Doctor of Philosophy
Department of Biology
Bellgard, Stanley Edward, The potential for vesicular-arbuscular mycorrhizal fungi to influence the recovery of Hawkesbury sandstone vegetation after disturbance, Doctor of Philosophy thesis, Department of Biology, University of Wollongong, 1992. http://ro.uow.edu.au/theses/1080
Disturbance to plant communities by agricultural and mining practices is an inevitable intrinsic part of civilization. Growing public, political and scientific concern about problems such as land degradation and the greenhouse effect is increasing interest in revegetation as a counter-measure. One of the primary stresses imposed upon seedlings colonizing disturbed sites is a lack of nutrients. Plants species colonized by vesicular-arbuscular mycorrhizal (VAM) fungi have higher tissue levels of some inorganic nutrients (especially phosphorus), greater biomass yield, and more rapid uptake of water, and they are often more tolerant of various forms of stress than non-mycorrhizal plants of the same species. Consequently, it has been hypothesized that the successful revegetation of disturbed plant communities may be dependant in part on the availability of viable mycorrhizal inoculum.
The studies described in this thesis were aimed at assessing the potential for VAM fungi to influence the revegetation of disturbed plant communities. VAM fungi were studied in detail in two quite different plant communities on Hawkesbury Sandstone soils in southeastern New South Wales. Although the sites supported vegetation of different physiognomy, 14 plant species were common to both sites. The following features ofthe ecology of VAM fungi are presented in this thesis: (i) the mycorrhizal associations of plant species in the two study sites, (ii) an examination of which part of an intact soil profile represents the major store of potential propagules of VAM fungi, (iii) an investigation of the relationship between the intensity of topsoil disturbance and the infectivity of VAM fungi, and (iv) an examination of which propagules of VAM fungi are capable of initiating VAM after topsoil disturbance.
At both sites, an assessment of the mycorrhizal status of each of the plant species was made. Roots from five representatives of each plant species were collected from random locations within both of the study areas during July-September 1989. Fine feeder roots were assayed for mycorrhizal infection. At the woodland site, 21 of the 32 plant species examined had mycorrhizal associations. At the shrubland site, 31 of the 47 plant species examined were mycorrhizal. Internal hyphae, vesicles, and cortical hyphal coils were discovered on the roots of two species of Cyperaceae and on the non-proteoid roots of nine species of the Proteaceae. Several species within genera and families previously known to be mycorrhizal were also found for the first time to have associations. Endomycorrhizal associations predominated at both sites, but several species had both ecto- and endomycorrhizal associations. The presence or absence of mycorrhizal associations was consistent in those plant species common to both sites.
The formation of VAM in intact soil profiles was measured in topsoil and subsoil using bioassay seedlings grown in intact soil cores. VAM most readily developed in the roots of bioassay seedlings grown in the topsoil. Limited VAM occurred in the roots grown in subsoil cores. Most colonization of roots by VAM fungi occurred in the soil cores collected and assayed during Spring and Summer.
Few spores were found in any soil sampled, though at least twice as many spores occurred in the topsoil than in the subsoil, for all seasons examined. As most of the propagules that could initiate VAM (i.e. spores, colonized root fragments and fungal hyphae), were observed in the topsoil, disturbances which involve the removal and storage of the top 15 c m will adversely affect these fungi.
Removal and storage of the surface layers of soil is known to decrease the infectivity VAM fungi. In previous studies investigating VAM fungi and soil disturbance, only two treatments have been examined viz. no disturbance v. profoundly disturbed soil. I investigated the relationship between increasing intensity of topsoil disturbance and the infectivity of VAM fungi. Intact soil blocks were treated with one of four levels of disturbance. Seeds of a bioassay species were sown into the blocks and harvested 14, 21, 28, 35, and 42 days after sowing. Colonization of roots by VAM fungi had commenced by 14 days in the intact, low, and intermediate disturbance treatments. Colonization of roots was delayed by up to six weeks for seedlings grown in the most disturbed of the soil blocks. Although the low and intermediate degrees of soil disturbance did not cause a delay in the initiation of infection, they did reduce the proportion of root length colonized by VAM fungi after 21 days. After 21 days, shoot biomass was significantly less in seedlings grown in the most disturbed of the soil blocks. The most severe experimental treatment probably disturbed the external hyphal network and the infected root fragments (containing VAM hyphae and vesicles), which in turn temporarily reduced the infective potential of the fungus to nil. The observed delay in the initiation of infection could therefore be explained by the time required for hyphae to grow from other propagules in the soil.
The propagules of VAM fungi include: (i) spores, (ii) root fragments containing VAM hyphae and vesicles, and (iii) soil hyphae. The viability of each type of propagule after soil disturbance will determine in part, the number of infective propagules available to initiate the VAM association with plants re-colonizing a disturbed site. The aim of this study was to examine which of the propagules of VAM fungi are capable of initiating VAM after topsoil disturbance. Soil from the open woodland site was wet-sieved through a tier of three sieves (1 mm , 250 μm, and 106 μm), and separated into: (i) root fragments, (ii) VAM hyphae, (iii) VAM spores. Each of these fractions was assayed to determine its potential to initiate VAM . Fungal hyphae grew from root fragments after 14 days. VAM hyphal fragments did not produce any VAM infection even after 42 days. The VAM spore fraction initiated VAM after 28 days.
Overall, this study showed that: (i) VAM fungi are a component of the soil environment in Hawkesbury Sandstone soils and mycorrhizal associations exist in a high proportion of the indigenous plant species, (ii) most of the propagules that can initiate V A M occur in the top 15 c m of soil, (iii) cutting soil blocks longitudinally into four and nine equal portions has no significant impact upon VAM fungi, but cutting blocks into 25 portions, temporarily can reduce the infective potential of VAM fungi to nil, and (iv) colonized root fragments and spores can be effective propagules initiating VAM after topsoil disturbance.