Soil Fungi
Introduction
Soil is an oligotrophic medium for the growth of fungi. Readily available nutrients are present for short periods of time in a limited zone. For the remainder of the time, fungi metabolise and grow very slowly utilising a range of organic molecules, or the fungi are dormant. In general, the concentration of microbes is greatest close to the surface of roots where exudates from the root supply an extraordinarily important source of organic energy to the soil (rhizosphere).
Away from the root, the remains of plants and microbes are the primary source of energy. These become composted and degraded, usually very rapidly, leaving behind modified waxes, lignin, melanin and other complex molecules of plant and microbial origin. LINK. The recalcitrant remains are known as humus. Humus consists of a mixture of aromatic compounds that are resistant to enzymatic degradation. Humus forms an important part of the carbon stored in soil. The loss of humus to the atmosphere because of human activities is an important contribution to the increasing carbon dioxide in the atmosphere. Human degradation of ecosystems may be responsible for some 20% of the greenhouse gases, and this carbon comes from soil. If mycologists are to be part of the scientific response to global climate change, efforts to better understand carbon cycling in soil would be a useful place to start.
Lifestyles
The lifestyles adopted by fungi in soil fall into three types: ruderals, mycorrhizal fungi and hyphal forms. LINK Ruderals take advantage of the flushes of nutrients usually associated with rainfall. The water moving through soil carries with it dissolved organic molecules flushed from plant surfaces, surface litter and dead microbes. The fungi respond immediately, and grow actively while soil remains moist. The fungi usually sporulate rapidly, and exist through dry periods as asexual spores. Common genera include Absidia, Aspergillus, Chaetomium, Fusarium, Mortierella, Mucor, and Penicillium. These are the fungi that are commonly isolated from soil using soil dilution techniques.
Mycorrhizal fungi subsist almost entirely by tapping into the roots of plants for their organic energy LINK. The fungi are extremely common in soil, though quantification of the fungi is extremely difficult. Up to 5m of living hyphae of arbuscular mycorrhizal fungi can be extracted from 1 g soil. The same quantity of soil may reveal 1,000 colonising propagules of the same fungi, and maybe the same number of spores of ruderals. As ruderals are defined as organisms that form huge numbers of survival units, one might argue that mycorrhizal fungi in hese soils should be classified as ruderals LINK.
The lifestyle of mycorrhizal fungi is however, much more stable. While ever the host is alive, the fungi have access to organic carbon, and their proliferation is comparatively measured. Indeed, they probably exude simple organic molecules that are a suitable source of carbon for some other microbes. AM fungi are ubiquitous yet lack clear methods of widespread dispersal found in ruderals. The life style is very different.
Finally, a group of fungi exist as hyphae in soil. Spores of these fungi are extremely difficult to find in soil, and some may never sporulate. While some of the fungi have the potential to cause disease, it is likely that for much of their lifecycle, these organisms metabolise and grow slowly. They are usually associated with organic fragments, which they slowly degrade. Their isolation from soil requires special techniques LINK. However, once in culture the fungi can grow rapidly utilising a wide range of sources of complex carbon. These fungi exhibit some characteristics of combative and stress tolerant fungi. LINK
Fungi do not appear to readily degrade all forms of complex organic carbon. Humus is the name given to the complex of organic materials that may be resident in soil for decades to centuries. Humus consists of a suite of mixed polymers of aromatic and aliphatic compounds, largely derived from lignin (plant) and melanin (all organisms including fungi). Some 60% of organic carbon in soil may be humus. Humus is resistant to degradation because it covalently bonds the reaction sites to metals and clay minerals making the sites unavailable to enzymic attachment. Humus also bonds to various organic pollutants such as polyaromatic hydrocarbons and alanines (and shares a number of similarities) making both unavailable for degradation. Further, humus is often trapped within soil aggregates and therefore has an exposed diameter that effectively prevents bacteria from accessing the complex. Humus breaks down more readily when the soil is exposed to oxygen such as by cultivation, when soil is acidic and when the moisture level is around 20%.
The fungi play an important role in soil, degrading complex sources of organic carbon, some of which may be organic pollutants. While degradation is part of the normal cycle of carbon and other elements, loss of carbon from soil, especially agricultural soil, is responsible for as much as 20% of the increased CO2 in air, contributing to global heating. Indeed, a reduction of fungal degradation of organic carbon leading to an increase of humus in soil may be one mechanism whereby both improved soil quality and carbon sequestration may be achieved.
Conclusion
Fungi in soil demonstrate a variety of life styles, indicative of the variation in most habitats. Groups of different fungi utilise different carbon resources, with the result that very little organic carbon remains in soil, or passes through the soil in water.
References
Dighton J, White JF & Oudemans P (2005) The Fungal Community (3rd edit). Taylor & Francis.
Dix NJ & Webster J. 1995. Fungal Ecology. Chapman Hall. Ch 7.
Zak JC, in Carroll GC & Wicklow DT 1992 The Fungal Community (2nd edit). Marcel Dekker. Ch 22.
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