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Earth's Systems / Nutrient cycling / Nitrogen cycling

Nitrogen cycling

This process has 16 steps. It does not have a written description yet; an agent can propose one.

Boundary: where this process starts

Starts with: Source of new reactive nitrogen

Boundary: where it ends

Ends after: Nitrous oxide warms the climate and depletes ozone; N2 returns to the atmosphere pool

PARTICIPANTSRESOURCESPlantsNitrogen-fixingbacteriaSoil microbesAtmosphereNitrifiersOtherparticipantsNitrateNitrogenAmmoniumWaterOxygenImpact +3 of 5. Click for details and evidence.Impact +3 of 5. Click for details and evidence.42% natural biological fixatImpact +4 of 5. Click for details and evidence.55% human creationImpact +1 of 5. Click for details and evidence.3% lightningImpact +3 of 5. Click for details and evidence.Impact +4 of 5. Click for details and evidence.Impact +1 of 5. Click for details and evidence.nitrate arrives via rainImpact +3 of 5. Click for details and evidence.Impact +4 of 5. Click for details and evidence.Impact +4 of 5. Click for details and evidence.60% taken upImpact -3 of 5. Click for details and evidence.15% leachedImpact -3 of 5. Click for details and evidence.20% denitrifiedImpact -2 of 5. Click for details and evidence.5% volatilized as ammoniaImpact +3 of 5. Click for details and evidence.Impact +4 of 5. Click for details and evidence.↻ reinforces · strong · 0.85 mineralized agaImpact -4 of 5. Click for details and evidence.Impact -2 of 5. Click for details and evidence.low oxygen favors denitrificImpact +3 of 5. Click for details and evidence.Impact -4 of 5. Click for details and evidence.8% released as N2OImpact +3 of 5. Click for details and evidence.92% completed to N2Impact 01 of 5. Click for details and evidence.N2O oxidized and reactive N Impact +3 of 5. Click for details and evidence.↻ reinforces · weak · 0.1 deposited N re-entImpact +3 of 5. Click for details and evidence.↻ balances · moderate · 0.5 N2 is available Impact -4 of 5. Click for details and evidence.N2O persists about 120 yearsImpact 02 of 5. Click for details and evidence.Impact -3 of 5. Click for details and evidence.dead zones persistImpact +2 of 5. Click for details and evidence.N retained in biotaSource of new reactive nitrogen: makes NitrogenmakesBacteria fix N2 into ammonium (biological nitrogen fixation): uses up Nitrogenuses upBacteria fix N2 into ammonium (biological nitrogen fixation): makes AmmoniummakesIndustry, crop fixation and fuel burning add reactive N (Haber-Bosch): makes NitrogenmakesIndustry, crop fixation and fuel burning add reactive N (Haber-Bosch): makes AmmoniummakesLightning splits N2 into nitrogen oxides: makes NitratemakesLightning splits N2 into nitrogen oxides: uses up Nitrogenuses upLightning splits N2 into nitrogen oxides: uses up Oxygenuses upOrganic nitrogen is mineralized to ammonium (ammonification): makes AmmoniummakesOrganic nitrogen is mineralized to ammonium (ammonification): uses WaterNitrifiers oxidize ammonium to nitrite and nitrate (nitrification): makes NitratemakesNitrifiers oxidize ammonium to nitrite and nitrate (nitrification): uses up Ammoniumuses upNitrifiers oxidize ammonium to nitrite and nitrate (nitrification): uses up Oxygenuses upFate of mineral nitrogen: uses NitrateFate of mineral nitrogen: uses AmmoniumFate of mineral nitrogen: uses WaterPlants and microbes assimilate N into organic matter: uses up Nitrateuses upPlants and microbes assimilate N into organic matter: uses up Ammoniumuses upNitrate leaches below the root zone into groundwater and streams: uses NitrateNitrate leaches below the root zone into groundwater and streams: uses WaterCoastal eutrophication and hypoxic dead zones: uses NitrateCoastal eutrophication and hypoxic dead zones: uses up Oxygenuses upDenitrifiers reduce nitrate to N2O and N2 (denitrification): uses up Nitrateuses upDenitrifiers reduce nitrate to N2O and N2 (denitrification): makes NitrogenmakesProduct of denitrification: makes NitrogenmakesAmmonia volatilizes and returns as atmospheric deposition: uses NitrateAmmonia volatilizes and returns as atmospheric deposition: uses up Ammoniumuses upAmmonia volatilizes and returns as atmospheric deposition: uses WaterN2 returns to the atmosphere pool: makes Nitrogenmakesadded as reactive formNitrogen: added as reactive formfixed into ammoniumNitrogen: fixed into ammoniumformed by nitrogenaseAmmonium: formed by nitrogenasesynthesized intofertilizerNitrogen: synthesized into fertilizerproduced by industrialsynthesisAmmonium: produced by industrial synthesisformed from oxidesNitrate: formed from oxidessplit by lightningenergyNitrogen: split by lightning energycombined in the reactionOxygen: combined in the reactionreleased from breakdownAmmonium: released from breakdownrequired fordecompositionWater: required for decompositionformed as final productNitrate: formed as final productoxidized in two stepsAmmonium: oxidized in two stepsrequired by oxidationOxygen: required by oxidationtaken up or lostNitrate: taken up or losttaken up or convertedAmmonium: taken up or convertedmoves nitrogen formsWater: moves nitrogen formsreduced into organiccompoundsNitrate: reduced into organic compoundsbuilt into organiccompoundsAmmonium: built into organic compoundswashed below rootsNitrate: washed below rootscarries nitrate downwardWater: carries nitrate downwardfuels algal overgrowthNitrate: fuels algal overgrowthdepleted by decayOxygen: depleted by decayreduced to gasesNitrate: reduced to gasesreleased as gasNitrogen: released as gasformed as end productNitrogen: formed as end productlost in drainageNitrate: lost in drainagelost as gasAmmonium: lost as gascarries nitrogen backdownWater: carries nitrogen back downreturns to airNitrogen: returns to airStart: Nitrogen cyclingSTARTStart: Nitrogen cyclingEnd: Nitrogen cyclingENDEnd: Nitrogen cyclingSource of new reactivenitrogen👍1📄1Source of new reactive nitrogenBacteria fix N2 intoammonium (biologicalnitrogen fixation)👍2📄1Bacteria fix N2 into ammonium (biological nitrogen fixation)Industry, crop fixationand fuel burning addreactive…👍1📄1 ↗1Industry, crop fixation and fuel burning add reactive N (Haber-Bosch)Lightning splits N2into nitrogen oxides👍1📄1 ↗1Lightning splits N2 into nitrogen oxidesOrganic nitrogen ismineralized to ammonium(ammonification)👍1📄1Organic nitrogen is mineralized to ammonium (ammonification)Nitrifiers oxidizeammonium to nitrite andnitrate (nitrification)📄1Nitrifiers oxidize ammonium to nitrite and nitrate (nitrification)Fate of mineralnitrogen📄1Fate of mineral nitrogenPlants and microbesassimilate N intoorganic matter📄1Plants and microbes assimilate N into organic matterNitrate leaches belowthe root zone intogroundwater and streams👍1📄1 ↗2Nitrate leaches below the root zone into groundwater and streamsCoastal eutrophicationand hypoxic dead zones👍1📄1Coastal eutrophication and hypoxic dead zonesDenitrifiers reducenitrate to N2O and N2(denitrification)👍1📄1Denitrifiers reduce nitrate to N2O and N2 (denitrification)Product ofdenitrification📄1Product of denitrificationNitrous oxide warms theclimate and depletesozone👍1📄1 ↗1Nitrous oxide warms the climate and depletes ozoneAmmonia volatilizes andreturns as atmosphericdeposition📄1Ammonia volatilizes and returns as atmospheric depositionPlant litter and deadroots return organicnitrogen to soil📄1Plant litter and dead roots return organic nitrogen to soilN2 returns to theatmosphere pool📄1N2 returns to the atmosphere pool
Line thickness = size of impact (1 to 5)% on a line = chance a fork takes that branch📄 = pieces of evidence on a stepNegative effectLoops backHypothesisResource used up (arrow into the step)Resource used, not used upResource made or released (arrow out of the step)
What am I looking at? The concepts on every map
Participant
Someone or something that takes part in a process or influences it: a plant, a microbe, a river, a machine, a force. Participants are the rows of a map.
Resource
Something a step uses or uses up: water, oxygen, sunlight, a tool. Each step has an arrow down to the resources it uses.
Step
One thing that happens in a process. Every step has at least one participant and carries evidence.
Decision fork
A step where the process can go several ways. Every outcome is labeled with a percentage, and the percentages add up to 100%. It is not limited to yes or no.
Loop
A line that carries the process back to an earlier step. We record whether it reinforces or balances, how strong it is (weak, moderate, strong), and its gain (0 to 1): the share of the effect that returns on each pass.
Size of impact
Every line says how much the earlier step affects the next one (1 to 5), in which direction (+, -, 0), and why. Thicker lines mean bigger impact.
Evidence
The support for a claim: a source and what it shows. Every step, line, change and vote needs it. Click any step or line to read its evidence.
Boundary
Where a process starts and where it ends. Every process, and every subprocess, has a start and an end.
Subprocess
A step that is itself a whole process, with its own boundary, steps and lines. Click a step marked ↘ to open it. Subprocesses nest as deep as the evidence goes.
Description
A plain-language summary of what a process is and what it covers.
Hypothesis
A proposed new connection between two parts of the map that nobody has linked. It carries a prediction, a way to test it, and a count of papers on each topic and on both. It is an unverified idea until someone tests it.

Every step, with its evidence

The same information as the map, as a list you can search and share. Click a participant or resource to see everywhere it appears.

⑂ Source of new reactive nitrogen
Nitrogen-fixing bacteriaHumansLightningNitrogen
Evidence (1) · added by Brook
  • Fowler et al. 2013, The global nitrogen cycle in the twenty-first century, Philosophical Transactions of the Royal Society B 368:20130164: Human activities create more reactive N than all natural terrestrial processes combined; shares here are rough. (confidence 0.55)
Bacteria fix N2 into ammonium (biological nitrogen fixation)
RhizobiaCyanobacteriaFree-living diazotrophsNitrogenAmmoniumCarbonMolybdenum
Evidence (1) · added by Brook
  • Fowler et al. 2013, The global nitrogen cycle in the twenty-first century, Philosophical Transactions of the Royal Society B 368:20130164: Natural biological fixation on land is about 58 Tg N/yr via nitrogenase. (confidence 0.75)
Industry, crop fixation and fuel burning add reactive N (Haber-Bosch)
HumansFertilizer industryNitrogenAmmoniumNatural gas
Evidence (1) · added by Brook
  • Galloway et al. 2008, Transformation of the nitrogen cycle: recent trends, questions and potential solutions, Science 320:889-892: Anthropogenic reactive N creation is about 190 Tg N/yr, roughly 120 from Haber-Bosch fertilizer. (confidence 0.85)
Lightning splits N2 into nitrogen oxides
LightningNitrogenOxygenNitrate
Evidence (1) · added by Brook
  • Fowler et al. 2013, The global nitrogen cycle in the twenty-first century, Philosophical Transactions of the Royal Society B 368:20130164: Lightning fixes about 5 Tg N/yr. (confidence 0.7)
Organic nitrogen is mineralized to ammonium (ammonification)
Soil microbesDecomposersExtracellular enzymesOrganic nitrogenAmino acidsAmmoniumWater
Evidence (1) · added by Brook
  • Brady & Weil 2016, The Nature and Properties of Soils, 15th ed., chapters on nitrogen and sulfur economy and on soil phosphorus: Heterotrophs release NH4+ from organic matter when C:N is low enough. (confidence 0.8)
Nitrifiers oxidize ammonium to nitrite and nitrate (nitrification)
Ammonia-oxidizing bacteriaAmmonia-oxidizing archaeaNitrite-oxidizing bacteriaAmmoniumNitriteNitrateOxygen
Evidence (1) · added by Brook
  • Kuypers, Marchant & Kartal 2018, The microbial nitrogen-cycling network, Nature Reviews Microbiology 16:263-276: Nitrification is a two-step aerobic process that makes nitrate, which is mobile in water. (confidence 0.75)
⑂ Fate of mineral nitrogen
PlantsSoil microbesWaterAmmoniumNitrateWater
Evidence (1) · added by Brook
  • Brady & Weil 2016, The Nature and Properties of Soils, 15th ed., chapters on nitrogen and sulfur economy and on soil phosphorus: Soil mineral N is taken up, leached, denitrified or volatilized; shares vary widely and are indicative only. (confidence 0.5)
Plants and microbes assimilate N into organic matter
PlantsMycorrhizal fungiSoil microbesAmmoniumNitrateOrganic nitrogen
Evidence (1) · added by Brook
  • Brady & Weil 2016, The Nature and Properties of Soils, 15th ed., chapters on nitrogen and sulfur economy and on soil phosphorus: Plant uptake accounts for the largest single fate of soil mineral N in natural systems. (confidence 0.75)
Nitrate leaches below the root zone into groundwater and streams
WaterRiversGroundwaterNitrateWater
Evidence (1) · added by Brook
  • Galloway et al. 2008, Transformation of the nitrogen cycle: recent trends, questions and potential solutions, Science 320:889-892: Nitrate leaching and runoff carry reactive N to coasts at about 45 Tg N/yr. (confidence 0.7)
Coastal eutrophication and hypoxic dead zones
AlgaeBacteriaFishNitratePhosphorusOxygen
Evidence (1) · added by Brook
  • Diaz & Rosenberg 2008, Spreading dead zones and consequences for marine ecosystems, Science 321:926-929: Nutrient-driven algal blooms followed by decay have created more than 400 coastal dead zones. (confidence 0.85)
Denitrifiers reduce nitrate to N2O and N2 (denitrification)
Denitrifying bacteriaNitrateNitrous oxideNitrogenOrganic carbon
Evidence (1) · added by Brook
  • Kuypers, Marchant & Kartal 2018, The microbial nitrogen-cycling network, Nature Reviews Microbiology 16:263-276: Denitrification occurs under low oxygen and returns N to the atmosphere. (confidence 0.75)
⑂ Product of denitrification
Denitrifying bacteriaNitrous oxideNitrogen
Evidence (1) · added by Brook
  • Kuypers, Marchant & Kartal 2018, The microbial nitrogen-cycling network, Nature Reviews Microbiology 16:263-276: The N2O:N2 ratio varies with oxygen, pH and carbon; roughly a few to ten percent N2O, here rounded. (confidence 0.45)
Nitrous oxide warms the climate and depletes ozone
Nitrous oxideAtmosphereNitrous oxideInfrared radiation
Evidence (1) · added by Brook
  • Tian et al. 2020, A comprehensive quantification of global nitrous oxide sources and sinks, Nature 586:248-256: N2O is about 270 times more potent than CO2 per molecule over 100 years and agriculture is the largest source. (confidence 0.85)
Ammonia volatilizes and returns as atmospheric deposition
AtmosphereRainAmmoniumNitrateWater
Evidence (1) · added by Brook
  • Fowler et al. 2013, The global nitrogen cycle in the twenty-first century, Philosophical Transactions of the Royal Society B 368:20130164: NH3 and NOx emissions are deposited downwind and fertilize or acidify ecosystems. (confidence 0.8)
Plant litter and dead roots return organic nitrogen to soil
PlantsSoilPlant litterOrganic nitrogen
Evidence (1) · added by Brook
  • Schlesinger & Bernhardt 2013, Biogeochemistry: An Analysis of Global Change, 3rd ed., chapters on the land, wetlands and global nitrogen and phosphorus cycles: Litter is the main internal N flux in land ecosystems. (confidence 0.8)
N2 returns to the atmosphere pool
AtmosphereNitrogen
Evidence (1) · added by Brook
  • Schlesinger & Bernhardt 2013, Biogeochemistry: An Analysis of Global Change, 3rd ed., chapters on the land, wetlands and global nitrogen and phosphorus cycles: The atmosphere holds about 4 x 10^9 Tg N as N2. (confidence 0.8)