IB ESS HL 2.3 Biogeochemical Cycles Questions

Analyse carbon and nitrogen cycle diagrams, storage data, bacterial transformations, ocean acidification and human impacts on cycle balance.

Exam points

classify and interpret organic/inorganic stores, stores/sinks/sources and long-term carbon storage in cycle diagrams
draw, label and trace carbon-cycle stores and flows, including photosynthesis, feeding, respiration, decomposition, sequestration and mitigation pathways
calculate and compare carbon storage or net carbon gain between ecosystems or treatment conditions, retaining units and interpreting sink effectiveness
explain ocean carbon exchange and acidification by linking atmospheric CO₂, dissolution, pH change and effects on marine calcifiers
explain how human activities alter carbon stores and flows, then outline mitigation through reduced fossil-carbon release or increased capture and storage
complete and interpret nitrogen-cycle diagrams by tracing bacterial transformations, plant assimilation and transfer into decomposer protein
analyse how agriculture, deforestation, aquaculture, urbanization, fertilizer use, combustion and sewage alter nitrogen stores and flows, including flow-versus-storage evidence and carbon-cycle comparisons

Covered syllabus points

2.3.1—Biogeochemical cycles purpose 2.3.2—Cycle components 2.3.3—Carbon stores 2.3.4—Carbon flows 2.3.5—Carbon sequestration 2.3.6—Ecosystems as carbon stores/sinks/sources 2.3.7—Fossil fuels as carbon stores 2.3.8—Agriculture as carbon store/source/sink 2.3.9—Ocean carbon exchange 2.3.10—Ocean acidification 2.3.11—Alleviating carbon cycle impacts 2.3.12 (HL)—Lithosphere carbon stores 2.3.13 (HL)—Carbonate fossilization 2.3.14 (HL)—Fossil fuel formation 2.3.15 (HL)—Methane production 2.3.16 (HL)—Methane cycle 2.3.17 (HL)—Nitrogen cycle stores 2.3.18 (HL)—Bacterial roles in nitrogen cycle 2.3.19 (HL)—Denitrification conditions 2.3.20 (HL)—Plant nitrogen fixation 2.3.21 (HL)—Nitrogen cycle flows 2.3.22 (HL)—Human impacts on nitrogen cycle 2.3.23 (HL)—Haber process 2.3.24 (HL)—Nitrogen planetary boundary 2.3.25 (HL)—Global collaboration needed

[Maximum number: 7]

Explain how a community of decomposers contributes to the stability of the whole ecosystem.

a. Decomposers will feed on dead organic matter/detritus (like dead leaves, wood, carcasses, feces)...;
b. ... reducing their accumulation / cleaning up Earth;
c. They will break down organic matter into inorganic matter…;
d. ...enhancing soil fertility / increasing nutrient availability;
e. The inorganic minerals (nitrates, phosphates, metals) will then feed plant communities / will be absorbed by plant roots...;
f. ...supporting high vegetation/plant abundance;
g. ...and may support high habitat/niche diversity;
h. Decomposers are responsible for nutrient cycling / e.g. nitrogen cycling (nitrogen fixation/ammonification/nitrification/denitrification/phosphorus cycling/carbon cycle;
i. ...high flow of matter may support complex food webs;
j. Decomposers are the basis of a detritus-based food web / important part of ecosystem flow of energy / may be eaten/pass biomass on to carnivore community...;
k. ...supporting/stabilizing carnivore abundance;
I. As decomposer numbers decrease due to predation, so do carnivores;
m. ... thereby contribute to regulation of the carnivore community / which is an example of negative feedback (leading to stability);
n. Decomposer will release organic matter through their own death/defecation...;
o. ... which would provide food for other decomposers;
p. Decomposers participate in humus formation...;
q. ...increasing water holding capacity of soil (and soil fertility = MPd);
r. Decomposers may compete with one another regulating their populations (another example of negative feedback = MPm);
s. Decomposers, like earthworms, loosen soil / disperse nutrients / create space for air and water circulation...;
t. ...helping root growth / improving soil structure;
u. Fragmentation: larger invertebrates initially fragment larger chunks of organic matter (then bacteria and fungi complete decomposition into nutrients = MPc);
v. After a disturbance, decomposers will increase their metabolic/respiration rate/feeding on detritus...;
w. ...contributing to a faster return to original state (resilience entailing stability);

[Maximum number: 7]

Figure Carbon cycle

(a)

Identify one inorganic carbon storage in Figure.

[ 1 ]

(b)

Draw a labelled diagram to illustrate the flows of carbon between plants and the atmosphere shown in Figure.

[ 2 ]

(c)

Using the data in Figure, calculate the net gain in carbon in the oceans in $10^{15} \mathrm{~g}$ .

[ 1 ]

(d)

Describe one advantage and one disadvantage of the oceans as a carbon sink.

Advantage:

Disadvantage:

[ 2 ]

Advantage:
oceans are large so absorb a lot of CO2 / increases primary productivity/photosynthesis / reduces global warming/climate change;

Disadvantage:
increasing $\mathrm{CO}_{2}$ in oceans causes acidification / ocean acidification harms sensitive marine species/ecosystems like coral reefs;

(e)

Outline one method to mitigate the effects of atmospheric carbon storage.

[ 1 ]

a. reforestation/afforestation initiatives so that more carbon can be absorbed;
b. carbon capture and storage/CCS to bury $\mathrm{CO}_{2}$ underground;
c. land use change/urban renewal leading to revegetation/green roofs to absorb $\mathrm{CO}_{2}$ ;
d. switch to renewable energy to reduce use of fossil fuels;

Note: There are multiple acceptable responses to this question i.e. any process that reduces release of $\mathrm{CO}_{2}$ or absorbs $\mathrm{CO}_{2}$ from atmosphere NB This is an "outline" question, not "identify", so requires a little more than "reforestation" (i.e. WHY reforestation?) or "reduce use of fossil fuels" (...need to state HOW fossil fuel use might be reduced),

[Maximum number: 7]

Figure Carbon cycle

(a)

Identify one carbon flow caused by human activities in Figure.

[ 1 ]

(b)

Draw a labelled diagram to illustrate the flows of carbon between plants and the atmosphere shown in Figure.

[ 2 ]

(c)

Using the data in Figure, calculate the net gain in carbon in the oceans in $10^{15} \mathrm{~g}$ .

[ 1 ]

$(92+0.8)-(90+0.1)=2.7\left(\mathrm{x}^{10} \mathrm{~g} \mathrm{C} \mathrm{yr}^{-1}\right)$

(d)

Describe one advantage and one disadvantage of the oceans as a carbon sink.

Advantage:

Disadvantage:

[ 2 ]

Advantage:
oceans are large so absorb a lot of $\mathrm{CO}_{2}$ / increases primary productivity/photosynthesis / reduces global warming/climate change;

Disadvantage:
increasing $\mathrm{CO}_{2}$ in oceans causes acidification / ocean acidification harms sensitive marine species/ecosystems like coral reefs;

(e)

Outline one method to mitigate the effects of atmospheric carbon storage.

[ 1 ]

a. reforestation/afforestation initiatives so that more carbon can be absorbed;
b. carbon capture and storage/CCS to bury $\mathrm{CO}_{2}$ underground;
c. land use change/urban renewal leading to revegetation/green roofs to absorb $\mathrm{CO}_{2}$ ;
d. switch to renewable energy to reduce use of fossil fuels;

Note: There are multiple acceptable responses to this question i.e. any process that reduces release of CO2 or absorbs CO2 from atmosphere NB This is an "outline" question, not "identify", so requires a little more than "reforestation" (i.e. WHY reforestation?) or "reduce use of fossil fuels" (...need to state HOW fossil fuel use might be reduced),

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Covered syllabus points

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