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ORIGINAL RESEARCH

Assessment of Root Zone Nitrogen Leaching as Affected by Irrigation and Nutrient Management Practices

^a Graduate School of Agriculture, Kyoto University, Kitashirakawa Oiwake-cho, Sakyo-ku, Kyoto, Japan, 606-8502
^b Dep. of Land, Air and Water Resources, University of California, Davis, CA, 95616
^c National Institute of Vegetable and Tea Science, Kanaya, Shizuoka, Japan, 428-8501
^d Division of Agriculture, Osaka Prefecture University, 1-1 Gakuen-cho, Sakai, Osaka, Japan, 599-8531
^* Corresponding author (ThHarter{at}ucdavis.edu)

Received 25 May 2004.

Multiple or split N fertilizer applications during a single cropping period is a recommended practice for controlling N (specifically NO₃–N) leaching into groundwater. Here, we demonstrate the benefit of split fertilizer applications in two typical upland soils of Japan (sand and Andosol) using a combination of a laboratory experiment and modeling. Soil water flow and N transport properties of the soils were measured using standard procedures. Transient N and water transport experiments were conducted in cylindrical soil columns under single (lumped) and split NH₄–N applications. The column experiments were successfully simulated using Richards' equation and an advection–dispersion model with equilibrium nonlinear sorption conditions and first-order transformation for N speciation. Using the model for the two soils, several irrigation and fertilizer management scenarios were then simulated based on 1992 through 2000 meteorological data to investigate the long-term effects of lumped and split fertilization schedules for a representative set of crop and irrigation conditions. In comparison with lumped applications, split fertilizer applications were found to consistently reduce the amount of N leaching, even though year-to-year differences of N leaching reductions between sand and Andosol were significant. For unstressed crops, the actual reduction in N leaching are shown to depend on the timing of precipitation and irrigation events, on soil type, and on plant N uptake behavior. However, across all scenarios, two split applications instead of a single, lumped application reduced the N leaching fraction by approximately one-third. In the sandy soil, a three-way split resulted in further leaching reductions compared with the two-way split. Six-way split applications did not result in further N leaching improvements in either sand or Andosol. After adjusting N application rates to account for reduced N use efficiency, N leaching rates for unstressed crops under lumped fertilization were found to be several times higher than under split applications.

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