Big picture: Agriculture is the largest human source of reactive nitrogen, through synthetic fertilisers, livestock waste, and the cultivation of nitrogen-fixing crops.
How agriculture disrupts the nitrogen cycle
- Synthetic fertiliser application adds massive reactive nitrogen to soils
- Only 30–50% of applied nitrogen is taken up by crops — the rest is lost
- Livestock waste releases ammonia and nitrous oxide
- Rice paddies create anaerobic conditions ideal for denitrification (Nâ‚‚O)
- Monoculture reduces soil microbial diversity affecting nitrogen cycling
Sustainable nitrogen management
- Precision agriculture — applying fertiliser only where and when needed
- Cover crops and crop rotation with legumes
- Buffer strips along waterways to capture runoff
- Improved animal waste management
- Organic farming practices
Key concept: Eutrophication is the enrichment of water bodies with nutrients (mainly nitrogen and phosphorus), leading to excessive algal growth, oxygen depletion, and ecosystem collapse.
The eutrophication process
- Excess nitrogen enters waterways through agricultural runoff
- Algal bloom occurs due to nutrient enrichment
- Algae block light, preventing photosynthesis by submerged plants
- When algae die, decomposing bacteria consume dissolved oxygen
- Hypoxic (low oxygen) or anoxic (no oxygen) conditions develop
- Fish and other aquatic organisms die — creating a dead zone
Major dead zones worldwide
- Gulf of Mexico — 15,000+ km², caused by Mississippi River agricultural runoff
- Baltic Sea — one of the largest marine dead zones
- Chesapeake Bay — nutrient loading from agriculture and urban areas
- Lake Erie — recurring algal blooms from agricultural runoff
IB exam tip: Be able to describe the steps of eutrophication in sequence and link each step to the next using cause-and-effect language.
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Non-agricultural nitrogen sources
- Fossil fuel combustion — releases nitrogen oxides (NOx) contributing to smog and acid rain
- Deforestation — releases stored nitrogen from biomass and soil
- Urbanisation — sewage and wastewater add nitrogen to waterways
- Aquaculture — fish waste and uneaten feed release nitrogen
- Industrial processes — manufacturing and chemical production
Consequences of excess atmospheric nitrogen
- Acid deposition damages forests, lakes, and buildings
- Photochemical smog in cities (NOx + VOCs + sunlight → ozone)
- Nitrous oxide (N2O) is 265× more potent than CO2 as a greenhouse gas
- Nitrogen deposition changes plant community composition, reducing biodiversity