Lecture 7 - Nitrogen and Phosphorus Cycling

Question 1

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

Answer 1

78%.

Question 2

What is the total mass of nitrogen in the atmosphere?

Answer 2

3.98 x 10¹⁸.

Question 3

Why is nitrogen important?

Answer 3

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.

Question 4

Why can’t atmospheric nitrogen be used by plants?

Answer 4

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

Question 5

What are the six ways nitrogen is biologically transformed?

Answer 5

Nitrogen fixation.

Ammonia Assimilation.

Nitrification.

Assimilatory nitrate reduction.

Ammonification.

Denitrification.

Question 6

What is nitrogen fixation?

Answer 6

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

Question 7

What is the equation for nitrogen fixation?

Answer 7

N₂ + 8H⁺ + 8e⁻ —> 2NH₃ + H₂

Question 8

What is most nitrogen fixation done by?

Answer 8

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

Question 9

What is ammonia assimilation?

Answer 9

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

Question 10

What is nitrification?

Answer 10

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

Question 11

What are the two steps of nitrification?

Answer 11

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⁻

Question 12

What is assimilatory nitrate reduction?

Answer 12

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

Question 13

What is ammonification?

Answer 13

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

Question 14

What is denitrification?

Answer 14

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

Question 15

What are the two reactions as part of denitrification?

Answer 15

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

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

Question 16

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

Answer 16

Two thirds.

Question 17

What can the bacteria that do nitrogen fixation be?

Answer 17

Aerobic

Anaerobic

Phototrophic

Chemotrophic

Question 18

What two types of bacteria can do nitrogen fixation?

Answer 18

Symbiotic bacteria

Free living bacteria.

Question 19

What proportion of nitrogen fixation occurs from human processes?

Answer 19

1/3

Question 20

How do humans nitrogen fix?

Answer 20

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.

Question 21

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

Answer 21

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

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

Question 22

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

Answer 22

Denitrification converts plant and animal matter back into N₂.

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

This process also produces N₂O.

Question 23

Why doesn’t the N₂O concentration keep increasing?

Answer 23

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%)

Question 24

What is dry deposition?

Answer 24

Deposition of particles via impact.

Question 25

What is turbulent diffusion?

Answer 25

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

Question 26

What is molecular diffusion?

Answer 26

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

Question 27

What is wet deposition?

Answer 27

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).

Question 28

Why is atmospheric chemistry important?

Answer 28

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

Question 29

How is NO₂ formed?

Answer 29

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

Question 30

What is the lifetime of NO₂?

Answer 30

12 hours so local sources dominate the abundance.

Question 31

How is NO₂ formed from NO?

Answer 31

NO + O₃ ---> NO₂ + O₂

Question 32

What health risks come from NO₂?

Answer 32

Respiratory health effects such as asthma and COPD.

Question 33

What are the detrimental effects of N₂O?

Answer 33

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.

Question 34

Where does most N₂O come from?

Answer 34

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

Question 35

What is the generic equation for the breakdown of ozone?

Answer 35

O₃ + X ---> XO + O₂ XO + O ---> X + O₂ Overall: O₃ + O ---> 2O₂

Question 36

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

Answer 36

O₃ + NO ---> NO₂ + O₂ NO₂ + O ---> NO + O₂

Question 37

Where is NO derived from?

Answer 37

The photolysis of N₂O.

Question 38

What else does NO react with?

Answer 38

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.

Question 39

Why is there more ozone depletion at the poles?

Answer 39

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

Question 40

What are the two types of smog?

Answer 40

Sulphurous smog. Photochemical smog.

Question 41

What conditions are required for photochemical smog?

Answer 41

Warm sunny days with busy traffic.

Question 42

What are the primary pollutants of smog?

Answer 42

NOₓ (mainly NO) and unburnt hydrocarbons.

Question 43

What are the secondary pollutants of smog?

Answer 43

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

Question 44

How is tropospheric ozone produced?

Answer 44

NO₂ + hv ---> NO + O O + O₂ ---> O₃ Overall equation: NO₂ + O₂ ---> NO + O₃

Question 45

How does phosphorus occur in the natural environment?

Answer 45

Very little in the atmosphere. Mainly found as tri-calcium phosphate in rocks.

Question 46

Why is phosphorus important?

Answer 46

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.

Question 47

Why is phosphorus produced on a large scale?

Answer 47

It is used as a fertiliser as it is often a limiting reagent in plants. This means it can cause eutrophication.

Question 48

What are sources of phosphorus to the atmosphere?

Answer 48

Dust from weathering/soil erosion. Dust from industrial processing (for example, fertiliser production). Sea spray.

Question 49

What are the comparisons between nitrogen and phosphorus?

Answer 49

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).