Nitrogen cycle – fixation, nitrification, denitrification and eutrophication
BiologyEcology & EnvironmentAges 17–18
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Sign in to playA box model of the nitrogen cycle in the soil of a grazed meadow: N₂ fixation by Rhizobium in legume root nodules, free-living bacteria, lightning and Haber-process fertiliser; uptake, feeding, death and excretion; ammonification, nitrification (Nitrosomonas, Nitrobacter), denitrification and leaching. Change soil aeration, waterlog the soil, add fertiliser, grow legumes or plough, and watch every pool and flux (kg/ha/yr) respond; follow a single N atom around the cycle and see leached nitrate and phosphate cause eutrophication in a lake. A phosphorus panel shows P moving from rocks into DNA and ATP and out to sediments.
Lesson: Biogeochemical cycles: the nitrogen and phosphorus cycles
What it shows
Air is 78 % nitrogen, yet plants and animals cannot use N₂ directly. Nitrogen enters living things only after fixation by bacteria, lightning or the Haber process. Plants absorb ammonium and nitrate ions to make amino acids, proteins and nucleic acids, animals get nitrogen by eating, and decomposers return it to the soil as ammonium. Nitrifying bacteria need oxygen, while denitrifying bacteria in waterlogged soil return nitrogen to the air. Excess nitrate and phosphate washed into lakes cause algal blooms and oxygen loss.
How to use
Press Play and drag Soil aeration, set Waterlogged, Nitrogen fertiliser and Legumes, or press Plough. Arrow widths and the flux table show each process in kg N/ha per year. Press Follow an N atom, then Next step, to trace one atom. Use Graph to plot soil, living or lake values, and switch Cycle to Phosphorus.
Parameters you can change
- Cycle Nitrogen, Phosphorus
- Soil aeration 0–100 %
- Waterlogged soil No, Yes
- Nitrogen fertiliser 0–300 kg N/ha/yr
- Legumes No, Yes (clover sown with the grass)
Questions to explore
- Why does soil that stays waterlogged lose nitrogen, and which bacteria are responsible?
- Why do legumes add nitrogen to the soil, and why do they fix less when lots of fertiliser is applied?
- How does heavy fertiliser use change the lake downstream, and why can the fish die?
