Mycorrhizal Response Research Articles

Nitrate leaching as a function of plant community richness and composition, and the scaling of soil nutrients, in a restored temperate grassland

Two, two-factor experiments manipulated species and functional form plant richness and the spatial scaling of either nitrogen (N) or phosphorous (P) in restored tallgrass prairie in North Dakota, USA. Nitrate (NO3−) leaching was measured in these plots and analyzed for its response to the treatment factors and measured plant community parameters. Nitrate extracted from anion exchange resin was regressed against the first principal component of species and functional form richness, the spatial scaling of N or P, the measured biomass of the functional forms used and the plot values for plant parameters based on weighted averages by species biomass. The treatments applied in the N and P experiments were 1, 2, 5, 10, or 20 plant species taxa, and the application of fertilizer in a random fractal pattern with either fine-scale or coarse-scale heterogeneity. Nitrate leaching decreased with plant diversity and increased by a factor of two going from fine-scale to coarse-scale N. It was also related to a number of plant functional parameters, and was positively correlated with the biomass of late successional C3 grasses (Koeleria cristata (Lam.) Beauv., Poa pratensis L., Stipa comata Trin. & Rupr., and Stipa viridula Trin.), which are known from previous studies to have negative mycorrhizal responsiveness and are characterized by high root lateral spread per unit of root biomass. Our results show that while plant diversity has a highly significant influence on plant community uptake of NO3−, this effect is mediated by the scaling of soil N and the functional traits of the species comprising the plant assemblage.

Agricultural management influences propagule densities and functioning of arbuscular mycorrhizas in low- and high-input agroecosystems in arid environments

Agricultural management is known to influence the abundance of arbuscular mycorrhizal (AM) fungi in the soil but relatively little is known about how this impacts the functioning of AM symbioses in crop plants, particularly in arid environments. We compared the abundance of spores and the mycorrhizal infection potential (a bioassay to measure total density of AM fungal spores, hyphae, and infected roots), in low-input and high-input agricultural systems managed by Native Americans in southwestern USA. These field studies were complemented with two greenhouse experiments to assess: (1) the mutualistic functioning of AM fungi and other soil organisms from high-input, low-input, and unmanaged ecosystems and (2) the mycorrhizal responsiveness of landrace and hybrid maize. We found significantly more AM fungal spores in high-input maize fields than in low-input maize fields, alfalfa fields, or unmanaged soils. However, the MIP bioassay indicated that the density of AM fungal propagules was not significantly different in the low- and high-input systems or the unmanaged-native soils. When maize was grown in field soil brought into the greenhouse, AM fungi and communities of other soil organisms did not benefit plant growth in high fertility soil (Experiment 1), but they did improve maize growth in low fertility soil (Experiment 2). Landrace maize was more responsive to mycorrhizas than hybrid maize, and novel soil inoculum was more beneficial than inoculum from sites where the crop and organisms have long coexisted. These findings suggest that agricultural management practices may influence the propagule density and symbiotic functioning of AM symbioses in arid environments; and that AM fungi and other soil organisms may play a more important role in crop performance in low fertility soil than in high fertility soil.

Effects of arbuscular mycorrhizal fungi on seedling growth and development of two wetland plants, Bidens frondosa L., and Eclipta prostrata (L.) L., grown under three levels of water availability

To identify the importance of arbuscular mycorrhizal fungi (AMF) colonizing wetland seedlings following flooding, we assessed the effects of AMF on seedling establishment of two pioneer species, Bidens frondosa and Eclipta prostrata grown under three levels of water availability and ask: (1) Do inoculated seedlings differ in growth and development from non-inoculated plants? (2) Are the effects of inoculation and degree of colonization dependent on water availability? (3) Do plant responses to inoculation differ between two closely related species? Inoculation had no detectable effects on shoot height, or plant biomass but did affect biomass partitioning and root morphology in a species-specific manner. Shoot/root ratios were significantly lower in non-inoculated E. prostrata plants compared with inoculated plants (0.381 ± 0.066 vs. 0.683 ± 0.132). Root length and surface area were greater in non-inoculated E. prostrata (259.55 ± 33.78cm vs. 194.64 ± 27.45cm and 54.91 ± 7.628cm(2) vs. 46.26 ± 6.8cm(2), respectively). Inoculation had no detectable effect on B. frondosa root length, volume, or surface area. AMF associations formed at all levels of water availability. Hyphal, arbuscular, and vesicular colonization levels were greater in dry compared with intermediate and flooded treatments. Measures of mycorrhizal responsiveness were significantly depressed in E. prostrata compared with B. frondosa for total fresh weight (-0.3 ± 0.18g vs. 0.06 ± 0.06g), root length (-0.78 ± 0.28cm vs.-0.11 ± 0.07cm), root volume (-0.49 ± 0.22cm(3) vs. 0.06 ± 0.07cm(3)), and surface area (-0.59 ± 0.23cm(2) vs.-0.03 ± 0.08cm(2)). Given the disparity in species response to AMF inoculation, events that alter AMF prevalence in wetlands could significantly alter plant community structure by directly affecting seedling growth and development.

TaqMan real-time PCR assays to assess arbuscular mycorrhizal responses to field manipulation of grassland biodiversity: effects of soil characteristics, plant species richness, and functional traits.

Large-scale (temporal and/or spatial) molecular investigations of the diversity and distribution of arbuscular mycorrhizal fungi (AMF) require considerable sampling efforts and high-throughput analysis. To facilitate such efforts, we have developed a TaqMan real-time PCR assay to detect and identify AMF in environmental samples. First, we screened the diversity in clone libraries, generated by nested PCR, of the nuclear ribosomal DNA internal transcribed spacer (ITS) of AMF in environmental samples. We then generated probes and forward primers based on the detected sequences, enabling AMF sequence type-specific detection in TaqMan multiplex real-time PCR assays. In comparisons to conventional clone library screening and Sanger sequencing, the TaqMan assay approach provided similar accuracy but higher sensitivity with cost and time savings. The TaqMan assays were applied to analyze the AMF community composition within plots of a large-scale plant biodiversity manipulation experiment, the Jena Experiment, primarily designed to investigate the interactive effects of plant biodiversity on element cycling and trophic interactions. The results show that environmental variables hierarchically shape AMF communities and that the sequence type spectrum is strongly affected by previous land use and disturbance, which appears to favor disturbance-tolerant members of the genus Glomus. The AMF species richness of disturbance-associated communities can be largely explained by richness of plant species and plant functional groups, while plant productivity and soil parameters appear to have only weak effects on the AMF community.

Salinity tolerance and mycorrhizal responsiveness of native xeroriparian plants in semi-arid western USA

Restoration of salt-affected soils is a global concern. In the western United States, restoration of salinized land, particularly in river valleys, often involves control of Tamarix, an introduced species with high salinity tolerance. Revegetation of hydrologically disconnected floodplains and terraces after Tamarix removal is often difficult because of limited knowledge regarding the salinity tolerance of candidate native species for revegetation. Additionally, Tamarix appears to be non-mycorrhizal. Extended occupation of Tamarix may deplete arbuscular mycorrhizal fungi in the soil, further decreasing the success of revegetation efforts. To address these issues, we screened 42 species, races, or ecotypes native to southwestern U.S. for salinity tolerance and mycorrhizal responsiveness. As expected, the taxa tested showed a wide range of responses to salinity and mycorrhizal fungi. This variation also occurred between ecotypes or races of the same species, indicating that seed collected from high-salinity reference systems is likely better adapted to harsh conditions than seed originating from less saline environments. All species tested had a positive or neutral response to mycorrhizal inoculation. We found no clear evidence that mycorrhizae increased salinity tolerance, but some species were so dependent on mycorrhizal fungi that they grew poorly at all salinity levels in pasteurized soil.

Phosphorus mobilization and uptake in mycorrhizal rice (Oryza sativa L.) plants under flooded and non-flooded conditions

Phosphorus (P) deficiency severely limits rice production in the world. Colonization of plant root with arbuscular mycorrhizal fungi (AMF) may have a considerable consequence for P uptake and plant growth. In contrast to other crop species, there is little experimental evidence about the role of mycorrhizal colonization in lowland rice plants. This study was undertaken to examine whether inoculation of rice plants can contribute in the mobilization and uptake of insoluble P form. In addition, an attempt was made to compare flooded plants with non-flooded ones in their mycorrhizal responsiveness. In one experiment, insoluble P was supplied for plants inoculated either with Glomus mosseae or Glomus intraradices, then growth and P uptake was determined. Results showed that colonization with AMF significantly improved uptake of P derived from insoluble P source. In other experiment, rice plants were inoculated with Glomus mosseae or Glomus intraradices in non-flooded nurseries and then transplanted either to flooded or non-flooded conditions. Root colonization by AMF was decreased due to flooding conditions from 43% to 27%. Nevertheless, the plant growth responded positively (117% increase) to inoculation when grown at flooded conditions, while dry matter of non-flooded plants was diminished up to 64% by inoculation. Mycorrhizal colonization had also a significant contribution in the uptake of P and K in flooded but not in non-flooded plants.

Evidence for the evolution of reduced mycorrhizal dependence during plant invasion

Introduced species inevitably experience novel selection pressures in their new environments as a result of changes in mutualist and antagonist relationships. While most previous work has examined how escape from specialist enemies has influenced herbivore or pathogen resistance of exotic species, post-introduction shifts in exotic dependence on mutualists have not been considered. In a common environment, we compared dependence on AM fungi of North American and European populations of Hypericum perforatum (St. John's Wort), a forb native to Europe. Introduced North American populations responded less to inoculation with AM fungi than did European populations. Root architecture was strongly correlated with mycorrhizal response, and introduced populations had finer root architecture than native populations. Finally, introduced populations exhibited decreased root and increased reproductive allocation relative to European populations, consistent with a transition to a weedier life history; however, biomass allocation patterns were uncorrelated with mycorrhizal response. These findings are the first demonstration of a genetically based reduction of mycorrhizal dependence and shift in root architecture in an introduced species.

Differential Growth Response of Various Crop Species to Arbuscular Mycorrhizal Inoculation

To investigate the growth response of various crop species to mycorrhizal inoculation, arbuscular mycorrhizal fungi were applied to Glycine max, Vigna angularis, Senna torn, Hordeum migare var. hexastichon. Zea mays, Sorghum bicolor, Allium tuberosum, Solanum melongena, and Capsicum annuum. The biomass of the inoculated crops was measured every two weeks for the 12-week growth period. By measuring biomass, we calculated the mycorrhizal responsiveness of the nine crop species. Among the nine crop species, four species showed a significant response to mycorrhizal inoculation. The shoot biomasses of V. angularis, C. annuum, A. tuberosum, and S. tora significantly increased with mycorrhizal inoculation.

Topographic position modulates the mycorrhizal response of oak trees to interannual rainfall variability

California coast live oak (Quercus agrifolia) forms tripartite symbiotic associations with arbuscular (AMF) and ectomycorrhizal (EMF) fungi. We selected oak individuals differing in topographic position and depth to groundwater (mesic valley vs. xeric hill sites) to investigate changes of tree mycorrhizal status in response to interannual rainfall variability. EMF root colonization, as well as hyphal abundance and viability in upper rhizosphere soil (0-30 cm), were negatively affected by severe multi-year drought, although not to the same extent in each topographic location. Oak trees growing in hill sites showed EMF colonization levels <1% in upper roots during drought. By contrast, oaks in valley sites maintained much higher EMF colonization (>19%) in upper roots during drought. EMF root colonization increased sharply at both topographic positions during the ensuing wet year (78% in valley, 49% in hill), which indicates that the mycorrhizal status of roots in upper rhizosphere soil is highly responsive to interannual rainfall variability. Across sites and years, percentage EMF colonization and soil hyphal density and viability were strongly positively correlated with soil moisture potential, but percentage AMF root colonization was not. Interestingly, changes in percentage EMF root colonization and density of viable hyphae between a wet and a dry year were proportionally much greater in xeric hill sites than in mesic valley sites. The mycorrhizal status of oak trees was particularly responsive to changes in soil moisture at the hill sites, where roots in upper rhizosphere soil shifted from almost exclusively AMF during severe drought to predominantly EMF during the ensuing wet year. By contrast, the mycorrhizal status of oaks in the valley sites was less strongly coupled to current meteorological conditions, as roots in upper soil layers remained predominantly EMF during both a dry and a wet year. Canopy shading and hydraulic lift by oaks in valley sites likely contributed to maintain the integrity and viability of EMF roots and extraradical hyphae in upper rhizosphere soil during extended drought. Our results suggest that oak woodlands in water-limited ecosystems may become increasingly reliant on the AMF symbiosis under future climate change scenarios for the U.S. southwest and other world regions.

Screening of wild emmer wheat accessions (Triticum turgidum subsp. dicoccoides) for mycorrhizal dependency

Mycorrhizal dependency was studied in 23 wild emmer (Triticum turgidum subsp. dicoccoides) accessions originating from ecologically and geographically different locations in the Fertile Crescent covering Israel, Turkey, Lebanon, Jordan, and Syria. Wild emmer accessions were grown with mycorrhizae (Glomus mosseae) and non-mycorrhizae under greenhouse conditions, and harvested according to the Zadoks scale at growth stage 33. Root, shoot and total dry weight, growth response, root infection, and mycorrhizal dependency were calculated. It was determined that mycorrhizal inoculation increased 4.1-, 3.9-, and 3.9-fold for root, shoot, and total dry weight, respectively, compared to the control. It was found that wild emmer wheat exhibited a wide range of mycorrhizal dependency (56.8%-90.5%) and growth response (144.0%-990.4%), except root colonisation (70.0%-75.0%). Based on these observations, the hypothesis whether or not wild wheat chromosomes have a gene(s) for mycorrhizal response was tested. The experiment was done on the Langdon-T. dicoccoides substitution lines, each having an individual chromosome from "wild emmer wheat", T. dicoccoides, substituted into the tetraploid wheat cultivar "Langdon" background. Among the Langdon-T. dicoccoides substitution lines, mycorrhizal dependency and growth response varied from 70.9% to 87.0% and from 261.1% to 690.0%, respectively, whereas most of the substitution lines were significantly lower than Langdon for these traits. Disomic substitution lines for B genome chromosomes of dicoccoides showed a significant reduction in mycorrhizal dependency and growth response when compared with disomic substitution lines for A genome chromosomes of dicoccoides. These results revealed that B genome chromosomes had a more detrimental effect on mycorrhizal dependency than did A genome chromosomes. The results of this study showed that wild emmer wheat may be used as a source of mycorrhizal dependency in wheat breeding.

Phosphorus mobilization and uptake in mycorrhizal rice (Oryza sativa L.) plants under flooded and non-flooded conditions

Phosphorus (P) deficiency severely limits rice production in the world. Colonization of plant root with arbuscular mycorrhizal fungi (AMF) may have a considerable consequence for P uptake and plant growth. In contrast to other crop species, there is little experimental evidence about the role of mycorrhizal colonization in lowland rice plants. This study was undertaken to examine whether inoculation of rice plants can contribute in the mobilization and uptake of insoluble P form. In addition, an attempt was made to compare flooded plants with non-flooded ones in their mycorrhizal responsiveness. In one experiment, insoluble P was supplied for plants inoculated either with Glomus mosseae or Glomus intraradices, then growth and P uptake was determined. Results showed that colonization with AMF significantly improved uptake of P derived from insoluble P source. In other experiment, rice plants were inoculated with Glomus mosseae or Glomus intraradices in non-flooded nurseries and then transplanted either to flooded or non-flooded conditions. Root colonization by AMF was decreased due to flooding conditions from 43% to 27%. Nevertheless, the plant growth responded positively (117% increase) to inoculation when grown at flooded conditions, while dry matter of non-flooded plants was diminished up to 64% by inoculation. Mycorrhizal colonization had also a significant contribution in the uptake of P and K in flooded but not in non-flooded plants.

Response of Two Maize Inbred Lines with Contrasting Phosphorus Efficiency and Root Morphology to Mycorrhizal Colonization at Different Soil Phosphorus Supply Levels

ABSTRACT A pot experiment investigated the response of two maize inbred lines with contrasting root morphology and phosphorus (P) efficiency to inoculation with Glomus mosseae or Glomus etunicatum compared with non-mycorrhizal controls. Soil phosphorus was supplied at rates of 10, 50, and 100 mg P kg −1 soil. Root length, specific root length, and specific phosphorus uptake of maize line 178 (P-efficient) were significantly higher than of line Hc (P-inefficient). Percentage of root length colonized showed the opposite trend regardless of soil P supply level. The two maize lines did not differ significantly in growth response to mycorrhizal colonization. Root colonization rate decreased with increasing soil phosphorus supply. The beneficial effect of the two AM fungi on plant growth and P uptake was greatest at low soil P level and the responses were negative at high P supply. Mycorrhizal responsiveness also decreased with increasing P supply and differed between the two mycorrhizal fungal isolates.

Mycorrhizal responses to biochar in soil – concepts and mechanisms

Experiments suggest that biomass-derived black carbon (biochar) affects microbial populations and soil biogeochemistry. Both biochar and mycor- rhizal associations, ubiquitous symbioses in terrestrial ecosystems, are potentially important in various ecosystem services provided by soils, contributing to sustainable plant production, ecosystem restoration, and soil carbon sequestration and hence mitigation of global climate change. As both biochar and mycor- rhizal associations are subject to management, under- standing and exploiting interactions between them could be advantageous. Here we focus on biochar effects on mycorrhizal associations. After reviewing the experimental evidence for such effects, we critically examine hypotheses pertaining to four mechanisms by which biochar could influence mycorrhizal abundance and/or functioning. These mechanisms are (in decreas- ing order of currently available evidence supporting them): (a) alteration of soil physico-chemical proper- ties; (b) indirect effects on mycorrhizae through effects on other soil microbes; (c) plant-fungus signaling interference and detoxification of allelochemicals on biochar; and (d) provision of refugia from fungal grazers. We provide a roadmap for research aimed at testing these mechanistic hypotheses.

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

Abstract 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.

Mycorrhizal responsiveness of aerobic rice genotypes is negatively correlated with their zinc uptake when nonmycorrhizal

Plant Zn uptake from low Zn soils can be increased by Zn-mobilizing chemical rhizosphere processes. We studied whether inoculation with arbuscular mycorrhizal fungi (AMF) can be an additional or an alternative strategy. We determined the effect of AMF inoculation on growth performance and Zn uptake by rice genotypes varying in Zn uptake when nonmycorrhizal. A pot experiment was conducted with six aerobic rice genotypes inoculated with Glomus mosseae or G. etunicatum or without AMF on a low Zn soil. Plant growth, Zn uptake and mycorrhizal responsiveness were determined. AMF-inoculated plants produced more biomass and took up more Zn than nonmycorrhizal controls. Mycorrhizal inoculation, however, significantly increased Zn uptake only in genotypes that had a low Zn uptake in the nonmycorrhizal condition. We conclude that genotypes that are less efficient in Zn uptake when nonmycorrhizal are more responsive to AMF inoculation. We provide examples from literature allowing generalization of this conclusion on a trade off between mycorrhizal responsiveness and nutrient uptake efficiency.

Mycorrhizal responses to nitrogen fertilization in boreal ecosystems: potential consequences for soil carbon storage

AbstractMycorrhizal fungi can contribute to soil carbon sequestration by immobilizing carbon in living fungal tissues and by producing recalcitrant compounds that remain in the soil following fungal senescence. We hypothesized that nitrogen (N) fertilization would decrease these carbon stocks, because plants should reduce investment of carbon in mycorrhizal fungi when N availability is high. We measured the abundance of two major groups of mycorrhizal fungi, arbuscular mycorrhizal (AM) and ectomycorrhizal (ECM) fungi, in the top 10 cm of soil in control and N‐fertilized plots within three Alaskan boreal ecosystems that represented different recovery stages following severe fire. Pools of mycorrhizal carbon included root‐associated AM and ECM structures; soil‐associated AM hyphae; and glomalin, a glycoprotein produced by AM fungi. Total mycorrhizal carbon pools decreased by approximately 50 g C m−2 in the youngest site under N fertilization, and this reduction was driven mostly by glomalin. Total mycorrhizal carbon did not change significantly in the other sites. Root‐associated AM structures were more abundant under N fertilization across all sites, and root‐associated ECM structures increased marginally significantly. We found no significant N effects on AM hyphae. Carbon sequestered within living mycorrhizal structures (0.051–0.21 g m−2) was modest compared with that of glomalin (33–203 g m−2). We conclude that our hypothesis was only supported in relation to glomalin stocks within one of the three study sites. As N effects on glomalin were inconsistent among sites, an understanding of the mechanisms underlying this variation would improve our ability to predict ecosystem feedbacks to global change.

Phosphorus Supply Alters Tuber Composition, Flower Production, and Mycorrhizal Responsiveness of Container-grown Hybrid Zantedeschia

One-year old tubers of two hybrid calla lily (calla) cultivars (Zantedeschia ‘Pot of Gold’ and ‘Majestic Red’) were inoculated with the arbuscular mycorrhizal fungus (AMF), Glomus intraradices, or not, and grown at three different rates of phosphorus (P) supply to asses the effects of AMF-inoculation on plant development (time of shoot emergence and flowering), flowering (number, length and rate of flowering), and tuber biomass and composition over two growing cycles (2002, 2003). Tubers and flowers of calla responded differently to AMF inoculation. Differences in mycorrhizal responsiveness between cultivars was related to differences in P requirements for flower and tuber production, and the influence of P supply on resource allocation to different reproductive strategies. Inoculation increased shoot production and promoted early flowering, particularly in 2003. Inoculated plants also produced larger tubers than non-inoculated plants, but only increased the number of flowers per plant in 2003. High P supply also increased tuber biomass, but decreased the number of flowers per plant in 2002. Plants grown at a moderate P-rate, produced the most flowers in 2003. For ‘Majestic Red’, benefits from AMF were primarily in terms of tuber yield and composition, and AMF effects on marketable flower production could potentially have negative impact on production strategies for growers. Inoculation of ‘Pot of Gold’ primarily influenced flower production and aspects of tuber quality that caused detectable enhancement of tuber yield and flowering in the second growing cycle following inoculation (2003). The results of this study show that the responses of calla to AMF are partially a function of how nutrient supply alters resource allocation to sexual and vegetative reproduction. Whether AMF-induced changes in resource allocation to flowering and tubers significantly alters commercial productivity and quality of calla depends on the crop production goals (e.g. tubers, cut flowers or potted plants).

Relationships among arbuscular mycorrhizas, root morphology and seedling growth of tropical native woody species in southern Brazil

The relationships between arbuscular mycorrhizal fungi and root morphological characteristics were studied under greenhouse conditions of 78 tropical native woody species and 47 seedling species collected in the field. Seedlings of native woody pioneer and early secondary species that generally exhibited fine roots with a dense cover of long root hairs showed higher mycorrhizal response and root mycorrhizal colonization than late-secondary and climax species with coarse roots with a sparse cover of short root hairs. Root-hair length and incidence decreased with the progression among the successional groups while fine-root diameter increased, both in the greenhouse and in the field. The mycorrhizal response was highly correlated to root mycorrhizal colonization in the greenhouse and in the field. These parameters were inversely correlated with the seed mass and fine-root diameter, but directly correlated with root-hair incidence, both in the greenhouse and in the field. Mycorrhizal response and root mycorrhizal colonization were also directly correlated with the root-hair length and root/shoot ratio of uninoculated plants. The seedling mycorrhizal status of the early successional woody species suggests that the root traits of these fast-growing species can be more receptive to attraction, infection and colonization by arbuscular mycorrhizas than root traits of late-successional species.

Species of plants and associated arbuscular mycorrhizal fungi mediate mycorrhizal responses to CO2 enrichment

AbstractIt has been suggested that enrichment of atmospheric CO2 should alter mycorrhizal function by simultaneously increasing nutrient‐uptake benefits and decreasing net C costs for host plants. However, this hypothesis has not been sufficiently tested. We conducted three experiments to examine the impacts of CO2 enrichment on the function of different combinations of plants and arbuscular mycorrhizal (AM) fungi grown under high and low soil nutrient availability. Across the three experiments, AM function was measured in 14 plant species, including forbs, C3 and C4 grasses, and plant species that are typically nonmycorrhizal. Five different AM fungal communities were used for inoculum, including mixtures of Glomus spp. and mixtures of Gigasporaceae (i.e. Gigaspora and Scutellospora spp.). Our results do not support the hypothesis that CO2 enrichment should consistently increase plant growth benefits from AM fungi, but rather, we found CO2 enrichment frequently reduced AM benefits. Furthermore, we did not find consistent evidence that enrichment of soil nutrients increases plant growth responses to CO2 enrichment and decreases plant growth responses to AM fungi.Our results show that the strength of AM mutualisms vary significantly among fungal and plant taxa, and that CO2 levels further mediate AM function. In general, when CO2 enrichment interacted with AM fungal taxa to affect host plant dry weight, it increased the beneficial effects of Gigasporaceae and reduced the benefits of Glomus spp. Future studies are necessary to assess the importance of temperature, irradiance, and ambient soil fertility in this response. We conclude that the affects of CO2 enrichment on AM function varies with plant and fungal taxa, and when making predictions about mycorrhizal function, it is unwise to generalize findings based on a narrow range of plant hosts, AM fungi, and environmental conditions.

Effects of Mycorrhizae and Nontarget Organisms on Restoration of a Seasonal Tropical Forest in Quintana Roo, Mexico: Factors Limiting Tree Establishment

Abstract We initiated a study of the effects of mycorrhizal fungal community composition on the restoration of tropical dry seasonal forest trees. Tree seedlings were planted in a severely burned experimental site (1995 fire) during the growing season of 1998 at the El Edén Ecological Reserve, in north Quintana Roo, Mexico. Seedlings of Leucaena leucocephala, Guazuma ulmifolia, Caesalpinia violacea, Piscidia piscipula, Gliricidia sepium, and Cochlospermum vitifolium were germinated in steam‐sterilized soil and either remained uninoculated (nonmycorrhizal at transplanting) or were inoculated with mycorrhizal fungi in soils from early‐seral (recently burned) or late‐seral (mature forest) inoculum. Inoculum from the early‐seral soil was largely Glomus spp., whereas a diverse community of arbuscular mycorrhizal fungi were reintroduced from the mature forest including species of Scutellospora, Gigaspora, Glomus, Sclerocystis, and Acaulospora. Plants grew better when associated with the mature forest inoculum, unlike a previous experiment in which plants grew taller with the early‐seral inoculum. Reasons for the different responses include a less‐intense burn resulting in more residual organic matter. In addition to mycorrhizal responses, plants were severely affected by deer browsing. One tree species, C. vitifolium found in the region but not in the reserve, was eliminated by a resident fungal facultative pathogen. Several practical conclusions for restoration can be made. The common nursery practice of soil sterilization may be detrimental because it eliminates beneficial mycorrhizal fungi; species not native to the site may not survive because they may not be adapted to the local pathogens; and herbivory can be severe depending on the landscape context of the restoration.