Chemosphere I

Question 1

Why is it important to have O2 in soil air?

Answer 1

Most plant roots, microorganisms, and soil animals depend on it to meet their metabolic needs or for respiration.

Question 2

During gas exchange between the atmosphere and soil, what gets exchanged and in what direction?

Answer 2

Oxygen travels from the atmosphere into the soil, while CO2 travels from the soil into the atmosphere (high to low concentration)

Question 3

Name 3 factors that influence diffusion of gases between soil and the atmosphere.

Answer 3

Amount of air-filled pore space, continuity of pores, temperature.

Question 4

How does diffusion rate compare between air and water?

Answer 4

It is 1000X faster in air than water.

Question 5

Name 4 factors that influence O2 availability in soils.

Answer 5

1. Pore space
2. Water content
3. Respiration rates
4. Soil depth

Question 6

Describe how pore space affects O2 availability.

Answer 6

Less pore space = less O2 availability

Question 7

Describe how water content affects O2 availability.

Answer 7

More water = slower diffusion, less O2 availability

Question 8

Describe how respiration rates affect O2 availability.

Answer 8

Higher respiration rate = more CO2 produced in a given time. Might overwhelm influx of O2.

Question 9

Describe how soil depth affects O2 availability

Answer 9

Deeper = less O2 available.

Question 10

What happened in the biosphere 2 crisis?

Answer 10

People were living in an enclosed ecosystem. Microbes in the soils were producing CO2 faster than the plants were producing O2, which led to people asphyxiating.

Question 11

Oxygen gradients occur at what scales?

Answer 11

It can range from landscape scale to plot to microsites, where there can be small anaerobic pockets.

Question 12

How do oxygen gradients affect biodiversity?

Answer 12

Having zones with different aeration levels creates different niches for different types of organisms, thus maintaining diversity.

Question 13

How does the O2 content vary throughout a soil aggregate?

Answer 13

It gets lower and lower as you move towards the center of the aggregate. The center of the aggregate is completely without O2. The outside has roughly 21% O2 content.

Question 14

During aerobic respiration, organic matter gets […] to […], and an [….] gets […]

Answer 14

Oxidized, CO2, electron acceptor (O2), reduced

Question 15

What are terminal electron receptors (TEAs)?

Answer 15

As organic matter gets oxidized in soil, it will release electrons that must be accepted.

Question 16

Name the 6 most common TEAs in soils from most to least energetically favourable.

Answer 16

CO2, NO3-, MnO2, Fe(OH)3, SO42-, CO2

Question 17

Which TEAs are used in aerobic respiration vs anaerobic respiration?

Answer 17

Aerobic: O2
Anaerobic: NO3-, MnO2, Fe(OH)3, SO42-, CO2

Question 18

Describe aerobic respiration in terms of the substrate, terminal electron acceptor(s), and end products.

Answer 18

Substrate: glucose
Terminal electron acceptor: O2
End products: glucose becomes CO2, O2 becomes H2O, energy

Question 19

Describe denitrification in terms of the substrate, terminal electron acceptor, and end products

Answer 19

Substrate: glucose
Terminal electron acceptor: NO3- (nitrate)
End products: glucose becomes CO2, NO3 becomes N2 or N2O, energy

Question 20

If an organism is capable or both aerobic and anaerobic respiration, will it prefer one mode over the other? Why or why not?

Answer 20

Yes, it will prefer aerobic respiration because it produces more energy than anaerobic respiration.

Question 21

In anaerobic conditions, what determines which alternate TEA will be used to oxygen?

Answer 21

Whether it’s present in the environment and the amount of free energy the reaction releases.

Question 22

What is the least energetically favourable respiration type? Where does it dominate?

Answer 22

Methanogenesis, which converts CO2 to CH4. This dominates in wet soils that are completely anaerobic.

Question 23

Why is decomposition slower in wet soils?

Answer 23

Because methanogenesis it the dominant respiration mechanism, which releases little free energy compared to other types of respiration.

Question 24

Explain why Mn reduction is done after nitrate reduction despite Mn reduction being more energetically favourable.

Answer 24

Because Mn is generally present in very low concentrations, so many organisms default to nitrate reduction first.

Question 25

Describe how TEAs change during flooding.

Answer 25

The area is going from an aerobic condition to an anaerobic condition. First, the O2 will be used up. Then, NO3- will be used up and N2O or N2 will start to be produced. Then, Fe3+, producing Fe2+. Then, SO32-, producing S2- or H2S. Then, Mn4+ or MnO2 to produce Mn2+. Finally, the CO2 produced by the O2 will be used and CH4 will be produced last.