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2.2 Physical transformations mediated by soil organisms

How do soils form?

Microorganisms participate in the early stages of soil formation (Ehrlich 1996). Within the regolith and above ground, rock surfaces become colonised by microscopic organisms. Lichens are often the earliest colonisers of exposed rock surfaces. These organisms release acids, which dissolve the rock, and enhance the accumulation of organic debris. Over time other organisms also colonise the same spot, facilitated by the changes brought about by the lichen. Eventually, the surface of the rock is etched, crumbled, cracked and eroded. Erosion from wind and rain, combined with the continued action of microorganisms, continues the break down of these smaller rock pieces.

Lichens also help stabilise soils in dry areas that have limited plant cover. In relatively undisturbed ecosystems some species of lichen form crusts on a soil. Although this crust is thin, it provides resistance to mild wind erosion and also rain drops and assists in stabilising particles of soil disturbed by animals. Because the soil is stabilised communities of many types of organisms can live in these soil crusts. Microbial crusts usually absence from agricultural ecosystems if they are frequently disturbed; the slow-growing organisms have difficulty in establishing a coherent film.

What is soil aggregation?

Soil aggregation is the adherence of soil particles . Soil aggregates range in size from greater than 2 mm to less than 2 Ám. The proportion of differently sized aggregates varies with soil type and management history.

Soil organisms, organic matter and roots all contribute to assist in the processes of aggregation. Soil organisms can create both strong and weak forces between soil particles, so the stability of aggregates can vary. A soil with a friable structure has aggregated particles of different sizes and large soil pores.

What is the role of soil organisms in aggregate formation?

Soil organisms are essential to the formation of soil aggregates. Comparison of sterile organic matter added to soil with organic matter that included microorganisms has been used to demonstrate this. Only organic material with its associated community of soil organisms aggregates soil particles. The processes involved in facilitating the clustering of soil particles into aggregates are closely linked to those involved in the breakdown of organic matter.

There is further evidence for the roles of soil microorganisms in the formation of soil aggregates:

• soil aggregation is increased by the addition of glucose (a readily available source of carbon for bacteria and fungi).

• the addition of well composted organic matter does not increase soil aggregation because the readily decomposable component of the organic matter has already been decomposed and carbon is not in a form that stimulates microbial activity.

Larger soil animals such as earthworms contribution significantly to soil aggregation through the formation of wormcasts of coalesced faecal pellets (Marashi and Scullion 2003). When earthworms feed on organic matter they ingest some soil, leading to aggregation of the soil particles when they are deposited in soil. Not all earthworms function in the same way; the structure, position and permanence of burrows vary between earthworm species. Some earthworms line their burrows with casts while others deposit casts on the soil surface. This helps to create a more heterogeneously structured soil.

Small soil animals enhance soil aggregation indirectly through their interactions with soil microorganisms during the breakdown of organic matter.

Microorganisms can facilitate soil aggregation in several ways. Some fungi and bacteria produce polysaccharide gums that cause soil particles to attach to each other. These polysaccharide chains of simple carbon molecules (Bettelheim and March 1991) are flexible and make many points of contact across the surface of soil particles. Polysaccharides are released onto the outer cell wall surface of some microorganisms and are relatively resistant to immediate degradation by other soil organisms. In contrast, polysaccharides that are released from plant roots play only a minor role in soil aggregation because they decompose quickly in soil. In contrast, the roots themselves, especially thin roots, physically bind soil particles together.

Evidence for the role of polysaccharides in soil aggregation includes the use of the compound sodium periodate. When added to soil, sodium periodate breaks the long-chain polysaccharide molecules and thus reduces their effectiveness in aggregating soil particles (Tisdall and Oades 1982).

The scale at which microbial polysaccharides assist in aggregating soil is very small and there are other ways that microorganisms help bind soil particles. Some bacteria attract soil particles because they have an electrostatic charge on their surface that can become polarised. This attracts the bacterium to a surface, such as a clay particle, which has an opposite charge. In this way, forces of attraction between bacteria and soil particles contribute to the formation of small soil aggregates. This occurs at the same time as mineralisation of organic soil matter because some of the same organisms are simultaneously involved in both processes.

Some fungi bind soil into temporary aggregates (Degens 1997). The two groups of fungi that are likely to be most importance to the process of soil aggregation are those that colonise fresh organic matter and those that form mycorrhizal associations with roots. Several studies have shown that soil aggregation can be related to the length of hyphae in the soil. Furthermore, fungi that form 'woolly' rough hyphae are more effective at binding soil than are fungi that produce smooth, thin hyphae.

In forest soils, mycorrhizal fungi are abundant at the soil surface and create mats that stabilise the soil. In agricultural soils, mycorrhizal fungi may be important in binding both sandy soils and clayey soils but are less likely to form stable hyphal mats if there is frequent soil disturbance.

Aggregation of soil particles generally involves specific effects of polysaccharides and binding by fungal hyphae, but there is also a role for other microbial processes. The products of microbial degradation of organic matter include degraded components of microbial cells and plant organic matter. These organic molecules form inorganic linkages with very small soil particles such as clay molecules. It is difficult therefore to define the exact mechanism of soil aggregation, because a range of microbial and inorganic processes contribute and act interdependently.

The relationship between types of organic matter, the stability of aggregates and the longevity of the aggregates is illustrated in a classic study where glucose, ryegrass and cellulose were added to a soil and the stability of aggregates was measured for 12 months (Tisdall and Oades 1982). Glucose provides a readily available source of carbon for microorganisms whereas cellulose is a relatively unavailable source of carbon due to its slow degradation. Rye grass is composed of a mixture of readily available carbon and other molecules more resistant to breakdown. The measure of aggregate stability used in this study was water-stable aggregation. Addition of glucose immediately increased the stability of the aggregates but this was only short-lived, only lasting about two weeks. Within 6 months, all of the increased aggregation stimulated by the glucose had disappeared. In contrast, cellulose had little effect within the first 6 months, but there was a gradual increase in aggregate stability up to 12 months. This would have occurred largely in parallel with the progressive breakdown of cellulose. Other factors include the formation of more stable molecules that had greater binding properties than those released during the rapid microbial breakdown of glucose, and the involvement of different types of bacteria and fungi in the presence of glucose and cellulose.

When ryegrass was added, there was a gradual and sustained increase in aggregate stability. The readily available sources of carbon in the ryegrass appear to have enhanced aggregation. This contrasts with the slower rate of increase in aggregation that occurred for the pure cellulose treatment. The presence of more available forms of carbon in the ryegrass would have extended the supply of carbon and energy for microbial growth, allowing some cellulose and lignin to be degraded. Consequently, this would have contributed to soil aggregation processes over a longer time than was possible for either glucose or cellulose alone. Furthermore, different breakdown products and microorganisms involved in the decomposition of ryegrass compared to those associated with glucose and cellulose may also have contributed to the differences in strength of aggregates.

How do organisms affect the porosity of soil?

Spiders, ants and other large soil animals, including earthworms, improve the water penetrability of soil by creating burrows though which water can infiltrate and flow. Very small animals influence the fine structure of soil through their movement, degradation of organic matter and production of faecal pellets. Furthermore, soil aggregation influences pore structure of soil.

How do organisms influence water-repellence of soil?

An important quality of soil is that it retains and distributes water evenly, a function which ensures that plant roots have adequate water access through the soil profile. However, under some conditions, soils become water repellent. For example, in sandy agricultural soils where legumes such as lupins and pasture species are grown, soils are more likely to become water repellent than under wheat. In natural ecosystems, water repellency develops in association with some tree species including species of Eucalyptus and Pinus.

Water repellence is related to the presence of soil particles that have hydrophobic organic, waxy substances on their surfaces, or the presence of litter with hydrophobic qualities on the soil surface (Doerr et al. 2007). Generally, there is no well-defined relationship between the amount of organic matter in a soil and its degree of water repellence. The type of organic matter, its microbial degradation products and the type of soil all combine to produce water-repellent properties. Water repellence is more prevalent in soils with low amounts of clay.

The origin of water repellence is not completely understood. But, it is clear that the degradation of certain types of organic matter by microorganisms can contribute to water repellence if the breakdown products of organic matter have hydrophobic characteristics. These organic molecules then coat sand particles, leading to the soil having water-repellent properties as a whole.

Water repellence in forest and heath habitats may also be due to fungal hyphal mats that spread over the soil surface beneath the litter layer. These mats repel water due to their hydrophobic characteristics, so that water droplets cannot penetrate them and reach the soil below.

Other microorganisms can reverse the water-repellent properties of soil by degrading the waxy molecules. Generally, a limited number of wax-degrading organisms are a normal component of the soil microbial biomass. However, under field conditions, the growth of these microorganisms is not usually enough to prevent or overcome water repellence.

Water repellence can also be alleviated by mechanical abrasion. This breaks up the organic coatings on the sand particles and minimises their hydrophobic properties. Alternatively, the addition of clay reduces water repellence (Blackwell 1993). In this study, the addition of a wetting agent or clay to a water-repellent soil increased wheat and lupin yields. In contrast, the water repellent conditions had less effect on barley growth, so a reduction in water-repellency did not increase yields much. The addition of clay might also make the soil more favourable for the survival and activity of soil microorganisms, which would also contribute to the increase in growth of wheat and lupin.

Summary Points (2.2)

• Soil organisms are major contributors to the aggregation of soil particles.

• Roots, especially thin roots, and organic matter are also involved in soil aggregation.

• Microorganisms help bind soil particles by producing gum-like exudates or hyphal networks that enmesh soil particles or form linkages between particles.

• Soil animals, such as earthworms, make significant contributions to soil aggregation by increasing microbial activity of microorganisms associated with the soil and organic matter that they ingest.

• Larger soil animals such as spiders and earthworms contribute to water penetrability of soil by creating pathways for water to flow or by increasing the bulk density of the soil.

• Water-repellence of soil is due to hydrophobic wax-like molecules coats on soil particles that are the breakdown products of certain types of organic matter.

• Soils contain a range of organisms, usually actinomycetes, that are able to decompose waxy, water repellent molecules, reducing their hydrophobic characteristics.

• In natural ecosystems, mats of fungal hyphae may have hydrophobic characteristics that prevent water penetrating to the soil below.

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