wunder beta🌱 How does nitrogen cycle through an ecosystem?
You are standing in a clover field after rain. The air is mostly nitrogen, yet the plants at your feet cannot drink it.
What you’ll learn
- Nitrogen enters living systemsExplain why fixation is needed before atmospheric nitrogen can enter most food webs.Fixing organisms convert atmospheric N2 into reactive nitrogen, often through symbioses with plants.
- Soil microbes change nitrogen formsTrace ammonification, nitrification, and denitrification as microbial transformations.Microbial metabolism changes nitrogen among organic matter, ammonium, nitrite, nitrate, and gases.
- Plants and animals move nitrogenDescribe assimilation, food-web transfer, detritus, and root-nodule mutualism.Plants build biomass from available nitrogen, and feeding, waste, and decomposition redistribute it.
- Fertilizer changes the cycleConnect fertilizer inputs to leaching, runoff, eutrophication, and management.Reactive nitrogen can increase crop production but also move downstream when supply exceeds uptake.
- The ecosystem-scale pictureUse a budget view to connect nitrogen reservoirs, rates, oxygen, and human alteration.The cycle is a network of inputs, internal loops, storage, transformations, and losses.
Questions this course answers
Why is atmospheric nitrogen not directly available to most plants?
The strong triple bond in N2 makes atmospheric nitrogen chemically inaccessible until fixation creates reactive forms.
Order the route by which nitrogen enters a food web.
Fixation opens atmospheric nitrogen, assimilation puts it into plant tissue, and feeding moves it to consumers.
Match each process with its nitrogen change.
Different microbial groups carry out distinct conversions in the cycle.
Why can nitrification increase nitrate loss from a field?
Nitrate generally travels with soil water, so rain or irrigation can leach or transport it as runoff.
Why is decomposition part of the nitrogen cycle?
Death and waste return nitrogen to forms that can be stored, transformed, or taken up again.
What is a root nodule in a legume?
Legumes provide carbon and habitat while symbiotic bacteria fix atmospheric nitrogen.
Grounded in trusted sources
- OpenStax, Biology 2e, 46.3 Biogeochemical Cycles, https://openstax.org/books/biology-2e/pages/46-3-biogeochemical-cycles
- OpenStax, Biology 2e, 22.3 Prokaryotic Metabolism, https://openstax.org/books/biology-2e/pages/22-3-prokaryotic-metabolism
- University of Delaware Cooperative Extension, Nitrogen Cycling in Agriculture, Amy L. Shober, revised 2025, https://www.udel.edu/academics/colleges/canr/cooperative-extension/fact-sheets/nitrogen-cycling-in-agriculture/
- U.S. Geological Survey Circular 1136, Nutrients in the Nation's Waters, https://pubs.usgs.gov/circ/circ1136/circ1136.html
- U.S. Environmental Protection Agency, Nutrients parameter fact sheet, https://www.epa.gov/system/files/documents/2021-07/parameter-factsheet_nutrients.pdf
- University of Florida IFAS Extension, An Overview of Key Soil Nitrogen Cycling Transformations, SL471/SS684, https://ask.ifas.ufl.edu/publication/SS684
- U.S. Environmental Protection Agency, Nitrous Oxide Emissions, https://www.epa.gov/ghgemissions/nitrous-oxide-emissions
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