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Soil Aggregation Questions

Question 1

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Answer 1

Particles of sand, silt, clay and organic matter can be bound into aggregates by chemical and physical processes. The aggregates are classified according to size and nature of structure into microaggregates and macroaggregates. Microaggregates appear to be individual soil particles chemically attached into a recognisable, stable structure. Microaggregates can be bound by physical and chemical processes into larger macroaggregates which are also stable. Macroaggregates are bound by roots and hyphae which excrete further mucilage chemically stabilising the units.

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Question 2

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Answer 2

Plants and microbes exude polysaccharides into their environment. The mucilage is chemically charged and can therefore bond with charges on clay and organic fragments. Fewer bonds occur on sand and silt particles and their “gluing” is reduced. Older microaggregates have been examined and found to be hollow indicating that the bacterial cell has died leaving the remnant shell of clay embedded in polysaccharide. Fungal hyphae and plant roots can also grow through soil pores, and in doing soil physically bind microaggregates together. The physical binding results in larger particles, which can be further stabilised by further release of mucilage, which may chemically bond the microaggregates.

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Question 3

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Answer 3

Fungal hyphae can penetrate pores as small as 1mm. They have the capacity to proliferate in particular sites where very high densities ensure all material caught in the mycelium remains within the network. Provided the hyphae are not fragmented, the proliferation will result in a stable structure.

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Question 4

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Answer 4

Hyphae of AM fungi have a constant source of organic carbon from their host providing them with a distinct competitive advantage in soil. As a consequence, AM fungi are found in much higher concentrations in soil. They play a very important role due to the almost ubiquitous presence of their host. EM fungi may also play an important role in some situations. Their hosts are less common. However, many EM fungi have the capacity to degrade the leaf litter released each year by trees, potentially resulting in an increased number of sites for chemical bonds. However, as forests are rarely tilled, the comparative stability of soil under trees, may be due to the increased buildup organic carbon than to the activity of EM fungi.

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Question 5

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Question 6

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Question 7

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