Jason R. Vogel 1, Glenn O. Brown 1,

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Nitrate Flux Under Long-Term Wheat Fields

• Jason R. Vogel 1, Glenn O. Brown 1,
• John S. Tyner 1, Jurgen Garbrecht 2
• 1 Oklahoma State University, Stillwater, OK, USA
• 2 USDA-ARS Grazinglands Research Station, El
  Reno, OK, USA
• 1999 American Geophysical Union Summer Meeting
• Boston, MA, June 2, 1999

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Objective

• Test Rauns Soil Plant Buffer theory, which
  holds that any nitrate applied up to the point of
  dryland wheat maximum yield, will not leach to
  ground water.

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Justification

• 5.2 million acres in Oklahoma in wheat
  production.
• 60 of the total harvested acreage and 12 of the
  total state land area.
• Large amounts of nitrogen are used as fertilizer
  in wheat production.

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Soil-Plant Buffer

• Proposed by Dr. William Raun
• Root zone provides a buffer for N
• Not a process or property per se
• Implies that producers can safely apply rates
  necessary to insure maximum yields.
• Controversial theory which goes against some
  conventional wisdom and other published research.

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Long-Term Plots

• Operated by the OSU Soil and Plant Sciences
  Department.
• Dryland winter wheat planted continuously for 28
  years with constant rates of 0 to 134 kg/ha NO3-N
  and uniform 45 kg/ha K2O (as KCl).
• Plot dimensions 20 x 60.
• Have had time to equilibrate to their respective
  management.

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Long-Term Plots

• Treatment
• 1 0 kg/ha
• 2 45 kg/ha
• 3 90 kg/ha
• 4 134 kg/ha
• Max Yield _at_
• 73 kg/ha

1
4
2
3
2
3
1
4
REP I
REP II
3
4
1
2
3
1
2
4
REP III
REP IV
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Sampling

• 67 mm (2-5/8 in) cores collected to approximately
  2.5 m depth.
• Sectioned and sealed in field.

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Analysis

• Analyzed in the lab for
• water content,
• NO3-N,
• chloride,
• particle size, and
• bulk density.

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Particle Size
10
Dry Bulk Density
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Moisture Content
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Moisture Content
Treatment M.C. t test 1 (0 kg/ha) 27.0
X 2 (45 kg/ha) 26.9 X 3 (90 kg/ha) 29.4
4 (134 kg/ha) 28.4
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Nitrate-N Concentration
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Nitrate-N Concentration
Treatment Nitrate-N t test 1 (0 kg/ha) 4.4
mg/L X 2 (45 kg/ha) 4.4 mg/L X 3 (90
kg/ha) 8.6 mg/L 4 (134 kg/ha) 32.2 mg/L
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Water Flux by Cl- Balance
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Water Flux
Treatment Flux t test 1 (0 kg/ha) 1.6 cm/yr
X 2 (45 kg/ha) 2.1 cm/yr 3 (90 kg/ha) 1.5
cm/yr X 4 (134 kg/ha) 4.0 cm/yr
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Nitrate Mass Flux
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Nitrate Mass Flux(mg/cm2/yr)
Treatment Flux t test 1 (0 kg/ha) 0.008
X 2 (45 kg/ha) 0.010 X 3 (90
kg/ha) 0.012 X 4 (134 kg/ha) 0.121
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Summary

• Concentration measurements indicate
• equivalent concentrations of nitrate-N under 0
  and 45 kg/ha NO3-N plots
• increased nitrate-N concentrations under 90 kg/ha
  NO3-N plots, but still below drinking water
  standard
• greatly increased nitrate-N concentrations under
  134 kg/ha NO3-N plots

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Summary

• Water flux calculations indicate
• increased recharge rate under 134 kg/ha NO3-N
  plots
• Nitrate mass flux calculations indicate
• equivalent small mass transport under 0, 45, and
  90 kg/ha NO3-N plots
• significant mass transport below 134 kg/ha NO3-N
  plots (9 of applied nitrate)

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Conclusion

• Results support Rauns Soil-Plant Buffer theory.
  Nitrate application up to maximum grain yield on
  dryland wheat does not increase nitrate leaching
  to the ground water.

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