Soil Science

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Which of the following is most likely to comprise the bulk of the heterotrophic biomass in a temperate grazed pasture system?

Correct! Soil microbes. Note that plants are autotrophic, not heterotrophic. They obtain energy from the sun via photosynthesis, rather than from oxidation of organic compounds.

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Some soil organisms live in air-filled soil pores, while others live in water-filled pores. Which of the following are most likely to be found in water-filled pores?

Correct! Bacteria. For any soil water content, there is a pore size below which all pores will be filled with water. Large soil organisms require large soil pores, which tend to be air-filled. These organisms include soil macrofauna, as well as mesofauna such as springtails and mites. Smaller mesotauna (eg nematodes) and soil microbes live primarily in soul water.

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The main contribution of soil mesofauna to nutrient cycling is:

Correct! Soil mesofauna have an important role in increasing the surface areas of plant residues by ingesting and excreting them. Soil microbes are the main source of soluble, mineralised nutrients. Only soil macrofauna are large enough to create mecropores and bury residues.

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Fungi called vesicular arbuscular mycorrhizas (VAMs) colonise plant roots, forming a symbiotic relationship with the plant. WAM hyphae penetrate the soil, effectively expanding the plant root system, and facilitating nutrient uptake. For which nutrient would VAM be of most value?

Correct! VAM associations are particularly beneficial for plant phosphorus uptake. Unlike the other nutrients shown, P is very immobile in soil, and so little phosphate is cared to the roots by diffusion or mass flow. Rather, roots must grow to where the P is available in the soil. The increase in effective rooting volume provided by VAM can therefore improve P uptake.

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