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BIOS1004 - Building A Habitable Planet > Lecture 7 - Nitrogen and Phosphorus Cycling > Flashcards

Lecture 7 - Nitrogen and Phosphorus Cycling Flashcards

1
Q

What percentage of all the molecules in the atmosphere are N₂?

A

78%.

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2
Q

What is the total mass of nitrogen in the atmosphere?

A

3.98 x 10¹⁸.

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3
Q

Why is nitrogen important?

A

It is important for photosynthesis (the more nitrogen present, the more photosynthesis occurs).

Nitrogen is required for amino acids and it is a vital component of the enzymes that control the rate of biochemical reactions.

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4
Q

Why can’t atmospheric nitrogen be used by plants?

A

The triple bond is too strong to break and is very hard to be converted into a different form.

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5
Q

What are the six ways nitrogen is biologically transformed?

A

Nitrogen fixation.

Ammonia Assimilation.

Nitrification.

Assimilatory nitrate reduction.

Ammonification.

Denitrification.

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6
Q

What is nitrogen fixation?

A

Processes in which N₂ in the atmosphere is converted into any other form of nitrogen.

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7
Q

What is the equation for nitrogen fixation?

A

N₂ + 8H⁺ + 8e⁻ —> 2NH₃ + H₂

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8
Q

What is most nitrogen fixation done by?

A

Prokaryotes but some can be fixed abiotically by lightning or certain industrial processes.

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9
Q

What is ammonia assimilation?

A

The process by which NH₃ or NH₄⁺ is taken up by an organism to become part of its biomass.

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10
Q

What is nitrification?

A

The oxidation of NH₃ or NH₄⁺ to NO₂⁻ (nitrate) or NO₃⁻ by an organism as a means of producing energy.

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11
Q

What are the two steps of nitrification?

A

Oxidation of ammonia to nitrate via an intermediate carried out by microbes.
NH₃ + O₂ + 2e⁻ —> NH₂OH + H₂O —> NO₂⁻ + 5H⁺ + 4e⁻

Nitrate to nitrite conversion by a separate group of microbes.
NO₂⁻ + 1/2O₂ —> NO₃⁻

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12
Q

What is assimilatory nitrate reduction?

A

The reduction of NO₃⁻, followed by the uptake of the nitrogen by the organism as biomass.

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13
Q

What is ammonification?

A

The breaking down of organic nitrogen compounds (amino acids, DNA etc.) to NH₃ and NH₄⁺.

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14
Q

What is denitrification?

A

The reduction of NO₃⁻ to any gaseous nitrogen species such as N₂ or N₂O.

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15
Q

What are the two reactions as part of denitrification?

A

Reducing nitrate to molecular nitrogen gas.
NO₃⁻ —> NO₂⁻ —> NO + N₂O —> N₂

Complete redox reaction of denitrification.
2NO₃⁻ + 10e⁻ + 12H⁺ —> N₂ + 6H₂O

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16
Q

What percentage of all nitrogen fixation do bacteria/microorganisms do?

A

Two thirds.

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17
Q

What can the bacteria that do nitrogen fixation be?

A

Aerobic

Anaerobic

Phototrophic

Chemotrophic

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18
Q

What two types of bacteria can do nitrogen fixation?

A

Symbiotic bacteria

Free living bacteria.

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19
Q

What proportion of nitrogen fixation occurs from human processes?

A

1/3

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20
Q

How do humans nitrogen fix?

A

Fertilisers (ammonium sulphate, ammonium nitrate, urea, liquid ammonia).

Combustion. Some N in fuel is oxidised to NO, NO₂. Some N₂ in the air is oxidised to NO, NO₂.

21
Q

What is the mass of nitrogen in the atmosphere and how fast is it being used?

A

4 x 10⁹ Mt.
The rate of use 200 Mt per year.

This means it should all be used up in 20 million years.

22
Q

Why does the mass of nitrogen in the atmosphere not decrease?

A

Denitrification converts plant and animal matter back into N₂.

5CH₂O + 4NO₃⁻ + 4H⁺ —> 2N₂ + 5CO₂ + 7H₂O

This process also produces N₂O.

23
Q

Why doesn’t the N₂O concentration keep increasing?

A

Photons with a wavelength of <250 nm (UV-C) break up the N₂O molecule in the stratosphere.

N₂O + O(¹D) —> 2NO (62%)
N₂O + O(¹D) —> N₂ + O₂ (38%)

24
Q

What is dry deposition?

A

Deposition of particles via impact.

25
Q

What is turbulent diffusion?

A

Bulk movement of large numbers of molecules or particles in eddies.

26
Q

What is molecular diffusion?

A

Deposition of particles by impaction from the wind onto surfaces such as leaves.

27
Q

What is wet deposition?

A

Particles incorporating into cloud droplets and then raindrops (rainout).

The gases can dissolve into the cloud water droplets. Falling raindrops can collect particles and dissolve more gases (washout).

28
Q

Why is atmospheric chemistry important?

A

It can cause climate change, stratospheric ozone and photochemical smog.

29
Q

How is NO₂ formed?

A

High-temperature combustion in air such as from vehicle engines or power generation.

30
Q

What is the lifetime of NO₂?

A

12 hours so local sources dominate the abundance.

31
Q

How is NO₂ formed from NO?

A

NO + O₃ —> NO₂ + O₂

32
Q

What health risks come from NO₂?

A

Respiratory health effects such as asthma and COPD.

33
Q

What are the detrimental effects of N₂O?

A

N₂O is a very potent greenhouse gas and has a long lifetime of 120 years.

Over 100 years, it’s GWP is 15 times more than methane.

34
Q

Where does most N₂O come from?

A

Bacterial denitrification which means that emissions are associated with land use and agricultural practice.

35
Q

What is the generic equation for the breakdown of ozone?

A

O₃ + X —> XO + O₂
XO + O —> X + O₂

Overall:

O₃ + O —> 2O₂

36
Q

What are the equations for ozone breaking down with nitric acid?

A

O₃ + NO —> NO₂ + O₂
NO₂ + O —> NO + O₂

37
Q

Where is NO derived from?

A

The photolysis of N₂O.

38
Q

What else does NO react with?

A

Hydroxyl radical to produce nitrous acid (HONO). This is not involved in ozone destruction so there is less of a chain reaction than chlorine radicals.

39
Q

Why is there more ozone depletion at the poles?

A

Polar stratospheric clouds form which allow a chlorine radical chain reaction to occur. This causes ozone depletion to happen at a very high rate.

40
Q

What are the two types of smog?

A

Sulphurous smog.

Photochemical smog.

41
Q

What conditions are required for photochemical smog?

A

Warm sunny days with busy traffic.

42
Q

What are the primary pollutants of smog?

A

NOₓ (mainly NO) and unburnt hydrocarbons.

43
Q

What are the secondary pollutants of smog?

A

O₃, NO₂, PAN (peroxyacyl nitrates) and aerosol.

44
Q

How is tropospheric ozone produced?

A

NO₂ + hv —> NO + O
O + O₂ —> O₃

Overall equation:
NO₂ + O₂ —> NO + O₃

45
Q

How does phosphorus occur in the natural environment?

A

Very little in the atmosphere.

Mainly found as tri-calcium phosphate in rocks.

46
Q

Why is phosphorus important?

A

Used in pyrotechnics and smoke bombs.

Used in the production of steel and specialised gas.

Essential component of all cell protoplasm, nerve tissue and bones.

47
Q

Why is phosphorus produced on a large scale?

A

It is used as a fertiliser as it is often a limiting reagent in plants.

This means it can cause eutrophication.

48
Q

What are sources of phosphorus to the atmosphere?

A

Dust from weathering/soil erosion.

Dust from industrial processing (for example, fertiliser production).

Sea spray.

49
Q

What are the comparisons between nitrogen and phosphorus?

A

Most of the nitrogen is present in the atmosphere whereas most of the phosphorus is present in rocks.

There isn’t any biological recycling for phosphorus.

To renew phosphorus plate tectonics needs to recycle sedimentary rocks by weathering. This causes the timescale for phosphorus cycling to be very long (10⁸ years).

BIOS1004 - Building A Habitable Planet (9 decks)

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