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Rehabilitation of Habitat

An Opinion: Application of AM to Rehabilitation of Habitats


Cartoon of seedlingwith many attached AM fungi.

AM fungi have diverse attributes: they increase plant uptake of nonlabile minerals and water from soil, aggregate soils, increase the organic matter carbon and complexity of carbon being stored in soil, change the pattern of root development, and increase diversity of plant communities among other ecosystem factors.

Each of these factors is complex and multifacetted. Increased uptake of minerals will lead to faster rates of growth of responsive plants in mineral-poor soils. However most plant species used in rehabilitation of degraded habitats in Australia are likely to be native species adapted to poor soils. The plants will respond to increased mineral uptake initially with increased rate of mineral uptake but the plant will show no change in rates of growth. The mineral will be stored, and the effect more likely to be continued plant growth utilising stores of mineral and therefore a greater proportion of seedlings that survive. The presence of mycorrhizas may also enable the plant to survive for longer as soil dries, and the presence of the fungi modifies the soil environment in which the root system is developing. Thus root development interacts with the development of aggregation, which is also modified by AM fungi.

AM fungi modify soil structure. In broad terms, provided the soil contains adequate organic matter, proliferation of hyphae will lead to the formation of more macro-aggregates and increased complexity of the pore space. Aggregation of soil will lead to faster rates of water penetration, maintenance or improvement of water holding capacity and aeration of soil. Improved water characteristics reduce water erosion, and increased aggregation may increase the storage of stable organic carbon.

Initiation of AM increases root proliferation. Greater densities of roots, especially fine roots, increases carbon contributing to organic matter in the soil, and increases the potential sources of attachment for AM fungi. Increased density of AM fungi increases the fungal carbon being added to the soil, and again, the potential for organic carbon to be stabilised and stored in a protected location. Neither plant nor AM fungus directly increases the storage of protected organic carbon in soil. AM fungi appear not to increase micro-aggregation, and AM fungi via "glomalin" do not appear to contribute to 'gluing' of the soil particles. Changes in the distribution of roots will increase the benefit attributable to AM fungi, and determine the location of survival propagules of AM fungi in the soil profile. Altogether, presence of AM fungi will initiate increased mineral movement and establish a cycling of minerals between the soil and host plant.

AM fungi influence the composition of the plant community. Two mechanisms are important. Nonmycorrhizal plant species tend to be suppressed in the presence of AM fungi. Thus the increasing density of AM fungi in a recovering habitat will reduce the the proportion of non-mycorrhizal and facultatively mycorrhizal plants in that habitat. Complex communities of AM fungi appear to promote greater complexity in mycorrhizal plant communities. The mechanism is still speculative, but theoretical constructions based on negative interactions between plant and AM fungus could account for the complex plant community response observed in experimental settings. As a consequence of greater complexity in plant comunities, complexity of communities of AM fungi may also lead to greater total plant biomass. In other words, more plant species survive and increased total organic carbon is fixed in soil. This is despite the cost to the plant of supplying organic carbon to the fungal symbiont. The mechanisms underlying these influences are unclear. However, the data indicate that the complexity of natural plant communities will be attained much more rapidly if a diverse suite of AM fungi are included in the outplanting stage. These attributes of AM fungi indicate that AM fungi must be considered during the restoration of severely disturbed habitats.

Many fungi
Different seedlings interconnected by different coloured AM fungi.

Deliberate inoculation of seedlings or soil is necessary to ensure the establishment and survival of AM fungi. Natural dispersal of AM fungi into highly disturbed areas is slow, and the weedy fungi that disperse may not necessarily provide greatest benefit to the plant community or the soil habitat. Further, deliberate inoculation allows the use of fungi which have other roles in the community. For instance, small animals that utilise fungal sporocarps in their diet will return much readily if a food source has become available. Thus deliberate inoculation of mixed populations of selected AM fungi will enhance the establishment of complex habitats in natural ecosystems.

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