Discuss how human activities effect the nitrogen cycle and the impacts of this on ecosystems [9 marks]

Indicative Content	Guidance
Human activities affecting nitrogen cycle: Production and use of nitrogen-based fertilisers Agricultural practices and biomass burning Emissions from fossil fuel-based transportation and power plants Over-harvesting and irrigation → reduction in mineral nitrogen in soils Discharge of sewage and livestock effluent → increase in nitrogen content in waterways Farming practices → affect nitrogen flows (aquatic and terrestrial) Impacts on ecosystems: Introduction of NO_x into the atmosphere Formation of acid rain and its consequences NO_x → powerful greenhouse gas → contribution to climate change Specific impacts on soil composition and nutrient leaching from soil Effects on water bodies, including eutrophication and damage to aquatic life → pollution management strategies needed for eutrophication Balanced discussion: Discussion of activities increasing and decreasing nitrogen flows and stores Consideration of various human-induced impacts on the nitrogen cycle Some consideration of possible solutions	You will always have to answer two 9 mark questions from Section B in your IB ESS SL Paper 2 exam This is a 'level of response' answer. The examiner will award you a combined mark out of 9 for knowledge, application, analysis and evaluation ‘Discuss’ (for 9 marks) requires: Knowledge of concepts, principles and models Application to the context using relevant and focused examples Analysis showing logical and coherent chains of reasoning of a range of issues Evaluation showing critical thinking linked to the context Depth is more important than breadth It is sensible to discuss three or four main points but to apply, analyse and evaluate them fully For top-level evaluation, informed judgement is required This can be demonstrated in a conclusion that synthesises and weighs up the arguments to form a supported answer to the question
Model Answer
Human activities can significantly disrupt the nitrogen cycle, which can then have various impacts across ecosystems. The widespread production of nitrogen-based fertilisers for agriculture is responsible for the conversion of huge amounts of atmospheric nitrogen (N₂) into nitrate (NO₃) and the introduction of excess of NO₃ into ecosystems [Ap]. The practice of biomass burning, often used for land clearing and agriculture, releases substantial amounts of nitrogen compounds into the atmosphere [Ap]. Overharvesting, another human-driven activity, significantly alters the nitrogen balance in soils [K]. As humans extract resources from ecosystems at a greater rate than they can be naturally replenished, the mineral nitrogen content in soils is continually reduced [Ap]. This reduction in nitrogen, essential for the health of soils and the organisms that rely on them, poses a threat to ecosystem stability. Overharvesting disrupts the delicate equilibrium of nitrogen flows within ecosystems, emphasising the need for sustainable resource management to mitigate these adverse effects [An]. The release of sewage and livestock effluent into waterways results in unnaturally high nitrogen content in aquatic systems, potentially leading to eutrophication and other water quality issues [Ap]. In addition, the atmospheric pollution resulting from human activities, particularly the emissions of nitrogen oxides, combines with water vapour to form acid rain [Ap]. This acid rain has far-reaching consequences, causing needle loss in pine forests, leaching essential minerals from soils, and contributing to the acidification of water bodies [An]. To mitigate the extensive and varied impacts of human-induced disturbances to the nitrogen cycle, a range of different approaches are required [An]. Using more sustainable agricultural practices that reduce reliance on nitrogen-based fertilisers and promote responsible land use, including a decrease in biomass burning, can help minimise the introduction of excess NO₃ into ecosystems [Ap]. Adopting practices that reduce the discharge of animal waste into waterways is another critical step towards restoring balance to the nitrogen cycle [Ap]. Recognising the complex relationships between human activities, the nitrogen cycle, and ecosystem impacts is needed in order to implement sustainable practices that mitigate the negative outcomes of these interactions [E].
Commentary
This answer is an example - yours may be different The answer discusses several specific examples of human impacts on the nitrogen cycle and how these effect ecosystems The response is annotated to show where the skills of knowledge (K), application (Ap), analysis (An) and evaluation (E) have been demonstrated

Site	Number of woodlice per m²	Net primary productivity / g m^-2 y^-1
X	240	1 320
Y	80	460

Flows of Energy & Matter (SL IB Environmental Systems & Societies (ESS))

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