Nitrogen Cycling in Soils

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Nitrogen Cycling in Soils
N2 fixation by plants
N2 (gas)
NOx (gas)
aerosol excess NO3- gt 0.5 ?m NH4 lt 0.5 ?m
aerosol NH4/NO3- (aq/s)
HNO3 (gas?liquid)
deposition
NH3, NOx
NOx (gas) ?15N?
N2O (gas) ?15N1-3
soil production
NH3 (gas)
sea-air flux
N2 (gas)
N2O NOx (gas)
dissolved NO3 (aq) ?15N?
dissolved NH4 (aq)
upwelled NO3 ?15N10-15
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Simplified View of Soil N Cycle
Nitrate, ammonium, org N
Nitrate, N20, N2
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N cycle within soil
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Model of Soil N
At steady state (inputsoutputs
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Geographical Distribution of Soil N

• Soil N linked to C (maybe other way round)
• they are still independent of each other
• N more effectively conserved during plant
  decomposition (C/N ratios decline with time)
• Soil N patterns follow global soil C patterns
• Inputs increase with precipitation (temp?)
• Losses increase with temperature, deficiency of
  other nutrients

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Soil N vs. Soil Age Input and decomposition for
San Joaquin Valley annual grasslands
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• illustrates trend that with increasing C content
  (due to both decreasing temp and increasing
  precip), the C/N ratio of the SOM increases.
• Reflects C/N values closer to plant
• Reflects lower degrees of decomposition

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Climate Controls Total N Amounts and C/N Ratios
How does it affect form of N losses

• N isotope composition of soil N reflects the form
  of N lost from soils
• Nitrate, N2O, N2 (forms of N lost from plant
  available forms) enrich remaining soil N in 15N)
• Dissolved organic N or erosion of soil organic N
  do not affect N isotopes of soil N
• Globally, the 15N increases with increasing
  temperature and decreasing moisture, which
  implies that plant avaible forms of N are
  increasingly lost as climate becomes becomes hot
  and/or hot and dry.
• Hot dry climates are limited by water rather than
  N, so plant available N can leak out
• Hot/wet environments (Brazil, etc.) are commonly
  limited by other elements (such as P) so plant
  available N forms can also leave..

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How have humans altered the global N cycle?

• Natural N Cycle
• lightning lt10 Tg N/yr (Tg1012g)
• biological N fixation 90 to 140 Tg N/yr
• Altered N Cycle (INPUTS)
• N fertilizer 80 Tg N/yr
• Fossil fuel burning gt 20 Tg N/yr
• N fixing crops 40 Tg N/yr
• Altered N Cycle (OUTPUTS)
• Land clearing/cultivation new lands 20 Tg N/yr
• Drainage wetlands and oxidation 10 Tg N/yr
• Total oxidation of N from all ag soils in world
  (sum) 4000 to 5000 Tg N

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Humans have doubled the N inputs to Earth
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Human impact varies with N form
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Ecosystem Response to Increased N Inputs

• Increased ecosystem productivity (areas with N
  limitation)
• Increased C sequestration (up to 1.3 Gt C
  estimated)
• N saturation
• increased NO3 leaching from soils/rivers
• Changes in species composition (loss of
  biodiversity)
• Decline in productivity
• Loss of Ca and Mg
• Increase in Al
• NE US, Europe

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N in Rivers
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N Effects on Biodiversity Minnesota
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Characterists of N Saturated Ecosystems