Biosphere VI

Question 1

What are the 4 major forms of N loss globally? Which is the most important?

Answer 1

Plant uptake, leaching, soil, erosion, gaseous emission. The most important is plant uptake.

Question 2

What is the Haber-Bosch process?

Answer 2

It is an industrial process of N fixation, thus taking N2 out of the atmosphere to create ammonium.

Question 3

What was the impact of the discovery of the Haber-Bosch process?

Answer 3

It made fertilizer to be made available, which increased food production and increased global inputs of reactive N.

Question 4

What are the major categories and types of N in the nitrogen cycle?

Answer 4

Soluble inorganic: ammonium (NH4+), nitrate (NO3-)
Soluble and insoluble organic N (amino and nucleic acids)
Gaseous: N2 and N2O

Question 5

What are the 5 N transformation in the N cycle?

Answer 5

Mineralization, immobilization, nitrification, denitrification, N2 fixation.

Question 6

What is mineralization?

Answer 6

It is the biological conversion of organic N compounds, such as amino acids and proteins, to inorganic forms (NH4+) that can be used by plants and other microbes

Question 7

What are the major N sources for mineralization?

Answer 7

The death of microbes and the grazing of soil organic matter (decomposition).

Question 8

What organisms carry out mineralization?

Answer 8

Microorganisms of a large variety, including aerobes, anaerobes, fungi, and bacteria. Soil fauna are also necessary because they do the preliminary fragmentation and moving of detritus.

Question 9

What inputs and outputs are required for mineralization to occur?

Answer 9

Microbes require the N to assemble proteins, nucleic acids, and cellular components. They also need energy and C from OM. The outputs are CO2, NH4, and H2O.

Question 10

What is immobilization?

Answer 10

It is the uptake of inorganic N (NH4+) into organic biomass, transforming it back into organic N and rendering inaccessible to plants.

Question 11

What is the relationship between mineralization and immobilization?

Answer 11

They occur in close proximity to one another and in rapid succession, as they are essentially the opposites of one another.

Question 12

What is the preferred source of N for microbes for immobilization?

Answer 12

NH4+

Question 13

What is the impact of immobilization on the mobility of nitrogen?

Answer 13

Because immobilization is constantly happening, NH4+ is not mobile for very long in the soil, so it has a short residence time and often can’t be taken up by plants despite being preferred over NO3-.

Question 14

What determines the balance of mineralization vs immobilization in soil? Explain why.

Answer 14

The organic input of N into the soil. If organic input is low in N (> 30 to 40 C:N), microbes will be more inclined to immobilize NH4+ to take it up into their own biomass. If the organic inputs are rich in N, then mineralization will dominate, as the microbial N needs are met.

Question 15

Explain the impact of high C:N ratio plant inputs being added to the soil on plants and why.

Answer 15

When high C:N (low N) is added, the microbes will increase immobilization to take the N for their own biomass. This will cause a nitrate depression period in the soil, which means that the plants will likely experience nitrogen deficiency. This depression will end and the soluble N in the soil will rise again once the microbes die.

Question 16

What is nitrification?

Answer 16

It is the rapid conversion NH4+ to NO2- and then to NO3-.

Question 17

What is the residence time of NO3- and why?

Answer 17

It is short because it is highly mobile and reactive. It is also easily lost.

Question 18

What consequence do high nitrification rates tend to have on the system? Why?

Answer 18

High rates of nitrification usually indicate a lot of loss of N through leaching, because NO3- is highly mobile.

Question 19

What are the two steps of nitrification? State which organisms are repsonsible for each step.

Answer 19

1. Oxidation of ammonia to nitrite (NO2-) by a few types of bacteria.
2. Oxidation of nitrite to nitrate (NO3-) by specific nitrite oxidizing bacteria.

Question 20

Are nitrifiers autotrophs or heterotrophs? Explain why.

Answer 20

They are autotrophs, as they fix their carbon through CO2.

Question 21

Describe nitrification in terms of the substrate, the electron acceptor, and the end products.

Answer 21

Substrate: NH4
TEA: O2
End products: NO2-, low energy yield

Question 22

Why is the energy yield of nitrification low?

Answer 22

Because it uses the reductive citric acid cycle to assimilate CO2.

Question 23

Name the 2 major controlling factors on nitrification.

Answer 23

Abundance of NH4+ (most important) and presence of oxygen, since the process is carried out by obligate aerobes.

Question 24

How do the mineralization and nitrification rates compare in wetlands and forests?

Answer 24

In wetlands, mineralization rates are veryhigh but nitrification rates are low due to lack of oxygen. In forests, mineralization rates are low and nitrification is also low, both because of the lack of N inputs.

Question 25

Do wetlands and forests mainly accumulate NH4+ or NO3-? Why?

Answer 25

They mostly accumulate NH4+ and have low NO3-. In wetlands, this is because there is a lack of oxygen for nitrification, while in forests, this is because they tend to be N-limited and thus have slow N-cycling.

Question 26

How do rates of nitrate leaching compare between annual crops and perennial crops? Why?

Answer 26

Perennial crops see less leaching than annual crops. This is because they have more extensive root systems that can sop up more of the NO3- before it gets leached out.

Question 27

What is denitrification?

Answer 27

it is the process that converts nitrate (NO3-) to nitrogen gas (N2 or N2O).

Question 28

What are the two possible outputs of denitrification? Explain the difference between them.

Answer 28

N2, which is the end product if denitrification is complete, or N2O, which is the intermediate product if denitrification doesn’t get completed. N2 belongs in the atmosphere, but N2O is a potent greenhouse gas.

Question 29

What types of organisms carry out denitrification?

Answer 29

Heterotrophic bacteria.

Question 30

What inputs are required for denitrification to occur?

Answer 30

A form of organic carbon must be supplied as energy for the bacteria, and NO3 must be present as a TEA during respiration. There also must be an absence of oxygen.

Question 31

Describe denitrification in terms of substrate, TEA, and end products.

Answer 31

Substrate: organic carbon (i.e. glucose)
TEA: NO3-
End products: CO2, N2, N2O

Question 32

As soil moisture and microbial activity increase, how does the balance between nitrification and denitrification change?

Answer 32

There will be more denitrification, as this is an anaerobic process and the conditions are tending towards a more anaerobic soil.

Question 33

Describe the pattern of N2O gas emissions from soil and a possible reason why.

Answer 33

The emissions tend to be episodic as opposed to consistent. This tends to happen after a bout of fertilizer application.

Question 34

What have the trends been in terms of N inputs and losses over time?

Answer 34

N inputs have dramatically increased from fertilizer and manure. N losses have somewhat increased, but mainly due to runoff into ecosystems.

Question 35

How can N loss be reduced?

Answer 35

By improving crop N use efficiency (NUE).

Question 36

What is NUE?

Answer 36

N use efficiency for crops. It is the Crop N uptake amount/total added.

Question 37

What are the most important source of ecosystem nitrogen?

Answer 37

Legumes