LEACHN simulations of nitrogen dynamics and water drainage in an Ultisol
1999
Johnson, A.D. | Cabrera, M.L. | McCracken, D.V. | Radcliffe, D.E.
Nitrate leached from soils can contaminate drinking water and pose a health risk at concentrations > 10 mg N L(-1). Computer models may be useful management tools for estimating NO(3) leaching, but they need to be calibrated and validated before use. The objective of this work was to calibrate and validate LEACHN to simulate soil NO(3), soil NH(4), water drainage, and NO(3) leaching in a Cecil sandy loam (fine, kaolinitic, thermic Typic Kanhapludults). The calibration was done by determining rate constants and parameters under laboratory conditions. The validation data was obtained from a two-year study with conventionally tilled corn (Zea mays L.) during summer and either a rye (Secale cereale L.) cover crop or fallow conditions during winter. Water drainage collected by tiles was automatically measured, subsampled, and analyzed for inorganic N concentrations. During the cold season, LEACHN underestimated soil NH(4) and NO(3) in at least half of the cases. During the warm season, the model correctly estimated soil NO(3) 75% of the time, but it overestimated soil NH(4) in an equal 75% of the cases. Also, LEACHN overestimated cumulative drainage and leached NO(3) at least 50% of the time during both cold- and warm-season periods. These results suggest that the soil hydraulic properties and N mineralization rate constants determined under laboratory conditions did not apply to field conditions. Also, results obtained by changing rate constants for N transformations indicate that LEACHN was not properly simulating N immobilization from fertilizer N, or nitrification under dry conditions.
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