Biogeochemical Cycles

Question 1

Definition of Flux

Answer 1

The rate of transfer of material within a system per unit of time (in units of mass/time)

Question 2

Definition of Reservoir

Answer 2

Areas where elements are accumulated or held for a long period of time

Question 3

Definition of Residence Time

Answer 3

Average time an element stays in a reservoir before cycling

Question 4

Definition of feedbacks

Answer 4

processes in a system that can either amplify (+ve) or dampen (-ve) the system’s response to external influences

Question 5

Example of Positive and Negative feedbacks

Answer 5

Runaway feedback = x amplifies or increases y which amplifies or increases x etc
EG: carbon-rich permafrost (frozen soil) and temperature, ice albedo feedback

Negative/Stabilizing feedback = reduces the effect of change and maintains balance
EG: CO2 and plant growth, silicate weathering feedback

Question 6

Explain what the Redfield ratio is and how it is used.

Answer 6

Atomic ration of C:N:P found in phytoplankton and throughout deep oceans
Redfield ratio numbers = 106:16:1
Dictates the biogeochemical impacts of phytoplankton growth and decay & allows us to estimate the impact of a process on one nutrient concentration from knowledge of the impact on another nutrient

Question 7

What is the importance of biology in the cycling of the elements relative to the sedimentary cycle and the anthropogenic effects on these cycles.

Answer 7

Important for degradation of biomatter and mineralisation of elements.

tends to accelerate elemental fluxes between reservoirs.

1) mining – accelerates chemical weathering lithospheric flux of elements (S & Fe) to the hydrosphere (rivers, ocean, lakes, atmosphere)
2) extraction of fertilizer (e.g., phosphate rock) for farming accelerates the rate of biomass production and degradation.

Question 8

Why is the exchange of CO2 between the atmosphere and ocean important?

Answer 8

1. Regulates earth’s temp
2. Maintain CO2 concentration in the atmosphere
3. Source of carbon for shell growth

Question 9

What is Ksp, what information can it tell us about a reaction and what considerations are there when using this equation?

Answer 9

The solubility product constant, Ksp​, is the equilibrium constant for a solid substance dissolving in an aqueous solution. It represents the level at which a solute dissolves in solution. The more soluble a substance is, the higher the Ksp value it has.

The K equation must always show the reversible reaction so that the value of K has meaning.
The concentrations used in the equation must be measured at equilibrium.
Solids, pure liquids & solvents are not included in the expression.
Products are favoured Ksp > 1
Reactants are favoured Ksp <1

Question 10

Give the equation for the dissolution of carbon dioxide in water.

Answer 10

CaCO3(s) + CO2(aq) + H2O Ca2+ + 2HCO3-

Question 11

Referring to the dissolution of CO2 equation, state what effect dissolving CO2 in the ocean has on the pH. What consequences does this have?

Answer 11

Acidifies the ocean. Drives the reaction to the right, dissolution of calcium carbonate (makes it harder for organisms to make their shells). Increased production of carbonate acid.

Question 12

How do microorganisms drive the uptake of CO2 into the ocean?

Answer 12

Through photosynthesis, respiration, and decomposition. coccolithophores (single-celled algae)

Question 13

Compare and contrast the amount of CO2 in the surface water of the ocean and the deeper ocean. In your answer include processes that are occurring, whether calcium carbonate is saturated, undersaturated or at equilibrium, and the pH relative to depth and the consequences for marine organisms.

Answer 13

Less CO2 at surface than at deep ocean. Carbon is exported from surface to depths where it is being respired, decreasing pH, and releasing nutrients. Calcium carbonate is saturated as reactants are favoured and SI is positive. The diatoms die and shells dissolve which returns the Carbon to the sea.

Question 14

What affect does increased continental weathering have on the rate of calcium carbonate burial?

Answer 14

Can deepen CCD (calcite compensation depth) and lysocline increase ocean floor area over which carbonate can accumulate which increases carbon sequestration.

Question 15

What does the term “biomass” mean?

Answer 15

total amount of living tissue within a given trophic level

Question 16

Give a half reaction equation for the decomposition of biomass (oxidation reaction).

Answer 16

{CH2O} + 5H2O > CO2(g) + 4H3O+(aq) + 4e-

Question 17

For oxidation to occur, the half reaction must be coupled to

Answer 17

A reduction half equation

Question 18

Explain why microorganisms are important for the degradation of organic matter.

Answer 18

Microorganisms almost exclusively control biomass degradation. They play a key role in driving oxidation-reducing (redox) reactions.

Question 19

If oxygen is not available in a system, how does organic matter degrade?

Answer 19

Oxidation of organic matter can still proceed via reactions involving other O-containing species (nitrate and sulphate). Biodegradation can also occur via fermentation in anoxic conditions.

Question 20

What conditions are required for biomass to be decomposed?

Answer 20

1. Naturally occurring oxidising agent
2. Oxidising agent needs to be in high concentrations
3. Reaction needs to be favourable (PE should be positive)

Question 21

What factors dictate which species will oxidise the biomass?

Answer 21

If there are no limiting factors, biomass oxidation follows the typical sequence of using Oxygen, nitrate, sulphate, fermentation.

Question 22

Define pE

Answer 22

the negative logarithm of the electron activity

Question 23

Define pE°

Answer 23

reduction potential at standard temperature and pressure (STP)

Question 24

Define pE(w)

Answer 24

A measure of pE˚ at pH7

Question 25

In a deep eutrophic lake, what is the likely O2 profile across the depth for summer and winter. You may wish to sketch diagrams to help explain your answer. Discuss the reasons for these profiles.

Answer 25

Excess nutrients with high biological activity. In winter, organic matter is oxidised in the deeper portion of the lake and consumes O2 which lowers concentration.

Question 26

Discuss the redox buffering regions. How does this relate to a pE/pH diagram?

Answer 26

The pE value is maintained at a relatively constant high value when O2 is present. It is held at a lower value by other oxidant species when O2 is not available.

Buffering by oxygen is higher region (~20 aqueous pE). pE maintain roughly constant value.

Sulfate buffering lower region (~4 aqueous pE). pE is roughly constant value.

Question 27

discuss the conditions required for ammonia to be present in the environment. Give examples of where this type of environment is likely to occur.

Answer 27

Anoxic conditions with high pH, such as swamps or peatlands

Question 28

What is nitrogen fixation?

Answer 28

The conversion of dinitrogen gas (N2) into a ‘fixed’ form associated with the hydrosphere and terrestrial environments.

Question 29

Why is nitrogen fixation important biologically?

Answer 29

Nitrogen is biologically important because it is necessary to sustain life on Earth. Plants and animals require it for their growth, so, therefore, need nitrogen in a form they can use

Question 30

Why do plants require nitrogen to be fixed?

Answer 30

The bonds between N-N is too strong for a plant to break

N2 is also inert (chemically inactive)

Question 31

What are the three ways that nitrogen can be fixed?

Answer 31

Atmospheric fixation (Lightning discharges, cosmic rays and meteor trails)
Industrial fixation (Haber-bosh process)
Biological nitrogen fixation

Question 32

What is the Haber-Bosch process used for? Explain how it works.

Answer 32

Nitrogen and hydrogen are passed over 4 beds of catalysts and only about 15% conversion occurs on each pass. The unreacted gases are recycled, and the overall conversion is 97%

Question 33

Biological Nitrogen Fixation

Answer 33

Conversion of atmospheric nitrogen into ammonia in the roots of plants, exclusively carried out by prokaryotes. Bacteria and plant have a symbiotic relationship. The process requires an anaerobic environment.

Question 34

Discuss the uptake of nitrogen by plants. Include the species of nitrogen and how uptake occurs.

Answer 34

Nitrogen assimilation immobilises N. Formation of organic nitrogen compounds from inorganic compounds present in the environment. Plants cannot fix nitrogen gas, so they depend on the ability to assimilate nitrate or ammonia. In the form of Nitrate (NO3-) and ammonium – preferred (NH4+). 
Aerobic soils (plenty of oxygen) where nitrification can occur, nitrate is usually the predominant form. Grasslands and flooded aerobic soils such as rice paddies = ammonia. 
To maintain charge balance across the plant cell wall NO3- uptake consumes acidity (assimilation). Not immobilised – forms weak complexes with metals. Weakly absorbed.
	NH4 uptake generates acidity (nitrification). Potential to immobilise – ion exchange.
lowering of pH can increase solubility of Al and Mn that interfere with root growth.

Question 35

Discuss the issue of nitrate leaching and give THREE examples on how to prevent leaching.

Answer 35

1. A abundant supply of easy decomposable organic matter
2. Correct bacteria present
3. Correct environmental conditions (No O2 present and a slow gas exchange)
   Wetlands and peatlands

Question 36

Why do Nitrate Ions leach but Ammonium does not?

Answer 36

NO3- leaches as mobile species in negatively charged soil.

NH4+ interacts with soil at the root cropping zone and therefore does not precipitate.

Question 37

What sulphate species is most likely to be present in swampland?

Answer 37

H2S or HS-

Under high-pE aerobic conditions, sulphide is unstable

Question 38

Biological fixation accounts for roughly how much of the total fixed nitrogen

Answer 38

65%

Question 39

Biological fixation

Answer 39

Bacteria in soil and water convert N2 to ammonia NH3.

Question 40

Where do nitrogen fixating bacteria live?

Answer 40

nodules of plant root (chickpea, clover, mustard seed and legumes)

Question 41

How to nitrogen fixating bacteria survive on plants

Answer 41

Plants provide carbon energy, while bacteria produce ammonia.