Mycorrhizal networks (MNs) are fungal hyphae that connect the roots of at least two plants, potentially providing a conduit for interplant resource transfer. Interior Douglas-fir (Pseudotsuga menziesii var. glauca (Beissn.) Franco) is an obligate ectomycorrhizal (EM) tree species that has high potential to form MNs with neighboring trees because of its receptivity to a diverse community of EM fungi. This MN potential is expected to be greatest among conspecific trees. In this thesis, I determined the influence of MNs formed by residual Douglas-fir trees on interplant carbon transfer and survival, growth, physiology, and EM status of neighboring naturally regenerated and planted Douglas-fir seedlings. To do this, I used MN-restricting treatments and isotope gas-labeling techniques on sites harvested with variable tree retention to investigate how varying: i) proximity to conspecific trees affects EM colonization and performance of planted seedlings; ii) ‘donor’ tree size affects seedling establishment and carbon or nitrogen transfer, and; iii) soil disturbance stress affects net carbon transfer between established seedlings. Because I used physical barriers (i.e., mesh bags) to control for the presence and characteristics of the MN, I also verified the effectiveness of different-sized mesh pores at reducing hyphal connections between plants in the greenhouse. In my experiments, I found that MN-mediated colonization was not the dominant mechanism responsible for EM colonization of planted seedlings; other sources of inoculm (e.g., spores, sclerotia, hyphal fragments) were more important. I found that mature trees not only competed for resources with seedlings but offered some facilitative effects at intermediate distances within their rooting zones. My key finding was that access to a MN with residual trees benefited seedling survival and that this corresponded with increased carbon and nitrogen transfer to seedlings. In addition, I found that there was consistently a net gain in carbon by one seedling in a MN and this net transfer increased with relative growth rate of the receiver seedling. These results indicate that MNs can facilitate interplant carbon transfer and be important in regeneration dynamics in dry Douglas-fir forests.

Arbuscular mycorrhizal (AM) fungi are an important component of agricultural ecosystems, and can directly influence the productivity of these systems. Unfortunately, conventional agricultural practices have been shown to adversely affect AM fungi. The use of more ecologically sustainable agricultural practices such as tree-based intercropping (TBI) may have the potential to reduce the negative impact of agricultural practices on AM fungi. The objectives of this thesis were to determine (1) if trees influence the structuring of AM fungal communities, (2) if TBI systems support a more diverse AM fungal community compared to conventional monocropping (CM) systems, and (3) if differences in AM fungal richness and composition between the two cropping systems have a functional effect on the growth of crops. Molecular analysis of the AM fungal community in the TBI system revealed 17 phylotypes that all belonged to the family Glomeraceae. Differences in richness and composition among the treatments indicated that trees had an effect on the structuring of AM fungal communities. Intercropping alleys adjacent to white ash and poplar tree rows had a significantly (P < 0.05) richer and different AM fungal community compared to intercropping alleys adjacent to Norway spruce tree rows. When comparing TBI and CM systems, AM fungal abundance was not significantly (P > 0.05) different between the two cropping systems. However, differences in both richness and community composition of AM fungi were observed between the two cropping systems. The TBI system had a significantly higher AM fungal richness and contained several taxa not found in the CM system. Controlled greenhouse experiments revealed that differences in AM fungal richness and community composition between the TBI and CM systems had no functional effect on the growth of three crops (i.e. barley, canola, and soybean). The similar growth response of crops to AM fungi from the two cropping systems may be due to the lack of functional complementarity among the AM fungi. Overall, the TBI system had a more diverse AM fungal community compared to the CM system and trees appear to be a significant factor in the structuring of these communities.

No description available.

Mycorrhizal fungi are a major component of agricultural systems and play a key role in plant nutrition. Little is known about the effects of tillage practices and manuring on arbuscular mycorrhizal fungi (AMF). The purpose of this study was to evaluate the effect of soil disturbance on winter survival, development and distribution of AMF in soil and on plant nutrient uptake and productivity. This research was conducted in long-term corn plots in two soils and under controlled conditions. / A growth chamber study with field soil demonstrated that most of the fungal hyphae with mycorrhizal plants were mycorrhizal rather than saprophytic. This result was extrapolated to subsequent experiments. Soil disturbance reduced corn nutrient uptake and growth by disrupting the AMF hyphal network. Similarly, fallow periods reduced density of AMF hyphae, leading to reduced mineral nutrients uptake and plant growth. Soil disturbance was also found to severely reduce winter survival of AMF hyphae in agricultural soil. AM hyphae could survive the winter in soil, even when they were not attached to roots. Their survival however, was improved when they remained attached to roots. / Under field conditions, indigenous AMF were more abundant in no-till soil, less abundant under reduced tillage and least abundant under conventional tillage. Under all tillage systems, most of AMF hyphae were located in the top 15 cm of the soil profile suggesting that deep plowing could result in dilution of AMF propagules in the seeding zone. There was a seasonal variation in the abundance of hyphae in soil. Soil hyphae and root colonization declined after the silking stage of corn. Hyphal abundance decreased further over the winter, to reach their lowest level in the spring. / The spatial distribution of fungal hyphae in the field was not homogenous. Hyphal density was maximal directly under the corn rows and decreased linearly up to the mid-row. Marked seasonal variations in hyphal densities were observed on the row but fluctuations at mid-row were not significant suggesting that little AMF hyphae were ever present between the rows. Liquid dairy manure had little effect on the abundance of hyphae and spores.

It may be possible to tailor crop management to encourage diverse soil microbial communities and beneficial microorganisms, and produce high quality food products. Studies were carried out in 2005-2007 to evaluate the impact of spring wheat (Triticum aestivum L.) cultivar choice and crop polycultures on soil microbial communities in organic and conventional systems, and subsequent wheat quality. Five wheat cultivars were grown organically and conventionally to evaluate grain breadmaking quality and micronutrient content and their impact on the soil microbial community. Organic grain yields were roughly half of conventional yields, but quality levels were all acceptable for Canadian Western Hard Red Spring wheat. Measured soil (0-15 cm) microbial profiles (by phospholipid fatty acid analysis) differed between the two management systems, and amongst cultivars in the conventional system. The most recent cultivar in the study, AC Superb, exhibited the highest levels of fungi suggesting that breeding efforts in conventionally managed environments may have resulted in cultivating mycorrhizal dependence in that environment. In general, many of the studied grain micronutrients were greater in the organically grown wheat system, possibly due in part to decreased grain yield and smaller grain size. Maximizing grain micronutrient content through wheat cultivar choice was dependent on management system. The presence of fungi biomarkers appears to have improved uptake of Mn and Cu. Monocultures and polycultures of common annual crops were grown organically and conventionally in 2006-2007. Intercrops exhibited an ability to overyield in an organic system, largely through weed suppression, but intercrops also overyielded in a conventional system where weeds were controlled through herbicides. As intercrop complexity decreased, the instances of improved weed suppression declined. Management systems and wheat cultivars can alter the composition of the soil microbial community. Annual crop polycultures did not alter soil microbial communities in this study, but showed evidence of agronomic benefits in both organic and conventional systems. / Plant Science

Mycorrhizal networks (MNs) are fungal hyphae that connect the roots of at least two plants, potentially providing a conduit for interplant resource transfer. Interior Douglas-fir (Pseudotsuga menziesii var. glauca (Beissn.) Franco) is an obligate ectomycorrhizal (EM) tree species that has high potential to form MNs with neighboring trees because of its receptivity to a diverse community of EM fungi. This MN potential is expected to be greatest among conspecific trees. In this thesis, I determined the influence of MNs formed by residual Douglas-fir trees on interplant carbon transfer and survival, growth, physiology, and EM status of neighboring naturally regenerated and planted Douglas-fir seedlings. To do this, I used MN-restricting treatments and isotope gas-labeling techniques on sites harvested with variable tree retention to investigate how varying: i) proximity to conspecific trees affects EM colonization and performance of planted seedlings; ii) ‘donor’ tree size affects seedling establishment and carbon or nitrogen transfer, and; iii) soil disturbance stress affects net carbon transfer between established seedlings. Because I used physical barriers (i.e., mesh bags) to control for the presence and characteristics of the MN, I also verified the effectiveness of different-sized mesh pores at reducing hyphal connections between plants in the greenhouse. In my experiments, I found that MN-mediated colonization was not the dominant mechanism responsible for EM colonization of planted seedlings; other sources of inoculm (e.g., spores, sclerotia, hyphal fragments) were more important. I found that mature trees not only competed for resources with seedlings but offered some facilitative effects at intermediate distances within their rooting zones. My key finding was that access to a MN with residual trees benefited seedling survival and that this corresponded with increased carbon and nitrogen transfer to seedlings. In addition, I found that there was consistently a net gain in carbon by one seedling in a MN and this net transfer increased with relative growth rate of the receiver seedling. These results indicate that MNs can facilitate interplant carbon transfer and be important in regeneration dynamics in dry Douglas-fir forests.

No description available.

Ultramafic soils enriched in nickel, such as found in Australia and New Caledonia, are associated with unique, diverse and poorly known vegetation communities. Re-establishment of these highly specific ecosystems is still a challenge for Ni mining companies. Ultramafic vegetation communities are the outcome of a long evolution process resulting in their adaptation to the extreme soil conditions found on ultramafic outcrops. Mycorrhizal fungi, a very common plant symbiont, are generally thought to be beneficial to plants in other ecosystems, providing plants with phosphorus and even promoting metal tolerance in plants in some cases. We examined the hypothesis that mycorrhizal fungi may contribute to the survival of plants in ultramafic soil conditions. Bandalup Hill, an ultramafic outcrop enriched in Ni (South West of Western Australia) was selected to assess the contribution of mycorrhizal fungi to ultramafic plants. Soil constraints, in particular the degree of Ni toxicity, were assessed at two sites with ultramafic soils within the outcrop. Total metal, nutrient, DTPA extractable Ni and available P were measured in soil while Ni, Ca and Mg were tested in the soil solution. In addition, nutrients and metals were analyzed in shoots of some plant species occurring at each site: Eucalyptus flocktoniae, Melaleuca pomphostoma, Melaleuca coronicarpa and Hakea verucosa. Topsoils in Bandalup Hill and plant shoots had high levels of Ni, and very low levels of P, K and N. Variation in DTPA extractable Ni between sites reflected the variation in shoot Ni level of E. flocktoniae and M. pomphostoma. Variations in soil solution Ni levels reflected variations in shoot Ni levels of M. coronicarpa and H. verucosa between sites. The germination requirements of the plant species used to assess the soil constraints was assessed. Species selected included Eucalyptus flocktoniae, Melaleuca coronicarpa, and Hakea verucosa. Seeds of E. flocktoniae and M. coronicarpa had a higher germination rate if pre-treated with smoke water, while no pre-treatment was required to germinate H. verucosa seeds. The unusual germination requirement of E. flocktoniae and M. coronicarpa involve complex chemical signals that may be present in the soil when the conditions are more favorable for plant establishment. Such unusual germination requirement may represent an adaptation to the hostile conditions of the ultramafic soils of Bandalup Hill. The mycorrhizal association and root characteristics of the selected plant species was also assessed after 8 weeks of growth in undisturbed ultramafic topsoil cores from Bandalup Hill. Roots of these species (including H. verucosa from a previously designated non-mycorrhizal family, Proteaceae) were associated with mycorrhizal fungi. Roots of E. flocktoniae and M. coronicarpa were colonized by both arbuscular mycorrhizal fungi (AMF) and ectomycorrhizal fungi (ECM), while roots of H. verucosa only contained some AM fungal structures. All species had high shoot to root ratios and their root characteristics reflected their association with mycorrhizal fungi. Based on the previous observations, uninoculated and inoculated E. flocktoniae seedlings were grown for 10 to 16 weeks in sand amended with Ni at 0, 0.2, 1 and 2.3 mg/kg. Mycorrhizal inoculum consisted of spores of Pisolithus sp. (ECM) or a mix of AMF spores and colonized root fragments, both originating from Bandalup Hill. Another inoculum consisted in Pisolithus sp. spores from a site with ultramafic soils in New Caledonia. Inoculation with AM and ECM fungi from Bandalup Hill was beneficial to E. flocktoniae. Benefits consisted mainly of a reduction of Ni shoot translocation at the highest Ni soil level. At 1 mg/kg soil Ni, E. flocktoniae exhibited a certain degree of tolerance to Ni. A substantial increase in growth and nutrient uptake with Pisolithus sp. from Western Australia was also observed. The contribution of AM fungi from Bandalup Hill to E. flocktoniae, M. coronicarpa, H. verucosa, and Trifolium subterraneum (clover) was then examined in ultramafic soil from Bandalup Hill.Steaming of ultramafic soil increased the availability and plant uptake of P. Consequently, uninoculated seedlings grew better, and inoculation with AM fungi decreased the growth of native plant species but did not affect their shoot Ni concentration. The presence of AM fungi increased the concentration of P in shoots of native plants species. Inoculation had no effect on the growth and nutrient content of subterranean clover. As mining activities have the potential to reduce the infectivity of AM fungi in topsoils, the effect of disturbance and storage practices on the AM infectivity of ultramafic topsoils collected in summer or winter from Bandalup Hill was investigated. Disturbance consisted in passing topsoil through a 2mm seive and cutting roots into 1cm fragments. Disturbed topsoil was then stored at room temperature in pots that were either sealed from the atmosphere or left open, and pots were maintained at field capacity. E. flocktoniae seedlings were planted in undisturbed and disturbed topsoil just after topsoil collect and then after 3, 6 and 9 months of topsoil storage. AM fungi present in the topsoil collected in summer was less susceptible to initial disturbance than AM fungi present in topsoil collected during winter. Also, storage of topsoil in sealed pots watered to field capacity was more detrimental to its infectivity than storage of topsoil in dry conditions. Mycorrhizal fungi can contribute to the survival of some native plant species in the ultramafic soils of Bandalup Hill and they may represent another strategy to improve the success of Ni mine revegetation. However, such contribution may not be the unique avenue for native plants to survive in ultramafic soils of Bandalup Hill.

[Truncated abstract] Genus Burkholderia encompasses a diverse collection of bacteria that inhabit rhizospheres throughout the world. Species can provide beneficial returns for eukaryotes, such as nitrogen fixation and nodule formation in plants and biocontrol of cropping systems. Burkholderia members can also cause disease in various animals, fungi and plants. These seemingly conflicting characteristics point to the capacity of Burkholderia spp. to interact with diverse eukaryotes. Within terrestrial ecosystems, Burkholderia spp. must negotiate favourable outcomes with both the primary producers and the primary decomposers, namely plants and fungi. It is these ongoing negotiations which govern many rhizosphere processes and lead to niche differentiation for Burkholderia spp. This research set out to design an in vitro model for investigating Burkholderiaeukaryote interactions. Surface and cellular interactions between Burkholderia spp. and both plants and fungi were then investigated. Specifically, mechanisms of adherence and invasion of plant and fungal cells were studied. The Burkholderia spp. B. vietnamiensis and B. pseudomallei were applied to mycorrhizal fungus spores as well as to several plant species. Bacterial inoculation had varying effects on germination of plant and fungal dormant forms. B. vietnamiensis-inoculation consistently increased Gigaspora decipiens spore germination, while B. pseudomallei produced no significant change. The effect of B. vietnamiensis on Acacia colei seed germination was density dependant, resulting in either increases or decreases in seed germination rates. ... Detection of B. pseudomallei in surface waters and soils was improved by the use of a rapid on-site molecular method. The related species B. thailandensis and B. ubonensis were also cultured from northern Western Australia. Mycorrhizal spores were isolated from soils of melioidosis-endemic regions. Burkholderia spp., including B. pseudomallei and B. vietnamiensis were detected in extracts of these mycorrhizal spores. Therefore, associations of Burkholderia spp. with mycorrhizal spores extend beyond the in vitro setting. These studies have increased our understanding of several specific interactions between Burkholderia spp. and eukaryotes of the rhizosphere. Common themes in adherence and invasion have emerged. Burkholderia spp. are able to closely associate with eukaryotes and to gain access to protected niches. Such access helps to explain the persistence of these bacteria in the environment during periods of desiccation and nutrient limitation.

Thesis (Ph. D.)--University of Washington, 2004. / Vita. Includes bibliographical references (leaves 120-141).