et al., 1995), center-pivot fertigation (Spalding et al., 2001), groundwater table control (Drury et al., 1997), and re-Multiple or split N fertilizer applications during a single cropping duced till or no-till practices (Power et al., 2001). Whatperiod is a recommended practice for controlling N (specifically ever management practices may be adopted, it is impor-NO 3-N) leaching into groundwater. Here, we demonstrate the benefit of split fertilizer applications in two typical upland soils of Japan tant to consider the simultaneous transport of water and (sand and Andosol) using a combination of a laboratory experiment N to evaluate the potential for NO 3 groundwater poland modeling. Soil water flow and N transport properties of the soils lution. were measured using standard procedures. Transient N and water Numerous models have been proposed for modeling transport experiments were conducted in cylindrical soil columns unthe transport of N in soils. The conceptual N model by der single (lumped) and split NH 4-N applications. The column experi-Tanji et al. (1977) is very simple with few input data ments were successfully simulated using Richards' equation and an and is based on the principles of mass balance and steady advection-dispersion model with equilibrium nonlinear sorption constate. The extended conceptual model proposed by Tanji ditions and first-order transformation for N speciation. Using the et al. (1979) is also simple and is applicable to transient model for the two soils, several irrigation and fertilizer management conditions. The Nitrate Leaching and Economic Analyscenarios were then simulated based on 1992 through 2000 meteorological data to investigate the long-term effects of lumped and split sis Package (NLEAP) model combined with GIS is used fertilization schedules for a representative set of crop and irrigation to identify potential NO 3 hot spots in shallow alluvial conditions. In comparison with lumped applications, split fertilizer aquifers under irrigated agricultural areas (Shaffer et al., applications were found to consistently reduce the amount of N leach-1995, 1996; Follett, 1995). The SOILN model simulates ing, even though year-to-year differences of N leaching reductions daily N and C fluxes in agricultural systems, including between sand and Andosol were significant. For unstressed crops, the plant growth and N uptake (Jabro et al., 2001). actual reduction in N leaching are shown to depend on the timing of Nitrate leaching is considered to occur mainly during precipitation and irrigation events, on soil type, and on plant N uptake high precipitation or during irrigation; hence, transient behavior. However, across all scenarios, two split applications instead dynamic models of water and N transport and N transof a single, lumped application reduced the N leaching fraction by formation are more adequate to evaluate the risk of approximately one-third. In the sandy soil, a three-way split resulted in further leaching reductions compared with the two-way split. Six-NO 3 leaching into ...