The Biology Of Soil Health 4

1.

When plant residues are left on soil, they feed the soil microbes, which is good for soil health. But from a sequestration perspective, what happens to the carbon in these residues?
- A.
  
  It is released into the atmosphere as methane gas (CH4).
- B.
  
  It is all sequestered as soil carbon.
- C.
  
  It is all consumed by microbes, then returned to the atmosphere as CO2 (microbe respiration).
- D.
  
  Some portion remains in the soil as SOM; the rest is lost to the atmosphere as CO2 (microbe respiration).
Correct Answer
D. Some portion remains in the soil as SOM; the rest is lost to the atmosphere as CO2 (microbe respiration).

Explanation
Microbes consume the organic residues and incorporate some of the carbon in their bodies, with the rest released as CO2. Over time, as these microbes live, breathe, and die, much of the remaining carbon will also be released as CO2. However, some portion will be converted into more stable forms of carbon, and some will be protected in soil aggregates. These latter forms of carbon will stay in the soil much longer, adding to the soil organic matter pool. The latest scientific evidence suggests that in healthy soils more carbon is retained in these more stable forms than is the case in less healthy soils. This is probably due to the higher levels of microbial activity and greater aggregate formation in healthy soils, which usually leads to higher levels of soil organic matter.

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2.

What happens to the ratio of bacteria to fungus (the relative amount of each present in soil) as natural ecosystems move from early to later successional stages (e.g., from an abandoned farm field to a young forest)?
- A.
  
  The ratio moves from fungal domination to bacterial domination.
- B.
  
  No general pattern has been determined.
- C.
  
  The ratio moves from bacterial domination to fungal domination.
- D.
  
  Bacteria grow more quickly than fungus, so they always dominate in soils.
Correct Answer
C. The ratio moves from bacterial domination to fungal domination.

Explanation
Farm fields are often bacterial dominated, mainly because of the management practices used (tillage, in particular, is hard on fungus). When soil health best-management practices are adopted (e.g., no-till, cover crops), the ratio eventually changes as fungal populations become better established. These types of farm fields more closely mimic mid-successional ecosystems such as grasslands. Several benefits arise from this change: for instance, higher fungal populations are closely associated with higher organic matter levels and better soil aggregation.

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3.

Studies have shown that mycorrhizal fungi can help plants prepare for disease or pest outbreaks by passing warning messages from infected plants to non-infected plants via their underground networks.
- A.
  
  True
- B.
  
  False
Correct Answer
A. True

Explanation
At least two studies have revealed this beneficial characteristic of mycorrhiza. In one, tomato plants were warned of the presence in nearby plants of a fungal disease; in the other, bean plants were warned of attacking aphids. In both cases, non-affected plants were able to set up defenses in advance of any infestation.

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4.

Which of the following statements are true?
- A.
  
  Some plants are able to enlist microbial help in fighting off disease.
- B.
  
  Some fungi are able to trap and destroy nematode pests.
- C.
  
  Some protozoa attack plant roots.
- D.
  
  Some nematodes attack plant roots.
Correct Answer(s)
A. Some plants are able to enlist microbial help in fighting off disease.
B. Some fungi are able to trap and destroy nematode pests.
D. Some nematodes attack plant roots.

Explanation
1 Plants put out root exudates to attract and grow beneficial microbes in their root zone. Some of these microbes help plants to prevent and/or fight disease organisms. 2 Some fungi can trap nematodes in hyphal loops; they then gradually consume the nematode (which are often root feeders). 3 Protozoa eat bacteria and some organic matter, but not living roots. 4 Some types of nematodes are root feeders and they can cause a lot of economic damage in crops.

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5.

Among the mineral components of soil, silt particles are larger than clay particles, but smaller than ________ particles.

Correct Answer(s)
sand

Explanation
Sand is defined as particles between 0.5 and 2 millimetres (mm) in size; silt is between 0.002 and 0.5 mm; and clay particles are very tiny -- smaller than 0.002 mm and down to the size of a bacterium -- less than 0.001 mm (one thousandth of a millimetre).

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4
6.

"Net primary productivity" is:
- A.
  
  The total amount of energy captured by an ecosystem in a year.
- B.
  
  The amount of energy used by all of the organisms in an ecosystem in a given unit of time.
- C.
  
  The ratio of above-ground energy to below-ground energy in an ecosystem.
- D.
  
  The total amount of solar energy captured by an ecosystem over a given period of time, minus the energy that the organisms in the ecosystem use to sustain themselves over the same period.
Correct Answer
D. The total amount of solar energy captured by an ecosystem over a given period of time, minus the energy that the organisms in the ecosystem use to sustain themselves over the same period.

Explanation
The primary productivity of an ecosystem is the amount of solar energy that the system converts into biochemical energy, for any given period of time (usually expressed as an annual figure). The word "net" means that the amount of that energy used up by the organisms in the ecosystem over that time (the energy they use to live) has been subtracted. This can be thought of as the gain in biomass over that period, as the biomass represents the stored energy.

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7.

The largest single organism ever discovered is a fungus.
- A.
  
  True
- B.
  
  False
Correct Answer
A. True

Explanation
Armillaria ostoyae, nicknamed the "Humongous Fungus", is an organism that covers 2,385 acres (almost 4 square miles) of the Malheur National Forest in Oregon.

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8.

Rhizobium is a type of bacteria that is symbiotic with legumes. They live in nodules on the roots and receive sugars and other carbon-rich substances from the plant. In return they fix atmospheric ________ into a form that can be used by the plant.

Correct Answer
nitrogen, N, N2

Explanation
In nature, almost all of the N used by plants comes from bacterial N-fixers. Many of these are symbiotic bacteria, such as Rhizobium, but others are free-living bacteria.

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4
9.

Which of the following statements is false?
- A.
  
  Both bacteria and fungi are important in aggregate formation in soils.
- B.
  
  Bacteria are better than fungi at breaking down woody materials, which are high in lignin.
- C.
  
  Bacteria are better than fungi at breaking down woody materials, which are high in lignin.
- D.
  
  Mites and other arthropods promote the nutrient-cycling processes of the soil food web by chewing and shredding organic residuals, reducing their size and making them more accessible to bacteria and fungi.
Correct Answer
B. Bacteria are better than fungi at breaking down woody materials, which are high in lignin.

Explanation
Actually, it is the fungi that are good at breaking down the tough-to-degrade organic materials such as lignin. Most bacteria cannot degrade these substances, or do so very slowly. This is one of the main reasons that forest soils are fungal dominated.

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10.

Select the practices below that are beneficial to soil health.
- A.
  
  Summer fallow
- B.
  
  Adding perennial pasture to a rotation
- C.
  
  Regular tillage
- D.
  
  Cover cropping
Correct Answer(s)
B. Adding perennial pasture to a rotation
D. Cover cropping

Explanation
1 - summer fallow (ploughing, followed by leaving the field bare) is usually done to destroy weeds and conserve moisture, but in fact it starves the soil food web (no residues or plant root exudates), reducing its size and diversity. 2 - adding perennial pasture to a rotation (putting some portion of a farm into pasture for a few years, before returning it to production) increases the inherent fertility (and optimizes the soil biology) due to the organic matter -- residues and exudates -- added to the soil over the pasture period. If animals are grazed obn the pasture, they will also fertilize the soil with their manure; 3 - regular tillage destroys soil structure and results in bacterial dominated fields. 4 - cover crops add carbon, retain nutrients, and protect and feed the soil food web.

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11.

The word "recalcitrant" means "high calcium levels".
- A.
  
  True
- B.
  
  False
Correct Answer
B. False

Explanation
"Recalcitrant" refers to the ability of an organic substance to resist degradation by microbes. For instance, fresh plant residues are not at all recalcitrant, whereas biochar (organic materials heated in the absence of oxygen, so that almost all of the non-carbon materials are driven off and only black carbon remains) is usually extremely recalcitrant. This means that decomposer organisms are not able to break it down further and release the carbon back into the atmosphere as CO2. Extremely recalcitrant substances can remain in soils for thousands of years.

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12.

Which of the following is NOT a benefit of earthworms in agricultural soils.
- A.
  
  Better soil aggregation due to "slime" produced by earthworm skins (biological glue).
- B.
  
  Transportation of microbes from one area to another within soils.
- C.
  
  Release of nutrients from minerals via the use of specialized enzymes.
- D.
  
  Better aeration of soils through the burrowing activities of worms.
Correct Answer
C. Release of nutrients from minerals via the use of specialized enzymes.

Explanation
Nutrient release from minerals can be accomplished by certain enzymes, but these enzymes are produced by plants and some microbes, rather than by earthworms.

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13.

The term "common ________ network" refers to the mass of interconnected filaments produced and maintained in the soil by mycorrhizal fungi.

Correct Answer
mycelial

Explanation
Mycorrhizal fungi are the most common fungi in healthy agricultural soils, dominating compared to the decomposer and pathogenic varieties. They form "common mycelial networks" with their hyphae, connecting with each other both within and between species, and these networks can pass chemical messages, nutrients, water, and maybe even things that we don't yet know about.

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4

The Biology Of Soil Health 4

When plant residues are left on soil, they feed the soil microbes, which is good for soil health. But from a sequestration perspective, what happens to the carbon in these residues?

What happens to the ratio of bacteria to fungus (the relative amount of each present in soil) as natural ecosystems move from early to later successional stages (e.g., from an abandoned farm field to a young forest)?

Studies have shown that mycorrhizal fungi can help plants prepare for disease or pest outbreaks by passing warning messages from infected plants to non-infected plants via their underground networks.

Which of the following statements are true?

Among the mineral components of soil, silt particles are larger than clay particles, but smaller than ________ particles.

"Net primary productivity" is:

The largest single organism ever discovered is a fungus.

Rhizobium is a type of bacteria that is symbiotic with legumes. They live in nodules on the roots and receive sugars and other carbon-rich substances from the plant. In return they fix atmospheric ________ into a form that can be used by the plant.

Which of the following statements is false?

Select the practices below that are beneficial to soil health.

The word "recalcitrant" means "high calcium levels".

Which of the following is NOT a benefit of earthworms in agricultural soils.

The term "common ________ network" refers to the mass of interconnected filaments produced and maintained in the soil by mycorrhizal fungi.