Assessment of root zone nitrogen leaching as affected by irrigation and nutrient management practices

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.