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SOIL & WATER CONSERVATION

LEACHN Simulations of Nitrogen Dynamics and Water Drainage in an Ultisol

^a Crop & Soil Sciences, Plant Sciences Bldg., Univ. of Georgia, Athens, GA 30602 USA
^b Crop & Soil Sciences, Georgia Station, Griffin, GA 30223 USA

mcabrera{at}arches.uga.edu

Received for publication September 27, 1997. 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 NO₃ 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 NO₃, soil NH₄, water drainage, and NO₃ 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 NH₄ and NO₃ in at least half of the cases. During the warm season, the model correctly estimated soil NO₃ 75% of the time, but it overestimated soil NH₄ in an equal 75% of the cases. Also, LEACHN overestimated cumulative drainage and leached NO₃ 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.

Abbreviations: CI, confidence interval • RMSE, root mean square error

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