Human activities disrupt the nitrogen cycle by converting atmospheric nitrogen into reactive fertilizer faster than ecosystems can process it. Fertilizer runoff transfers excess nitrates into aquatic systems, causing eutrophication, oxygen depletion and biodiversity loss.
This content belongs to IB ESS Subtopic 2.3: Biogeochemical cycles and is HL only.
How the disruption occurs
Industrial fertilizer production uses the Haber process to convert atmospheric nitrogen into ammonia. This greatly increases the movement of nitrogen from the atmosphere into agricultural soils, adding reactive nitrogen beyond natural rates of nitrogen fixation.
Plants absorb some fertilizer, but excess nitrogen may be converted into soluble nitrates by nitrification. Nitrates can then be leached through soil into groundwater or carried by surface runoff into rivers, lakes and coastal waters.
| Stage | Effect on the nitrogen cycle and ecosystem |
|---|---|
| Fertilizer application | Adds large quantities of reactive nitrogen to soil stores |
| Runoff and leaching | Transfer nitrates into freshwater and marine systems |
| Nutrient enrichment | Produces rapid phytoplankton or algal growth called eutrophication |
| Algal death and decomposition | Decomposers consume more dissolved oxygen, increasing biochemical oxygen demand (BOD) |
| Oxygen depletion | Hypoxic or anoxic conditions kill aquatic organisms and reduce biodiversity |
A named example is the Gulf of Mexico dead zone. Nitrogen-rich runoff from agriculture in the Mississippi River basin contributes to seasonal algal blooms and hypoxia near the river’s mouth. The impacts include damaged fisheries and unequal economic costs for coastal communities, linking the scientific process to environmental justice.
A common misconception is that fertilizer directly removes oxygen from water. It does not: fertilizer stimulates algal growth, and the later decomposition of dead organic matter consumes dissolved oxygen.
Exam technique
For an explain question, present a complete causal chain: fertilizer use, nitrate runoff, algal growth, decomposition, increased BOD, hypoxia and biodiversity loss. Use precise nitrogen-cycle terminology and add a named example to strengthen higher-mark responses.