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Soil Aggregation And AM Fungi

Introduction

Soil consists of particles interprersed by pores. The ultimate textural units are usually considered to be clay, silt and sand, and organic fragments of various sizes, state of decomposition and chemical reactivity. In soil, these particles are aggregated together to varying degrees. The pores created by aggregation are necessary for the storage and movement of air and water in the soil profile. Aggregation also determines biological activity, especially growth of plant roots and microbes, 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 (organic carbon), plants and arbuscular mycorrhizal 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 total charge is determined by the type of clay. Negative charges attract positively charged particles: divalent cations like Ca and Mg function as bridges among clay particles, and bewteen clay and organic matter.

A18 SEM
SEM of colonised mine spoil showing hyphal proliferation and deposition of mucilage.

The widely accepted model of aggregate hierarchy postulates different binding agents at different stages of the aggregate hierarchy. Implicit in the hierarchical model is the sequential formation of each stage. That is, fine structural elements such as clay and organic matter initiate micro-aggregates (up to approx 250 µm), which in turn, are bound by mucilage, hyphae of arbuscular mycorrhizal fungi and fine roots to form macro-aggregates. Macro-aggregates may also form around organic matter and then fragment along planes of weakness into micro-aggregates. Processes such as shrink-swell during wet-dry cycles, the elongation, expansion and death of roots will fragment aggregates. Critically, the fragmentation process posits the enclosure of fine organic matter with encrustation by clay particles, that is the nuclear assemblage of micro-aggregates. Aggregates may be subject to many cycles of breakdown and re-formation especially in cultivated soil. Micro-aggregate formation is argued to be crucial for the stabilisation of OC in soil because the micro-aggregate is most resistant to further disruption and fragmentation.

Fungal transformants and exudates act in two ways: by coating fine particles with a layer of organic matter and by gluing the particles embedded and held on the surface. The variable rates of respiration within these materials initially creates micro-anaerobic sites around which form micro-aggregates. Because the core of the micro-aggregate is anaerobic, polyaromatic carbon is protected from oxidation and the structure of the aggregate matures. Hyphae of arbuscular mycorrhizal fungi continue to colonise the micro-aggregate, leaving organic matter behind. Hyphae of other fungi colonise the aggregate, utilise the remnants of the hyphae of arbuscular mycorrhizal fungi, transform some of the carbon to melanin and other polyaromatic compounds, and then withdraw in a process called autolysis. As oxidation is required for breakdown of aromatic carbon such as melanin, the melanised walls remain as hyphal ghosts inside the micro-aggregate, leading to an increase of protected organic carbon in the aggregate.

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 compost is essential to start the process of carbon turnover and soil aggregation.


Photo of hyphae and assorted rubbish in soil.
aggregtn
Aggregation of soil, no compost in the left tube, and AM in the right tube only.

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. An enormous length of hyphae of these fungi is present even in cultivated 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. Soils also takes a long time 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, some isolates have a greater binding effect due to greater proliferation, longer persistence in soil, binding clay particles more effectively, or growing through soil more thoroughly. However, macro-aggregates have a limited life because of the life of the mycelium.

g warcuppiiMucilage
Few AM funi have extracellular materials. G warcuppii, shown above has a mucilaginous outer layer.

AM fungi probably have a limited and indirect contribution to the stores of recalcitrant carbon. With few exceptions, the fungi are not known for form polyaromatic materials.

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 aggregation. They act in concert with 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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