Introduction

Soil consists of particles of various size and charge. The textural units are usually considered to be clay, silt and sand, that are classified according to size, and organic fragments of various sizes and composition. In soil, these particles are aggregated together to varying degrees. The pores created by aggregation are necessary for the storage in and movement of water through the soil profile, and the diffusion of air into the soil. Aggregation also determines biological activity, especially growth of plant roots and movement of fauna. Aggregation is an important determinant of soil function.

Aggregation

Soil is aggregated by essentially two processes, chemical bonding or gluing, and physical binding or enmeshment. Microbial exudates and breakdown products of litter "glue" fine particles together to form microaggregates (defined as being smaller than 250 µm). AM fungi specifically release glomalin, a putative glycoprotein within the group called heat shock proteins, that is long lived in soil. Concentrations of glomalin have been correlated with the development of microaggregates, though note that measures of glomalin are, at this stage, extremely inaccurate. Concentrations of glomalin are greater in soils under perennial vegetation and pastures than crops.

Glomalin acts in two ways: by coating fine particles with a layer of hydrophobic materials and by gluing the particles together. The action of glomalin creates microanaerobic sites, where organic matter is protected from oxidation and microbial attack.

Fungal hyphae and plant roots bind microaggregates to form medium and larger sized particles called macroaggregates (larger than 250 µm). The role of fungi appears to be one where growth through the soil pores may stabilise the collection of microaggregates and that polysaccharides may chemically bond the associated structures. The degree of binding is dependent on several factors, including the texture of the soil, nature of vegetative cover and charge of particles.

Arbuscular mycorrhizal fungi are an important component of the soil mycota. AM fungi contribute an enormous proportion of the hyphae in soil. While up to 20 m of hyphae have been found per g of soil, lengths of less that 5 m are more common, and this decreases to less than 0.5 m in cultivated soils. Even so, an enormous distance of hyphae is present in soil. Soil is essentially oligotrophic. Except for exudations from plant roots, nutrients are present in low concentrations, and simple organic molecules are rapidly removed. AM fungi have a constant supply of organic carbon from their host, which places them at a distinct competitive advantage over saprotrophic species. Both hyphae of AM fungi and roots to which they are attached form a considerable network through soil. As a consequence, they contribute to most of the macroaggregation of soil. 

Stability of macroaggregates is directly related to the amount of organic matter as well. Thus, soils under permanent pasture are more stable than soils under cropping; soils under direct-drill are more stable than those under traditional tillage. However, it also takes a long time for soils to regain stability following degradation.

The stability of macroaggregates is directly related to the length of hyphae of AM fungi. In stable aggregates, the hyphae may be important because they are present in very high densities. While these discussions have concentrated on AM fungi as a group, it is likely that some isolates will have a greater binding effect due to greater release of exudates, longer persistence in soil, binding clay particles more effectively, forming more hyphae in soil or growing through soil more rapidly. Indeed, AM fungi are likely to interact with other groups of microbes, resulting in complex contributions to soil stability.

The direct role of hyphae of AM fungi is also reliant on the plant root to which it is attached. While some hosts appear to influence the fungal hyphae differently, the host itself has direct effects. Fine and very fine roots influence aggregation differently. Fine roots aid aggregation directly, and very fine roots appear to support AM fungi more effectively. These interactions need to be examined more closely to clarify the mechanisms.

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Conclusion

Hyphae of AM fungi are important determinants of soil aggregation. They act in concert with other fungi, other microorganisms and plants. The microbes compete for space, organic nutrients and water. Most plants and microbes release polysaccharides and other organic materials into soil which are simultaneously sources of nutrients for microbes and binding agents in soil. The array of processes and the microbes interacting in the processes that bind soil particles are diverse and poorly understood.

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References

Tisdall JM  in Robson AD, Abbott LK & Malajczuk N 1994 Management of Mycorrhizas in Agriculture, Horticulture and Forestry. Kluwer, Ch 11.

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