Practicing HL Essay Questions: The Nitrogen Cycle - Markscheme
These are the suggested marking points for this practice essay on the Nitrogen Cycle.
a) Outline two transfers and two transformations in the Nitrogen Cycle. [4]
Transfers...
absorption of NO3 from soil to plant roots;
absorption of NO3 from root nodules into plant tissue;
leaching of NO3 from soil to water bodies;
runoff of NO3 from soil to water bodies;
Transformations...
Consumption of plant proteins by animals involves digestion, absorption and assimilation;
Lightning / Nitrogen Fixation of N2 into NH3 from atmosphere to soil;
Nitrogen Fixation of N2 into NH3 by soil bacteria / cyanobacteria in water;
Nitrogen Fixation of N2 into NH3 by bacteria in leguminous root nodules;
Nitrification of NH3 into NO2 and then NO3 by bacteria in soil/root nodules;
decomposition/ammonification of proteins by bacteria/fungi of animal excreta and dead organic plant and animal matter into NH4;
Denitrification of NO3 in (waterlogged) soils into N2 in the atmosphere;
b) Explain the role of bacteria, fungi, and other microorganisms in the Nitrogen Cycle. [7]
Bacteria are crucial for the transformation of N2 into a usable form by (complex organisms);
Nitrogen is necessary for the formation of proteins, DNA, RNA and other molecules crucial for life;
The availability of nitrogen for plants is a major limiting factor of their growth;
Bacteria are found in the soil, water and root nodules of some plants;
Plants in the Fabaceae (formally known as the Leguminosae) family have mutualistic relationships with bacteria that are found in their root nodules;
These plants have a competitive advantage in low-nitrogen environments;
Bacteria in the soil (and water) convert N2 into NH3 in the process of nitrogen fixation;
Bacteria in the soil (and water) convert this NH3 into NO2 and then NO3 which can then be absorbed by plants;
These nitrogen fixation and nitrification processes also take place by bacteria in root nodules;
Fungi and other microorganisms decompose dead organic matter of plants and animals and animal excreta;
Bacteria then convert the NH4 into NH3 (which can then be converted into NO2 and NO3);
Without these bacteria, plants and animals (living organisms) would not be able to use nitrogen;
Marking note: the chemical formulae can be replaced by the correct names - nitrogen, ammonia, ammonium, nitrite, nitrate
c) Discuss the impact that economics can have on the human impact on the Nitrogen Cycle. [9]
HINT: Remember that the command term discuss should include positive and negative impacts.
This part (c) question is asking you to incorporate your HL Lens understanding of Environmental Economics.
| Marks | Level Descriptor |
|---|---|
| 0 | The work does not reach a standard described by the descriptors below. |
| 1-3 | The response contains: • minimal evidence of knowledge and understanding of ESS issues, concepts or HL lenses content • fragmented knowledge statements that are poorly linked to the context of the question • some appropriate use of ESS terminology • no examples, where required, or examples with insufficient explanation or relevance • superficial analysis that amounts to no more than a list of facts or ideas • judgements or conclusions that are vague or not supported by evidence or argument. |
| 4-6 | The response contains: • some evidence of sound knowledge and understanding of ESS issues, concepts and HL lenses content • knowledge statements that are effectively linked to the context of the question • largely appropriate use of ESS terminology • some use of relevant examples, where required, but with limited explanation • clear analysis that shows a degree of balance • some clear judgements or conclusions that are supported by limited evidence or arguments. |
| 7-9 | The response contains: • substantial evidence of sound knowledge and understanding of ESS issues, concepts and HL lenses content • a wide breadth of knowledge statements that are effectively linked to each other and to the context of the question • consistent, appropriate and precise use of ESS terminology • effective use of pertinent, well-explained examples, where required, showing some originality • thorough, well-balanced, insightful analysis • explicit judgements or conclusions that are well supported by evidence or arguments and that include some critical reflection. |
Answers may demonstrate (HL in red):
- understanding concepts & terminology and HL Lenses content deforestation, agriculture, aquaculture and urbanisation impacts of humans on nitrogen cycle; Haber Process; Wastewater Production; planetary boundary of nitrogen flow globally, eutrophication and dead zones; biodiversity loss; economic negative externalities and market failure; polluter pays principle; IPAT. Kuznet's Curve; Tragedy of the Commons; Valuing ecosystem services; taxation/fines; incentives/subsidies;
- breadth in addressing and linking economic concepts with causes of human impacts on nitrogen cycle or the solutions to reducing use of (too much) nitrogen; incorporation elements of HL lenses;
- examples of ways that humans are impacting the nitrogen cycle and the economic incentives / disincentives that can be used to address the problems;
- balanced analysis evaluating the impact that economics is part of the problem or part of the solution;
- a conclusion that is consistent with, and supported by analysis and examples given, e.g. the traditional economic model that incentivises growth at all costs and externalises the problems, in this case, eutrophication, exceeding the planetary boundary for nitrogen flow, dead zones, biodiversity loss etc, has been a large part of the problem in causing the nitrogen flow to exceed its safe operating space however it can also be part of the solution with the removal of subsidies for the use of inorganic fertiliser and fossil fuels and providing subsidies to promote, sustainable agriculture, efficient fertiliser use, reducing nitrogen emissions, supporting nutrient recovery from waste water and enhancing ecosystem services.
[Total 20 Marks]