Introduction

Soil consists of particles interprersed by pores. The textural units are usually considered to be clay, silt and sand, that are classified according to size, and organic fragments of various sizes, state of decomposition and chemical recalcitrance. 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 in soil. Aggregation also determines biological activity, especially growth of plant roots and movement of fauna. Aggregation is an important determinant of soil function.

Aggregation

Aggregation of soil is directly related to the content of organic matter (carbon), plants and fungi. Soil is aggregated by essentially two processes, chemical bonding or gluing, and physical binding or enmeshment. All ultimate particles of the soil carry a charge on their surface, clay much more than sand and silt. Indeeed, while clay may carry a net negative charge, the quantity is determined by the type of clay. Negative charges attract positively charged particles, and divalent cations like Ca and Mg may function as bridges among clay particles. These charged particles interact with one another and the chemically active surfaces of organic compounds. Microbial exudates and breakdown transformants of litter "glue" these fine particles together to form micro-aggregates (defined as being smaller than 250 µm). AM fungi specifically release glomalin. Concentrations of glomalin have been correlated with the development of microaggregates, though note that we do not really know what glomalin is, measures of glomalin are, at this stage, inaccurate and probably include polyphenolic materials.

 

Fungal transformants and exudates, possibly including glomalin, act in two ways: by coating fine particles with a layer of hydrophobic materials and by gluing the particles embedded and held on the surface. The action of these materials creates micro-anaerobic sites within micro-aggregates, where organic carbon is protected from oxidation and microbial attack.

Fungal hyphae and fine plant roots bind and enmesh micro-aggregates forming medium and larger sized particles called macro-aggregates (larger than 250 µm). The role of fungi appears to be one where hyphal growth through the soil pores may stabilise the collection of micro-aggregates. The degree of enmeshment is dependent on several factors, including the texture of the soil, nature of vegetative cover and its contribution to organic matter in the soil, the quantity and the degree of degradation of organic matter, and surface charge of enmeshed particles. In the rehabilitation of mine sites, addition of composy is essential to start the process of carbon turnover and aggregation.

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 length of hyphae is present in soil.

Soil is essentially oligotrophic. Except for exudations from plant roots, organic 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 macro-aggregation of soil. 

Stability of macro-aggregates 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. This process of forming macro-aggregates can be manipulated by adding compost to soil and encouraging growth of mycorrhizal plants.

The stability of macro-aggregates is directly related to the length of hyphae of AM fungi. Very high densities of hyphae stabilise macro-aggregates. 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 proliferation, release of exudates, longer persistence in soil, binding clay particles more effectively, or growing through soil more rapidly. However, macro-aggregates have a limited life because of the life of the mycelium. Longer term stability is likely to be related to the storage of organic carbon. The most important stores of carbon in soil are held within longer-lived micro-aggregates where the carbon is protected.

AM fungi probably have a limited and indirect contribution to the stores of recalcitrant carbon. With few exceptions, the fungi lack an outler mucilaginous layer. They are not known for form polyphenolic materials. The experimental testing for the role of glomalin in carbon sequestration has so far not revealed any changes associated with the fungi.

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 macro-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 macro-aggregation. They act in concert with other fungi, other fungi, organic matter and plants to develop micro-aggregation. The microbes compete for space, organic nutrients and water. Most plants and microbes release complex polymers 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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