mycorrhizal (VAM) fungi. Paspalum notatum Flugge and VAM fungi were grown in a sandy soil low in N and available P. Hoagland nutrient solution without P enhanced sporulation in soil and root colonization of Acaulospora longula, Scutellospora heterogama, Gigaspora margarita, and a wide range of other VAM fungi over levels produced by a tap water control or nutrient solutions containing P. However, Glomus intraradices produced significantly more spores in plant roots in the tap water control treatment. The effect of the nutrient solutions was not due solely to N nutrition, because the addition of NH₄NO₃ decreased both colonization and sporulation by G. margarita relative to levels produced by Hoagland solution without P.     

Nutrient uptake in mycorrhizal symbiosis

The role of mycorrhizal fungi in acquisition of mineral nutrients by host plants is examined for three groups of mycorrhizas. These are; the ectomycorrhizas (ECM), the ericoid mycorrhizas (EM), and the vesicular-arbuscular mycorrhizas (VAM). Mycorrhizal infection may affect the mineral nutrition of the host plant directly by enhancing plant growth through nutrient acquisition by the fungus, or indirectly by modifying transpiration rates and the composition of rhizosphere microflora. A capacity for the external hyphae to take up and deliver nutrients to the plant has been demonstrated for the following nutrients and mycorrhizas; P (VAM, EM, ECM), NH₄ ⁺ (VAM, EM, ECM), NO₃ ^- (ECM), K (VAM, ECM), Ca (VAM, EM), SO₄ ^2- (VAM), Cu (VAM), Zn (VAM) and Fe (EM). In experimental chambers, the external hyphae of VAM can deliver up to 80% of plant P, 25% of plant N, 10% of plant K, 25% of plant Zn and 60% of plant Cu. Knowledge of the role of mycorrhiza in the uptake of nutrients other than P and N is limited because definitive studies are few, especially for the ECM. Although further quantification is required, it is feasible that the external hyphae may provide a significant delivery system for N, K, Cu and Zn in addition to P in many soils. Proposals that ECM and VAM fungi contribute substantially to the Mg, B and Fe nutrition of the host plant have not been substantiated. ECM and EM fungi produce ectoenzymes which provide host plants with the potential to access organic N and P forms that are normally unavailable to VAM fungi or to non mycorrhizal roots. The relative contribution of these nutrient sources requires quantification in the field. Further basic research, including the quantification of nutrient uptake and transport by fungal hyphae in soil and regulation at the fungal-plant interface, is essential to support the selection and utilization of mycorrhizal fungi on a commercial scale.     

Effects of simulated fire on vesicular-arbuscular mycorrhizae in pinyon-juniper woodland soil

Effects of fire on vesicular-arbuscular mycorrhizal fungi were tested using microcosms constructed from soil, litter, and duff collected beneath canopies of pinyon pine, Utah juniper, and in the open space (interspace). Burning was conducted over wet and dry soils. Soil temperatures were monitored continuously throughout the microcosms during burning. Plants grown in soils burned when dry had a lower VAM colonization than soils burned when wet. Juniper soils demonstrated the greatest reduction, over 95%, compared to their respective controls. Plants grown in interspace soils burned when wet were least affected. There was a positive correlation (r²=0.90) between the decrease in VAM colonization and the soil temperature as a result of the fire. Temperature effects, and associated reductions in VAM, were related to amount of litter burned in each microcosm and the moisture content of the soils.     

Soil Phosphorus Availability and Transformation Rates in Relictic Pinsapo Fir Forests from Southern Spain

Abies pinsapo fir forests are remnant of temperate-like coniferous forests currently subjected to the typical seasonal constraints of Mediterranean-type climates. We have examined for the first time, P availability and cycling in A. pinsapo forests from southern Spain by using both, measurements of concentrations of soil P fractions (resin extractable P, bicarbonate extractable P and NaOH extractable P) and rates of P supply to (P solubilization, P mineralization and phosphomonoesterases (PMEase activity) and P immobilization from the soil solution. Soils from A. pinsapo stands differing in lithology (serpentinitic and calcareous substrates) and successional status (from young-agradative to old-growth stages) were chosen for this study. Labile organic P fractions, PMEase activity and rates of P gross mineralization and immobilization were significantly higher in agradative stands on serpentines than in successionally comparable calcareous stands. This suggests an important role of the organic P subcycle in the A. pinsapo stand on serpentine. On calcareous lithology, PMEase activity and all soil P transformation rates significantly increased throughout the successional series. Similarly, all organic and inorganic P fractions measured in old-growth forests showed the maximum values of the series. These trends fit the predictions of standard patterns of P cycling changes along with succession, in which P supply to plants greatly depends on solubilization from mineral forms at early-to-mid successional stages, whereas the importance of processes related to the organic P subcycle increases as succession progresses.     

ROLE OF MYCORRHIZAL FUNGUS ORIGIN IN GROWTH AND NUTRIENT UPTAKE BY GERANIUM ROBERTIANUM

Prior field studies have shown that populations of forest herbs on relatively nutrient poor soils have higher vesicular-arbuscular mycorrhizal (VAM) infection intensity than plants on rich soils. However, the growth responses and ability to take up P against the soil nutrient gradient are often not linearly related to infection intensity. To determine if intraspecific differences among populations of the common VAM fungus Glomus occultum could differentially affect growth and nutrient uptake, Geranium robertianum seedlings were inoculated with Glomus occultum isolated from four forest types along a gradient of soil fertility, and grown in a greenhouse at P levels typical of the extremes of that gradient. Plants given inoculum from relatively infertile forest sites generally produced greater root, shoot, and total mass than plants given inoculum from fertile sites or uninoculated plants, especially at the low P supply rate. Total P uptake and both P and N uptake efficiency were also highest in plants given inocula from low fertility sites. These results indicate that local adaptation and intraspecific variations in the ability of VAM fungi to induce growth and nutrient uptake effects on host plants may be as important as interspecific differences among VAM fungus species.     

Plant phenology and life span influence soil pool dynamics: Bromus tectorum invasion of perennial C3–C4 grass communities

In water-limited ecosystems, small rainfall events can have dramatic impacts on microbial activity and soil nutrient pools. Plant community phenology and life span also affect soil resources by determining the timing and quantity of plant nutrient uptake, storage, and release. Using the replacement of C₃–C₄ perennial grasses by the invasive annual grass Bromus tectorum as a case study, we investigated the influence of phenology and life span on pulse responses and sizes of soil carbon (C) and nitrogen (N) pools. We hypothesized that available and microbial C and N would respond to small rainfall events and that B. tectorum invasion would increase soil C and N pools by reducing inter-annual plant C and N storage and alter seasonal pool dynamics by changing the timing of plant uptake and litter inputs. We tested our hypotheses by simulating small rainfall events in B. tectorum and perennial grass communities three times during the growing season. Microbial pools responded strongly to soil moisture and simulated rainfall events, but labile C and N pools were affected weakly or not at all. All pools were larger beneath B. tectorum than perennial grasses. Soil C and N pools increased after senescence in both communities. Our results suggest that transforming a perennial into a B. tectorum dominated community increases the overall size of soil C and N pools by decreasing plant C and N storage and changes seasonal pool dynamics by altering dominant plant phenology. Our results indicate strong roles for water, life span and phenology in controlling soil C and N pools and begin to elucidate the biogeochemical effects of altering plant community phenology and life span.     

Alteration of growth ofSolanum opacum andPlantago drummondii and inhibition of regrowth of hyphae of vesicular-arbuscular mycorrhizal fungi from dried root pieces by manganese

Addition of MnSO₄ or MnCl₂ to a fine sandy soil from South Australia had a negative effect on shoot growth and root elongation ofSolanum opacum in the absence of significant presence of vesicular-arbuscular mycorrhiza (VAM). VAM ameliorated the reduction of plant growth by Mn, even though mycorrhizal development was decreased. Mn inhibited infection of roots by a fine endophyte less than that by some coarse endophytes. High concentrations of available Mn inhibited growth of hyphae of VAM fungi from dried root pieces, a significant source of infection by mycorrhizal fungi in the soil used.     

Arbuscular mycorrhizal fungal communities change among three stages of primary sand dune succession but do not alter plant growth

Plant interactions with soil biota could have a significant impact on plant successional trajectory by benefiting plants in a particular successional stage over others. The influence of soil mutualists such as mycorrhizal fungi is thought to be an important feedback component, yet they have shown benefits to both early and late successional plants that could either retard or accelerate succession. Here we first determine if arbuscular mycorrhizal (AM) fungi differ among three stages of primary sand dune succession and then if they alter growth of plants from particular successional stages. We isolated AM fungal inoculum from early, intermediate or late stages of a primary dune succession and compared them using cloning and sequencing. We then grew eight plant species that dominate within each of these successional stages with each AM fungal inoculum. We measured fungal growth to assess potential AM functional differences and plant growth to determine if AM fungi positively or negatively affect plants. AM fungi isolated from early succession were more phylogenetically diverse relative to intermediate and late succession while late successional fungi consistently produced more soil hyphae and arbuscules. Despite these differences, inocula from different successional stages had similar effects on the growth of all plant species. Host plant biomass was not affected by mycorrhizal inoculation relative to un‐inoculated controls. Although mycorrhizal communities differ among primary dune successional stages and formed different fungal structures, these differences did not directly affect the growth of plants from different dune successional stages in our experiment and therefore may be less likely to directly contribute to plant succession in sand dunes.     

Effects of P-fertilization and inoculation by two vesicular-arbuscular mycorrhizal fungi on growth and nodulation ofCalopogonium caeruleum

The growth response ofCalopogonium caeruleum, a leguminous covercrop in plantation agriculture, to inoculation with two vesicular-arbuscular mycorrhizal (VAM) fungi was investigated in five phosphorus (P)-deficient soils supplied with various levels of rock phosphate. Significant shoot yield increases over the uninoculated controls were obtained in most sterilised or unsterilised soils at all applied P levels, although the inoculant VAM fungi differed in their effectiveness in the soils used. Responses in mycorrhizal root infections, P and nitrogen (N) concentrations in tops and plant nodulation varied. The results are discussed in relation to the edaphic environment of the mycorrhizal association.     

Interaction of vascular plants and vesicular-arbuscular mycorrhizal fungi across a soil moisture-nutrient gradient

Summary Abundance and distribution of vascular plants and vesicular-arbuscular mycorrhizal (VAM) fungi across a soil moisture-nutrient gradient were studied at a single site. Vegetation on the site varied from a dry mesic paririe dominated by little bluestem (Schizachyrium scoparium) to emergent aquatic vegetation dominated by cattail (Typha latifolia) and water smartweed (Polygonum hydropiperoides). Plant cover, VAM spore abundance, plant species richness, and number of VAM fungi represented as spores, had significant positive correlations with each other and with percent organic matter. The plant and VAM spore variables had significant negative correlations with soil pH and available Ca, Mg, P and gravimetric soil moisture. Using stepwise multiple regression, Ca was found to be the best predictor of spore abundance. Test for association between plant species and VAM fungal spores indicated that the spores of Glomus caledonium are associated with plants from dry, nutrient poor sites and spores of gigaspora gigantea are positively associated with plants occurring on the wet, relatively nutrient rich sites. Glomus fasciculatum was the most abundant and widely distributed VAM fungus and it had more positive associations with endophyte hosts than the other VAM fungi. We found no relationship between beta niche breadth of plant species and the presence or absence of mycorrhizal infection. However, our data suggest that some plant species may vary with respect to their infection status depending upon soil moisture conditions that may fluctuate seasonally or annually to favor or hinder VAM associations.     

Interactions between Nitrogen Fixation, Mycorrhizal Colonization, and Host-Plant Growth in the Phaseolus-Rhizobium-Glomus Symbiosis

Bean (Phaseolus vulgaris L. cv. Dwarf) roots were inoculated with Rhizobium phaseoli and colonized by the vesicular-arbuscular mycorrhizal (VAM) fungus Glomus fasciculatum Gerd. and Trappe or left uncolonized as controls. The symbiotic associations were grown in an inert substrate using 0, 25, 50, 100, or 200 milligrams hydroxyapatite (HAP) (Ca₁₀[PO₄]₆[OH]₂) per pot as a P amendment. Plant and nodule dry weights and nodule activity increased for both VAM and control plants with increasing P availability, but values for VAM plants were significantly lower in all parameters than for controls. Inhibition of growth and of N₂ fixation in VAM plants was greatest at the lowest and highest P regimes. It was smallest at 50 milligrams HAP, where available P at harvest (7 weeks after planting) was 5 micrograms P per gram substrate. At this level of P availability, the association apparently benefited from increased P uptake by the fungal endophyte. Percent P values for shoots, roots, and nodules did not differ significantly (p > 0.05) between VAM and control plants. The extent of colonization, fungal biomass, and the fungus/association dry weight ratio increased several fold as HAP was increased from 0 to 200 milligrams. It is concluded that intersymbiont competition for P and photosynthate was the primary cause for the inhibition of growth, nodulation, and nodule activity in VAM plants. Impaired N₂ fixation resulted in N stress which contributed to inhibition of host plant growth at all levels of P availability.     

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.