Numerical Evaluation of Nitrate Distributions in the Onion Root Zone under Conventional Furrow Fertigation

HYDRUS (2D/3D) model was used to simulate spatial and temporal distributions of nitrate-nitrogen (${\mathrm{NO}}_3\text{-}\mathrm{N}$) within and below the onion root zone under conventional furrow fertigation with the urea-ammonium-nitrate (UAN) liquid fertilizer. The simulated water contents in the furrow irrigated onion field agreed well with the measurements. Simulations produced similar patterns of the measured ${\mathrm{NO}}_3\text{-}\mathrm{N}$ concentration profiles throughout the growing season. ${\mathrm{NO}}_3\text{-}\mathrm{N}$ concentrations remained higher and accumulation of ${\mathrm{NO}}_3\text{-}\mathrm{N}$ was observed within the root zone. Higher ${\mathrm{NO}}_3\text{-}\mathrm{N}$ within the root zone was dependent on the rate of the UAN fertilizer application, quantity of ${\mathrm{NO}}_3\text{-}\mathrm{N}$ removed by root uptake, and ${\mathrm{NO}}_3\text{-}\mathrm{N}$ drainage fluxes below the root zone. Simulations also suggested that ${\mathrm{NO}}_3\text{-}\mathrm{N}$ below the root zone during different growth stages remained much higher than a recommended (for drinking water) standard concentration level ($10\mathrm{mg}{\mathrm{L}}^{-1}$). This resulted in higher ${\mathrm{NO}}_3\text{-}\mathrm{N}$ drainage fluxes, particularly during the fertigation events between the establishment and vegetative growth stages. This indicates the need to apply most fertigation events at an early stage of bulb formation to provide the maximum ${\mathrm{NO}}_3\text{-}\mathrm{N}$ demands by onions and to reduce potential ${\mathrm{NO}}_3\text{-}\mathrm{N}$ leaching.