Exam 3 - Nitrogen Cycle

Question 1

What is the most limiting nutrient in the soil for plant growth?

Answer 1

Nitrogen

Question 2

N inputs… Where the soil gets its nitrogen

Answer 2

Biological N fixation, Industrial N fixation, Lightening fixation (not influenced by humans), Combustion, Plant residues and animals wastes, and atmospheric deposition.

Question 3

N Losses…where the soil loses nitrogen

Answer 3

Denitrification, Volatilization, runoff, leaching, removal of crop residues, and plant and microbial uptake.

Question 4

Denitrification versus Volatilization

Answer 4

Denitrification: the removal of nitrate. it is a biological reaction and requires microbes.
NO3- to N2 gas
Volatilization: Ammonium to NH3 gas. Does not require microbes, and is an abiotic reaction.

Question 5

Describe the global pool size of nitrogen.

Answer 5

Lithosphere has the largest pool of N (1x10^23), the atmosphere has the second largest pool of N, and Biota N has the smallest pool of N.

Question 6

What are the major reactions of the nitrogen cycle?

Answer 6

nitrification, denitrification, ammonification, nitrate ammonification or dissimilatory nitrate reduction to ammonium, biological N fixation

Question 7

Nitrification reaction

Answer 7

NH4 + -> NO2 -> NO3-

Question 8

Denitrification reaction

Answer 8

NO3 -> N2 under anaerobic conditions

Question 9

Ammonification (N mineralization)

Answer 9

Organic N -> NH4+

Question 10

Nitrate ammonification or dissimilatory nitrate reduction to ammonium (DNRA)

Answer 10

NO3- -> NH4+ under anaerobic conditions

Question 11

Biological nitrogen fixation

Answer 11

N2 -> NH3

Question 12

What are the oxidation states of key nitrogen compounds?

Answer 12

Organic N (R-NH2): -3, most reduced
Nitrous Oxide (N2O): +1
Nitrate (NO3-): +5, most oxidized

Question 13

Amounts and forms of N in the soil

Answer 13

Total N: 0.02-0.5% in the surface soil
Amino acids: 20-50%
Amino sugars: 5-10%
Nucleic acids: 1%
Available inorganic N: <1-2%

Question 14

Overall process of immobilization and mineralization (define the two terms, too)

Answer 14

Mineralization: Process of going from organic N to inorganic N
Immobilization: Process of going from inorganic N to organic N
Both depend on C/N ratio
R-NH2 -> R-OH + OH + NH4 -> NO3-
Going forward is mineralization and going backwards is immobilization

Question 15

C/N ratios of microorganisms?

Answer 15

Bacteria -> 3-5:1
Fungi -> 4.5-15:1
On avg. 8:1

Question 16

Critical C/N ratio

Answer 16

20-30:1
Net immobilization if above this number
Net mineralization of below this number

Question 17

Describe the C/N ratio change during plant residue decomposition

Answer 17

High C/N residues:
Once the high C/N residue hits the soil, decomposition starts and microbes break down the residue and release CO2. This is not enough nitrogen to support microbial growth, though. The microbes use the nitrogen in the soil solution, and then decomposition can proceed. This leads to a reduction in C/N ratio, leading to a depression period in the soil. During this time, plants cannot get nitrogen from the soil solution. As the C/N ratio gradually decreases, nitrogen from the residue is released into the soil solution and then plants can get inorganic N.
Low C/N residues:
there is enough nitrogen in the residue for microbial growth, so the microbes will break it down and release the nitrogen that they don’t need into the soil solution. Inorganic N increases in the soil solution as the crop residue decomposes.

Question 18

C/N ratios in soils:

Answer 18

Cultivated surface soil:
Range: 8-15:1
Median: 12:1

Question 19

Legumes have _________ C/N ratio than other non legume plants.

Answer 19

Lower

Question 20

Specific Processes of immobilization:

Answer 20

Ammonium -> amino acids:
Nitrate must be converted to ammonium first.
Nitrate Reductase & Nitrite Reductase:
Nitrate reductase is an enzyme that reduces nitrate to nitrite
Nitrite reductase is an enzyme that reduces nitrite to ammonium.
Two Pathways for ammonium assimilation:
Glutamate dehydrogenase pathway (at high ammonium levels) starts with alpha-Ketoglutarate + ammonium and uses glutamate dehydrogenase to catalyze the reaction and produce glutamate.
Glutamine synthetase glutamate pathway (at low ammonium levels) uses glutamine synthetase to catalyze its reaction.
Transamination: glutamate + pyruvate -> alpha-Ketoglutarate + alanine, uses transaminase to catalyze the reaction.

Question 21

Specific process of mineralization:

Answer 21

Organic form to inorganic form. Ammonium is converted to nitrate. Extracellular enzymes that are involved are proteases and proteinases.
Lysozyme breaks down peptidoglycan.

Question 22

Nitrification process:

Answer 22

Autotrophic process
NH4+ -> NO2- -> NO3-
In the first step, ammonium oxidizers are involved and in the second step nitrite oxidizers are involved.

Question 23

microorganisms involved in the nitrification process

Answer 23

aerobic chemolithotrophs… get carbon from inorganic carbon and get their energy from oxidizing ammonium.
Gram negative organisms.
Ammonium oxidizers:
-Beta proteobacteria
-two genera and 14 species:
Nitrosomonas : N. europea, N. communis
Nitrosospira

Question 24

heterotrophs

Answer 24

these guys dont get their energy from nitrification of ammonium and nitrite. most of the time happens in acidic soils. can remove nitrite and reduce it and form gaseous form of nitrogen… N2

Question 25

ammonia oxidation

Answer 25

ammonia is oxidized to form nitrite, water, and hydrogen ions. the hydrogen ion in the reaction tells you that the pH drops, meaning more acidity. There are 6e- transferred in this reaction, meaning a lot of energy is generated from this reaction.
It is a rate limiting equation.

Question 26

nitrite oxidation

Answer 26

nitrite is oxidized to form nitrate.
catalyzed by nitrite oxidase.
smaller amount of energy released, so it has to proceed faster in order to keep up with the first step.

Question 27

By product formation

Answer 27

nitrous oxide N2O and nitric oxide NO

Question 28

reaction rates

Answer 28

heterotrophic and autotrophic reactions occur slowly, but autotrophic nitrification is faster than heterotrophic nitrification.

Question 29

Environmental factors affecting nitrification.

Answer 29

Temperature, pH, aeration, moisture, and ammonium availability.
Aeration: nitrification requires O2
Moisture: When there’s more water, aeration decreases. 60-70% is optimum water holding capacity.
Temperature: Optimal temp is 30C, and the range is 5-40.
pH: optimal pH is 6.6-8. nitrifiers produce acidity in their overall reaction, but they still need a neutral pH.
ammonium availability: Nitrifiers use ammonium as their substrate, so it is important that it is available. If ammonium is present in the NH3 form and there’s too much of it, & the soil pH is too high, then Nitrobacter can be inhibited and so we will end up with accumulation of nitrite and won’t end up w/ nitrate.

Question 30

Complete ammonium oxidation (comammox)

Answer 30

Some bacteria can complete nitrification in one step, instead of two.
NH4 +2O2 —-> NO3 + H2O + 2H+
The Nitrosospira species can complete this.

Question 31

Nitrate pollution

Answer 31

inorganic fertilizer, sewage sludge, septic tanks, decomposition of organic residue leads to a source of inorganic N in the soil.

Question 32

Negative effects of nitrate pollution.

Answer 32

Eutrophication (water pollution): The excess nutrients in the water contributes to harmful algae blooms. Once the algae die, decomposers in the water become very active and use up the oxygen in the water. This results in a depletion of oxygen and other organisms in the water die.
Human Health: methemoglobinemia: when there’s excess nitrate in the human system, it will compete with oxygen for hemoglobin. This is not good because hemoglobin transports oxygen around the body. This occurs more in babies and young animals (mostly cattle). An excess of nitrate can also cause nitrosoamines. these are mutagens and carcinogens. Reduction of nitrosoamines occurs in the stomach.

Question 33

Runoff of nitrate

Answer 33

Nitrate is negatively charged, so it tends to move around in the soil and be moved to water. This does not happen with ammonium because it is positively charged, so it is retained in the soil and does not tend to move around.

Question 34

Control of autotrophic nitrification:

Answer 34

Slow release fertilizers, nitrification inhibitors, and management practices.
Nitrification inhibitors:
Organic -> Nitrapyrin (N-serve) and acetylene
Inorganic: azides and chlorate
Slow release fertilizers: ammonium won’t be released right away so hopefully this gives time for the plants to use it rather than lose it.
Management practices: manage cover crop residues, avoid overuse of fertilizers, and correct timing and placement of fertilizers (don’t fertilize if the plants aren’t growing there and don’t fertilize in the winter).

Question 35

Fate of ammonium

Answer 35

can be used for plant uptake, microbial growth, nitrification, amammox (anaerobic ammonium oxidation) (lose nitrogen into the atmosphere -> N2 gas) (NH4 -> N2), volatilization at high pH, fixed by clay, reacts with SOM to produce quinone-NH2 complexes,

Question 36

Fate of Nitrate

Answer 36

Physical processes: runoff and leaching
Chemical processes: chemodenitrification… does not require microbes, happens under aerobic conditions and when pH is low.
Biological processes:
Assimilative nitrate reduction -> uptake by organisms. needs to be converted to ammonium before plants can use it. first step catalyzed by nitrate reductase. second step catalyzed by nitrite reductase… both enzymes are inhibited by ammonium
Dissimulative nitrate reduction -> not used for growth, used as an energy source. Reducing nitrate to produce energy for microbes. under anaerobic conditions. nitrate is reduced to nitrite and then further reduced to nitric oxide and nitrous oxide and the final product is N2 gas. uses the enzymes dissimilative nitrate reductase and dissimilative nitrite reductase. Prokaryotes ONLY.