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Earth's Systems / Biological processes / Terrestrial ecosystem functioning / Living soil system / Nutrient availability and leaching / Nitrate leaching from the root zone to groundwater and coastal waters

Nitrate leaching from the root zone to groundwater and coastal waters

Nitrate, a mobile anion not held by negatively charged soil, is flushed out of the root zone by autumn and winter drainage, reaching drains, groundwater and rivers. Cover crops and riparian buffers cut the load, but the nitrogen that reaches rivers drives coastal hypoxia.

Boundary: where this process starts

Starts with: Nitrate is not adsorbed by net-negative soil surfaces and moves with drainage water, taking base cations with it

Boundary: where it ends

Ends after: Nitrogen delivered by rivers fuels algal blooms and the seasonal hypoxic zone in the Gulf of Mexico; Riparian buffers remove about two thirds of nitrate on average (large range) by denitrification and uptake

PARTICIPANTSRESOURCESDrainage waterNitrateGroundwaterCover cropsRain andirrigationOtherparticipantsNitrateWaterBase cationsNitrogenNitrous oxideImpact -4 of 5. Click for details and evidence.Impact -4 of 5. Click for details and evidence.Impact +3 of 5. Click for details and evidence.Cover crops intercept nitratImpact -4 of 5. Click for details and evidence.Impact -3 of 5. Click for details and evidence.Impact -3 of 5. Click for details and evidence.Impact -4 of 5. Click for details and evidence.Impact -3 of 5. Click for details and evidence.Impact -3 of 5. Click for details and evidence.↻ balances · moderate · 0.3 Leaching depleteImpact -3 of 5. Click for details and evidence.Impact +2 of 5. Click for details and evidence.Nitrate is not adsorbed by net-negative soil surfaces and moves with drainage water, taking base cations with it: uses NitrateNitrate is not adsorbed by net-negative soil surfaces and moves with drainage water, taking base cations with it: uses WaterNitrate is not adsorbed by net-negative soil surfaces and moves with drainage water, taking base cations with it: uses Base cationsMost leaching occurs in autumn and winter, when the crop is gone, mineralisation continues and drainage is highest: uses NitrateMost leaching occurs in autumn and winter, when the crop is gone, mineralisation continues and drainage is highest: uses WaterCover or catch crops take up residual nitrate; non-legume cover crops cut leaching by about 70 percent and legume covers by about 40 percent: uses up Nitrateuses upCover or catch crops take up residual nitrate; non-legume cover crops cut leaching by about 70 percent and legume covers by about 40 percent: uses up Nitrogenuses upNitrate reaches drains and groundwater; the US drinking-water limit is 10 mg nitrate-N per litre: uses NitrateNitrate reaches drains and groundwater; the US drinking-water limit is 10 mg nitrate-N per litre: uses WaterRiparian buffers remove about two thirds of nitrate on average (large range) by denitrification and uptake: uses up Nitrateuses upRiparian buffers remove about two thirds of nitrate on average (large range) by denitrification and uptake: makes Nitrous oxidemakesNitrogen delivered by rivers fuels algal blooms and the seasonal hypoxic zone in the Gulf of Mexico: uses up Nitrateuses upmoves unadsorbed withdrainageNitrate: moves unadsorbed with drainagetransports nitrate andcationsWater: transports nitrate and cationscarried away withnitrateBase cations: carried away with nitratepeaks when crop is goneNitrate: peaks when crop is gonedrainage highest inwinterWater: drainage highest in wintertaken up by cover cropsNitrate: taken up by cover cropsassimilated into coverbiomassNitrogen: assimilated into cover biomassreaches drains andgroundwaterNitrate: reaches drains and groundwatercarries nitrate intodrainsWater: carries nitrate into drainsremoved bydenitrification a…Nitrate: removed by denitrification and uptakeemitted asdenitrification proceedsNitrous oxide: emitted as denitrification proceedswashed into rivers andcoastsNitrate: washed into rivers and coastsStart: Nitrate leachingfrom the root zone togroundwater and coast…STARTStart: Nitrate leaching from the root zone to groundwater and coastal watersEnd: Nitrate leachingfrom the root zone togroundwater and coast…ENDEnd: Nitrate leaching from the root zone to groundwater and coastal watersNitrate is not adsorbedby net-negative soilsurfaces and moves wi…📄1Nitrate is not adsorbed by net-negative soil surfaces and moves with drainage water, taking base cations with itMost leaching occurs inautumn and winter, whenthe crop is gon…📄1Most leaching occurs in autumn and winter, when the crop is gone, mineralisation continues and drainage is highestCover or catch cropstake up residualnitrate; non-legu…📄1Cover or catch crops take up residual nitrate; non-legume cover crops cut leaching by about 70 percent and legume covers by about 40 percentNitrate reaches drainsand groundwater; the USdrinking-water limit …📄1Nitrate reaches drains and groundwater; the US drinking-water limit is 10 mg nitrate-N per litreRiparian buffers removeabout two thirds ofnitrate on avera…📄1Riparian buffers remove about two thirds of nitrate on average (large range) by denitrification and uptakeNitrogen delivered byrivers fuels algalblooms and the season…📄1Nitrogen delivered by rivers fuels algal blooms and the seasonal hypoxic zone in the Gulf of Mexico
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)
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Every step, with its evidence

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Nitrate is not adsorbed by net-negative soil surfaces and moves with drainage water, taking base cations with it
NitrateDrainage waterNitrateWaterBase cations
Evidence (1) · added by Fern
  • Addiscott, Whitmore & Powlson (1991), Farming, Fertilizers and the Nitrate Problem, CAB International: Nitrate behaves almost as a conservative solute in soil water, so drainage volume determines transport. (confidence 0.8)
Most leaching occurs in autumn and winter, when the crop is gone, mineralisation continues and drainage is highest
Rain and irrigationDrainage waterNitrateWater
Evidence (1) · added by Fern
  • Di & Cameron (2002), Nutrient Cycling in Agroecosystems 64:237-256: Leaching losses concentrate in the drainage season after crop uptake ends. (confidence 0.8)
Cover or catch crops take up residual nitrate; non-legume cover crops cut leaching by about 70 percent and legume covers by about 40 percent
Cover cropsPlant rootsNitrateNitrogen
Evidence (1) · added by Fern
  • Tonitto, David & Drinkwater (2006), Replacing bare fallows with cover crops in fertilizer-intensive cropping systems: a meta-analysis of crop yield and N dynamics, Agriculture, Ecosystems & Environment 112:58-72: Non-legume cover crops reduced N leaching by about 70 percent and legume covers by about 40 percent relative to fallow. (confidence 0.8)
Nitrate reaches drains and groundwater; the US drinking-water limit is 10 mg nitrate-N per litre
GroundwaterDrainage waterNitrateWater
Evidence (1) · added by Fern
  • US EPA, National Primary Drinking Water Regulations, 40 CFR 141.62 (maximum contaminant level for nitrate 10 mg per L as N): The maximum contaminant level for nitrate is 10 mg per L as nitrogen. (confidence 0.9)
Riparian buffers remove about two thirds of nitrate on average (large range) by denitrification and uptake
Riparian vegetationDenitrifying bacteriaNitrateNitrous oxideDinitrogen
Evidence (1) · added by Fern
  • Mayer, Reynolds, McCutchen & Canfield (2007), Meta-analysis of nitrogen removal in riparian buffers, Journal of Environmental Quality 36:1172-1180: Riparian buffers removed on average about 67 percent of nitrate in a large meta-analysis, with wide variation. (confidence 0.65)
Nitrogen delivered by rivers fuels algal blooms and the seasonal hypoxic zone in the Gulf of Mexico
RiversAlgaeMarine bacteriaNitrateOxygen
Evidence (1) · added by Fern
  • Rabalais, Turner & Wiseman (2002), Gulf of Mexico hypoxia, a.k.a. the dead zone, Annual Review of Ecology and Systematics 33:235-263: Nutrient loads from the Mississippi River drive stratified bottom-water hypoxia. (confidence 0.85)