Option C: Ecology and conservation (AHL) :C.6 Nitrogen and phosphorus cycles SL Paper 3

Date	November 2016	Marks available	1	Reference code	16N.3.SL.TZ0.14
Level	Standard level	Paper	Paper 3	Time zone	TZ0
Command term	Outline	Question number	14	Adapted from	N/A

Question

In 1997 in South Africa, a decision was made to decrease the use of mosquito-killing pesticides due to their negative effect on the environment. Mosquitoes are known to be responsible for the spread of malaria. In 2001 the decision was reversed and the use of pesticides was increased. The graph shows the estimated numbers of people with malaria in each year.

Outline the trend in the number of people with malaria during the period when the use of pesticides was decreased in South Africa.

[1]

One pesticide used in killing mosquitoes was DDT. Considering its harmful effects, discuss whether the decision to reintroduce it was justified.

[4]

Markscheme

the number of people with malaria increased

a. choice has to be made between damage to environment or increase in malaria

b. DT may lead to biomagnification/bioaccumulation in food chains
OR
taken up by species in lower trophic levels becoming more concentrated at higher trophic levels

c. causes harm to consumers at end of food chain
OR
example «eg: thin egg shells of falcons»

d. DDT is shown to be effective in reducing malaria

e. possible partial solution to be selective in areas sprayed with DDT

f. may kill insects that are not pests

Date	May 2015	Marks available	3	Reference code	15M.3.SL.TZ2.18
Level	Standard level	Paper	Paper 3	Time zone	Time zone 2
Command term	State	Question number	18	Adapted from	N/A

Question

State the role of Rhizobium, Nitrobacter and Azotobacter in the nitrogen cycle.

Rhizobium: ……………………………………………………………
Nitrobacter: ……………………………………………………………
Azotobacter: ……………………………………………………………

[3]

Explain the production of methane from biomass.

[4]

Markscheme

a. Rhizobium: nitrogen fixation;
b. Nitrobacter: oxidizes/changes nitrites to nitrates;
c. Azotobacter: nitrification / bind atmospheric nitrogen / nitrogen fixation;

a. anaerobic digestion of biodegradable material;
b. fermentation (of carbohydrates) by bacteria;
c. methanogens produce methane;
d. methane/biogas used as energy;
e. waste products used as fertilizer;
f. CO2 produced (as a by-product);

Date	November 2015	Marks available	2	Reference code	15N.3.SL.TZ0.17
Level	Standard level	Paper	Paper 3	Time zone	TZ0
Command term	Outline	Question number	17	Adapted from	N/A

Question

Outline the process of nitrogen fixation by a named free-living bacterium.

Markscheme

a. atmospheric nitrogen is converted to ammonia;

b. by Azotobacter;

Do not accept Rhizobium.

Date	May 2013	Marks available	4	Reference code	13M.3.SL.TZ1.18
Level	Standard level	Paper	Paper 3	Time zone	Time zone 1
Command term	Explain	Question number	18	Adapted from	N/A

Question

Explain the consequences of releasing raw sewage and nitrate fertilizer into rivers.

Markscheme

a. raw sewage contains pathogens;
b. can contaminate drinking water / cause disease/death;
c. nitrate fertilizers cause algal blooms/(aquatic) plant development;
d. algae release toxins that can contaminate drinking water;
e. more organic matter results / eutrophication;
f. more oxygen required to decompose organic matter / increase in BOD;
g. (leading to) death of aquatic animals/organisms;
h. recovery follows since algae/plants release more oxygen;

Date	May 2013	Marks available	1	Reference code	13M.3.SL.TZ2.18
Level	Standard level	Paper	Paper 3	Time zone	Time zone 2
Command term	State	Question number	18	Adapted from	N/A

Question

State one soil condition that favours denitrification.

Markscheme

a. lack of oxygen / anaerobic conditions;
b. excess water / bog/marsh conditions;

Date	May 2011	Marks available	1	Reference code	11M.3.SL.TZ2.18
Level	Standard level	Paper	Paper 3	Time zone	Time zone 2
Command term	State	Question number	18	Adapted from	N/A

Question

State one condition that favours denitrification.

[1]

Explain the consequences of releasing raw sewage and nitrate fertilizer into rivers.

[4]

Markscheme

denitrification is favoured by anaerobic conditions in soil;
poor drainage/waterlogged soils;
slightly alkaline;
suitable temperature;
the presence of denitrifying bacteria/Pseudomonas (denitrificans);

pH changes;
raw sewage often contains pathogens that cause disease;
increase in nutrients/nitrates (from fertilizer) leads to eutrophication;
decomposition of organic matter releases ammonia into water;
ammonia is converted into nitrate;
nitrate causes algal bloom;
bacteria that feed on sewage cause high BOD/lack of (dissolved) oxygen;
lack of oxygen may kill fish/other aquatic life;
as algae release oxygen into water via photosynthesis, water recovers downstream;

Date	May 2012	Marks available	1	Reference code	12M.3.SL.TZ1.7
Level	Standard level	Paper	Paper 3	Time zone	Time zone 1
Command term	Estimate	Question number	7	Adapted from	N/A

Question

In a long-term experiment in Harvard Forest, Massachusetts in northeastern USA, nitrogen was added to the soil in two different areas of the forest containing either hardwood or softwood trees and the effects on release of CO2 from the soil were measured.

Estimate the difference between the lowest and highest rates of release of CO2 from the soil in the hardwood area, giving the units.

[1]

Suggest one process occurring in tree roots that could cause the release of CO2 from the soil.

[1]

Describe the relationship between rates of nitrogen addition and release of CO2 from soil in the hardwood area.

[2]

c (i).

Suggest a reason for this relationship.

[1]

c (ii).

Compare the effects of the nitrogen addition treatments on the hardwood and softwood areas of the Harvard Forest.

[2]

Markscheme

(255–184/183 =) 71/72 mg CO2 m–2 hr–1 (units required)

respiration (in tree root cells/mycorrhizal fungi/bacteria/other microorganisms)

as nitrogen addition increases, release of CO2 decreases;
greater decrease from zero to 50 kg ha–1 yr–1 than from 50–150 kg ha–1 yr–1 / from zero to 50 kg ha–1 yr–1 decrease is 43 mg CO2 m–2 hr–1, from 50–150 kg ha–1 yr–1 decrease is 28 mg CO2 m–2 hr–1;

c (i).

increased nitrogen addition leads to less root/fungal/bacterial growth/alters pH/osmotic potential of soil so affecting/decreasing respiration of organisms resulting in less CO2 production

c (ii).

as nitrogen addition treatment increases, release of CO2 from soil decreases in the hardwood area whereas there is no significant change in the softwood area;
release of CO2 from soil is higher in hardwood area than in softwood area at all nitrogen addition treatments;
at nitrogen addition treatment of 50 kg ha–1 yr–1 hardwood shows (large) decrease in CO2 release from zero treatment whereas softwood shows (slight) increase / other valid numerical comparison;

Date	May 2012	Marks available	2	Reference code	12M.3.SL.TZ1.17
Level	Standard level	Paper	Paper 3	Time zone	Time zone 1
Command term	Outline	Question number	17	Adapted from	N/A

Question

Denitrification is part of the nitrogen cycle. Outline the conditions that favour denitrification in the environment.

Markscheme

anaerobic conditions / absence of air/oxygen;
(facultatively) anaerobic bacteria / Bacillus/Thiobacillus/Pseudomonas genera;
soil is waterlogged/bogs/marshes/other valid example;

Date	May 2012	Marks available	1	Reference code	12M.3.SL.TZ2.18
Level	Standard level	Paper	Paper 3	Time zone	Time zone 2
Command term	State	Question number	18	Adapted from	N/A

Question

Explain the principles involved in the generation of methane from biomass.

[3]

State the role of Rhizobium in the nitrogen cycle.

[1]

c (i).

Markscheme

organic matter/manure/waste/agricultural material/seaweed used;
bacteria in digester transform biomass/raw material;
anaerobic conditions / constant temperature / neutral pH in the digester;
bacteria convert organic material to organic acids/alcohol;
other bacteria convert organic acids/alcohols into acetate;
methanogenic bacteria convert acetate to methane

nitrogen fixation / changes (free) nitrogen to ammonia

c (i).

Date	May 2010	Marks available	2	Reference code	10M.3.SL.TZ1.23
Level	Standard level	Paper	Paper 3	Time zone	Time zone 1
Command term	Outline	Question number	23	Adapted from	N/A

Question

Below is a diagram of the nitrogen cycle.

Indicate the processes occurring at A and B.

A: …………………………………………………………

B: …………………………………………………………

[1]

Draw an arrow to indicate where in the cycle Azotobacter plays a role.

[1]

b (i).

State the role of Nitrobacter in this cycle.

[1]

b (ii).

Outline the consequences of releasing nitrate fertilizer into rivers.

[2]

Markscheme

Both required for [1].
A: denitrification
B: putrification/decay/decomposition

arrow pointing from nitrogen in atmosphere to ammonium compounds

b (i).

nitrification / converts nitrite to nitrate

b (ii).

adding nitrates causes increase in growth of phytoplankton/algal blooms;
decomposition (of increased biomass/dead plants/animals) uses up more oxygen;
can result in death of other aquatic organisms due to lack of oxygen;
plant/algal biomass increases causing turbidity of water;

Date	November 2011	Marks available	1	Reference code	11N.3.SL.TZ0.16
Level	Standard level	Paper	Paper 3	Time zone	TZ0
Command term	Label	Question number	16	Adapted from	N/A

Question

The diagram below shows the nitrogen cycle.

Using the letter X, label where the process of denitrification occurs in the nitrogen cycle.

Markscheme

letter X labelled anywhere on the arrow between Nitrate (NO3) and Nitrogen (N2)