Agronomic Perspective on Modeling Denitrification

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Agronomic Perspective on Modeling Denitrification

• R. W. Skaggs, M. Youssef, and J.W. Gilliam

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DRAINAGE WORLD-WIDE

• Cropland NeedingDrainage 170 Million Ha

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Avg. annual nitrogen yield
Mississippi River of streams for 1980-1996
Drainage Basin



Hypoxic Zone
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Locations of elevated nitrate levels in surface
water and the distribution of tile-drained soils
in Illinois
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Effect of Subsurface Drainage Intensity on N Loss
to Surface Waters
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q 4 Ke m (2de m)/ L2 DI q _at_ m D DI 4
Ke D (2de D)/ L2
m
D
d
L
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MODEL DESCRIPTION NITROGEN CYCLE
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MODEL DESCRIPTION MODES OF OPERATION

• DRAINMOD-N II operates in three modes with
  different levels of complexity
• Mode 1 Mineral-N NO3-N
• Mode 2 Mineral-N NO3-N NH4-N(a,s)
• Mode 3 Mineral-N NO3-N NH4-N(a,s)
  NH3-N(a,g,s)

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MODEL DESCRIPTION

• DRAINMOD-N II simulates N reactive transport
  using finite difference solution to a multi-phase
  form of the one-dimensional advection-dispersion-r
  eaction equation.
• It includes a fertilizer component capable of
  modeling the application of the most widely used
  N fertilizers including urea and anhydrous NH3.

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MODEL DESCRIPTION

• It allows the application of animal manure and
  plant residues to the soil-plant system.
• It simulates N mineralization/immobilization
  processes as a consequence of C cycling in the
  system.
• It simulates processes of Nitrification,
  denitrification, and urea hydrolysis using
  Michaelis-Menten kinetics
• It simulates temporal change in soil pH due to
  fertilizer application, nitrification, and N
  plant uptake.

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FIELD STUDY
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Observed and Predicted WTD (Field 2, 1994)
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Observed and Predicted WTD (Field 2, 1995)
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Observed and Predicted Cumulative Subsurface
Drainage and NO3-N Loss (Field 2, 1995)
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Observed and Predicted Cumulative 6-Year
Subsurface Drainage and NO3-N Loss (Plot 2,
1992-1997)
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CHARACTERISTICS OF AGRICULTURAL DRAINAGE WATER
TOTAL NITROGEN TRANSPORT AVERAGE OF 14 SOILS
STUDIED
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31.1
25
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Kg/Ha
17.3
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13.8
10
7.6
5
5.0
0
UNDEVELOPED
SUBSURFACE
SURFACE
NO CONTROL
CONTROLLED
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Eh with Depth for D75 cm
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Summary

• Experimental evidence indicates nitrate losses
  from agricultural drains depends strongly on
  drainage intensity
• The dependence is apparently due to the effect of
  drainage on denitrification
• Models have been developed to predict the
  processes
• Field measurements of denitrification are needed
  to rigorously test the models so that nitrogen
  losses to surface waters can be objectively
  considered in the design and management of
  drainage systems