Outline how soil can be viewed as an ecosystem.

Compare and contrast the impact of humans on the carbon and nitrogen cycles.

Discuss the role of humans in the destabilization of ecological systems.

Like an ecosystem, soil is an open system with inputs and outputs;
(inputs of) eg water/O₂ (and outputs of) eg CO₂/nitrogen;
...and storages and flows/processes;
(storages of) eg nitrates/water (and flows of) eg leaching/decomposition;
like an ecosystem, soil is a community of living/biotic and abiotic elements;
(biotic) eg bacteria/fungi/earthworms (and abiotic) eg clay/sand/silt/water/heat;
...with the many complex interactions/interrelationships/eg mineral cycling/leguminous plants;
interacts with/supports other systems/eg vegetation growth, animal movement/burrowing/human development;

Award marks as above for identifying relevant common features, and also for giving examples (provided it is clear what is being exemplified).

in both cycles combustion (of forests/fossil fuels) increases concentration of oxides in atmosphere;
in both cycles deforestation/agriculture/SDW lead to decomposition that also releases oxides;
...but carbon dioxide released (by respiration) into atmosphere/(whereas) nitrous oxides are released into soil water (by nitrification);
both oxides will increase impact of global warming/climate change;
...but NOx to a smaller degree;
both oxides result in the acidification of water/aquatic bodies;
...but only NOx may cause acid deposition/acidify soils;
deforestation removes organic storages of both N and C (stored in plant biomass);
…and reduces absorption of C from atmosphere (via photosynthesis) (but not N);
…causes soil erosion which reduces inorganic N storages in soil (but not C);
use of inorganic fertilisers increases N in soil (but not C);
…and run-off may cause excessive inorganic N in aquatic systems (but not C);
pesticide/herbicide use in agriculture might kill organisms thus reducing both C and N organic storages (stored in their biomass);
… thus reducing nitrification/denitrification/decomposition process/(whereas) effect on C cycle is limited to reducing respiration by soil animals;
extraction of oil/coal/gas reduces underground (ancient) C storages/transfers C storages on surface (for human use)/(whereas) effect to N cycle is limited to a few organic compounds/aromatics found in oil;

Award 5 max if only similarities or only differences are identified.

The following guide for using the markbands suggests certain features that may be offered in responses. The five headings coincide with the criteria given in each of the markbands (although “ESS terminology” has been conflated with “Understanding concepts”). This guide simply provides some possible inclusions and should not be seen as requisite or comprehensive. It outlines the kind of elements to look for when deciding on the appropriate markband and the specific mark within that band.

Answers may include:

• understanding concepts and terminology of steady state and dynamic equilibria, stability, resilience, tipping-points, diversity, storage size, productivity, negative/positive feedback, complexity, community interdependence/interaction, human impacts of overexploitation/unsustainable harvesting, pollution, habitat degradation/destruction, unsustainable development, etc
• breadth in addressing and linking factors that provide stability/resilience eg storage size, diversity, productivity, complexity, etc with human activities that either weaken/reduce them eg overfishing, intensive agriculture, C emissions/global warming, water/atmospheric pollution, deforestation, urbanisation, mining/resource extraction, etc or activities that strengthen/protect them eg legislation setting pollution limits/standards/fishing quotas, sustainable development schemes, alternative/renewable energies, conservation efforts/reserves, etc
• examples of specific negative impacts on stability of ecological systems e.g. overfishing reduces the storage size of targeted fish populations reducing their stability, C emissions lead to climate change that reduces primary productivity of systems, eutrophication will interfere with negative feedback loops in freshwater systems, atmospheric pollutants may eliminate sensitive species of lichen reducing diversity, etc and positive impacts on stability e.g. in-situ conservation protects complexity of relationships in system, agricultural techniques conserving soils maintain high productivity, etc
• balanced analysis of the extent to which human activities promote or prevent destabilisation of ecological systems with acknowledgement of relevant counter-arguments/alternative viewpoints.
• a conclusion that is consistent with, and supported by, analysis and examples given eg “The role of humans in destabilising ecological systems is very diverse, and with the current size and growth of population, the magnitude of that role is becoming immense, calling for urgent and wide-scale efforts to adopt a proactive role in re-stabilising those systems.” NB This is only an example of a possible conclusion. Candidates’ conclusions do not have to agree. Their value should be assessed simply according to the criteria given in the markband descriptors.

Refer to paper 2 markbands, available under the "your tests" tab > supplemental materials.

Q7 was the second most popular option in Section B. Most candidates could identify some key features of a system or ecosystem and exemplify them within soil, but few identified sufficient for full credit.

A good majority of candidates had some knowledge of C and N cycles but few could identify human impacts on these much beyond the burning of fossil fuels to increase atmospheric storage of carbon.

The most successful responses to this question demonstrated a sound grasp of the principles of ecological stability and the concepts of resilience and tipping points and were able to develop a strong line of analysis and argument. However, the majority of candidates simply catalogued a range of human impacts limiting their scores to the lower markbands. This raises an issue of question selection. With a “Discuss” command term a candidate needs to be confident of producing a balanced argument and analysis to gain full credit, and not simply a long list of relevant knowledge statements.