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ENVB 210 > Chemosphere IV > Flashcards

Chemosphere IV Flashcards

1
Q

Does soil pH measure the pH of solution or of the exchange complex?

A

It measures the pH of the exchange complex.

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2
Q

Give 2 examples of soils that are typically acidic.

A

Forest soils and acid peats.

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3
Q

Give 2 examples of soils that are typically alkaline.

A

Calcareous soils and sodic soils.

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4
Q

Name 3 reasons why pH is important in soils.

A
  1. Greatly influences availability of plant nutrients
  2. Influences the activity of microorganisms and type of vegetation that will grow
    3.Affects the mobility of many pollutants in soil via rate of biochemical breakdown, solubility, adsorption to colloid
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5
Q

In acid soils, what nutrients are typically lacking?

A

Ca, Mg, K, P, N, S, Mo, B.

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6
Q

In acid soils, what nutrients are typically in excess?

A

Iron, aluminum, manganese zinc.

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7
Q

In alkaline soils, what nutrients are typically lacking?

A

Fe, Mn, Zn, Cu, and Co.

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8
Q

In alkaline soils, what nutrients are typically in excess?

A

Ca and Mo.

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9
Q

What two processes promote soil acidification?

A
  1. The leaching of nonacid cations
  2. The production of H+ ions
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10
Q

Is Al3+ considered an acid or nonacid cation? Why?

A

It is considered an acid cation because it tends to split the water molecule and combine with the OH- ion, leaving H+ behind in solution.

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11
Q

What are the two ways in ways the loss of nonacid cations occurs?

A

Through leaching or through plant biomass removal.

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12
Q

What cations are typically lost through leaching?

A

Ca2+, Mg2+, K+, and Na+

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13
Q

Describe 3 steps in the process by which leaching occurs.

A
  1. Addition of H+ ions from acid-forming processes
  2. Exchange of H+ ions for nonacid cation(s)
  3. Leaching loss of nonacid cations along with anions of acids
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14
Q

After leaching, the exchange complex and soil solution become dominant in which two cations?

A

H+ and Al3+.

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15
Q

Why might humid regions have more acidic soils?

A

Higher water inputs can lead to more leaching.

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16
Q

What is the geographic relationship between soil acidity and annual precipitation?

A

Generally, humid climates and northern climates tend to have more acidic soils.

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17
Q

Why might northern latitudes have more acidic soil?

A

Because there is more precipitation (?).

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18
Q

Explain how plant biomass removal causes the loss of nonacid cations.

A

Crops normally absorb mostly non-acid ions for their nutrition. When they are removed from the field, many ions responsible for counteracting soil acidity are lost.

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19
Q

What effect does crop death have on soil pH? Why?

A

It tends to prevent acidification, because the crops are rich in nonacid cations, which can then be returned to the soil.

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20
Q

Name the 4 major sources of H+ ion production.

A
  1. Organic acids and CO2
  2. Plant uptake of cations
  3. Nitrification
  4. Acid raid and deposition
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21
Q

Explain how CO2 is a source of H+ ions in soil.

A

CO2 is produced by the roots and by microbial respiration, and some dissociates in water to form carbonic acid, leaving H+ behind to contribute to acidity.

22
Q

Explain how organic acids are a source of H+ ions in soil.

A

They come from the breakdown of organic matter, inputs from plants, and microbial excretions. These can also be deprotonated.

23
Q

What is the most common source of H+ ions in soil?

A

CO2 (carbonic acid)

24
Q

What types of ecosystems typically have a lot of organic acids in their soils? Why?

A

Forests dominated by conifers, as they have lots of organic acids in their needles. This is partly why they tend to have very acidic soil.

25
Q

Explain how plant uptake of cations is a source of H+ ions.

A

To maintain charge balance in the plant during the soil ion uptake process, H+ or OH- ions must pass out of the root into the surrounding soil. So, when the root takes up cations, H+ is released into the soil.

26
Q

How can/is plant uptake of cations as a source of acidity be mitigated by plants?

A

When they take up anions, they release OH- into the soil solution, which helps to neutralize the pH again.

27
Q

What is nitrification?

A

It is the oxidation of ammonium (NH4+) to nitrate (NO3-)

28
Q

What is the largest input of H+ ions into soils?

A

Nitrification.

29
Q

Write the chemical equation of nitrification.

A

NH4+ + 2 O2 -> NO3- + H2O + 2H+

30
Q

Explain how nitrification serves as an input of H+ into the soil. Name the major inputs in the process.

A

Ammonium gets added into soils through fertilizer, which results in an increased rate of nitrification, which releases more H+ ions into solution. There are also inputs by atmopsheric deposition and legume N2 fixation.

31
Q

What is the consequence of ammonium deposition on atmospheric emissions?

A

It leads to emissions of ammonia (NH3). This ammonia then reacts with water in the atmosphere and turns into ammonium, which gets deposited in soils nearby.

32
Q

What can be done to reduce NH3 emissions (3)?

A
  1. Incorporate manure into soil
  2. Cover stock piles
  3. Lower temperature and pH
33
Q

What is the consequence on ecosystems of NH+ deposition?

A

Ecosystems surrounding the N source appear to decline. The farm released NH3, which turns into NH4 in the atmosphere, then deposits in the ecosystems. The consequence of the extra N kills organism from the top to the bottom of the food web as it accumulates and surpasses its critical load.

34
Q

Explain how acid rain and deposition serve as sources of H+ ions.

A

Sulphur oxides and nitrous oxides get released by sources like power plants, fossil fuel combustion, and vehicle emissions. These dissociate in rain and release H+ into ecosystems when acid rain falls - this is wet deposition. The pollutants might also be deposited directly - this is dry deposition.

35
Q

How are soils able to (somewhat) resist sources of acidification?

A

They are naturally buffered because of the exchange of acid ions between solution and colloids. So, colloids can retain acidic ions to keep active acidity (acidity of soil solution) low.

36
Q

How does CEC affect buffering capacity?

A

Higher CEC = higher buffering capacity

37
Q

What is the difference between residual, exchangeable, and active acidity in soil?

A

Residual: the acidic ions (Al3+ and H+) bound to the colloids
Exchangeable: the ions held near the colloid surface
Active: the Al3+ and H+ ions in soil solution

38
Q

Name 3 ways to amend low pH soils and increase their pH.

A
  1. Liming
  2. Increase organic matter
  3. Return more crop residues back to soil in croplands
39
Q

Explain how liming can increase soil pH.

A

Calcium and magnesium carbonate are added to the soil. Their base cations (Ca2+, Mg2+, CO32-) react with H+ ions in soil solution.

40
Q

Explain how increasing organic matter increases soil pH.

A

It increases the buffering capacity of the soil.

41
Q

What is the most common problem associated with strongly acid soils? What is the threshold pH?

A

Aluminum toxicity. This occurs at < 5.2 pH.

42
Q

What are the major symptoms of Al toxicity in plants?

A
  • Stunted root system that show little branching, brown root tips
  • Leaves may show chlorosis (yellowing) and show symptoms are drought stress and P deficiency
43
Q

How does the balance of bacteria and fungi change as pH changes?

A

Bacteria dominate at moderate to high pH, while fungi dominate at low pH.

44
Q

What are the four major drivers affecting whether soils are susceptible to acidification?

A

Climate, parent material, biological activity, time

45
Q

How does climate affect vulnerability acidification?

A

High rainfall environments tend to be prone to acidification

46
Q

How does parent material affect vulnerability to acidification?

A

Parents materials low in base cations like Ca2+, Mg2+, K+, and Na+ are more prone to acidification

47
Q

How does biological activity affect vulnerability to acidification?

A

A high degree of biological activity will result in more OM decomposition, releasing H2CO3 that will release H+.

48
Q

How does time affect vulnerability to acidification?

A

Older soils where there is more time for loss of base cations via leaching or weathering are more prone to acidification.

49
Q

Are tropical forest soils typically acidic? Why?

A

They are slightly to strongly acidic. This is mainly because they are hot and humid, but also because they experience rapid mineral weathering (precipitation), they are highly leached soils, and have high biological activity.

50
Q

Are drylands-semiarid ecosystem soils typically acidic? Why?

A

They are typically alkaline. This is because there is low precipitation and high evapotranspiration (minimal water), there is sparse vegetation and thus minimal OM decomposition, and the products of soil weathering accumulate as salts or carbonates that serve as buffers.

51
Q

Are temperate grasslands and intercontinental ecosystems soils typically acidic? Why?

A

They are typically neutral to alkaline. This is because they are often formed on sedimentary rocks, which are high in base cations, there is less precipitation, leaching is low or moderate, and they tend to accumulate base cations.

52
Q

Are boreal/coniferous forest soils typically acidic? Why?

A

They are typically very acidic. This is because they have an acidic parent material, they are dominated by conifers, there is slow decomposition and abundant organic acids produced, and loss of base cations due to lower plant uptake and leaching.

ENVB 210 (16 decks)

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