Maintaining intensive agriculture overlying aquifers using the threshold nitrate root-uptake phenomenon

Reducing nitrate leaching from agricultural land to aquifers has been a high priority concern for more than a half century. This study presents theory and observations of a threshold concentration of nitrate in the root zone (Cmax), which the leachate concentration increases at higher rates with increasing root-zone nitrate concentration. The value of Cmax is derived both by direct results from container experiments with varying nitrogen (N) fertigation and as a calibration parameter in N-transport models beneath commercial agricultural plots. For five different crops, Cmax ranged between 20 and 45 mg L^-1 of NO₃ -N derived from experiments and models. However, for lettuce (Lactuca sativa L.), which was irrigated with a large leaching fraction, Cmax could not be defined. In crops irrigated and fertilized in the warm/dry season (corn [Zea mays L.] and citrus), the experiments show a dramatic change in leachate-concentration slope, and simulations reveal a wide range of sensitivity of leachate NO₃ -N concentration to Cmax. In annual crops irrigated and fertilized in the cool/wet season (e.g., potato [Solanum tuberosum L.] in a Mediterranean climate), the experiments show a distinct Cmax that is less dramatic than that of the summer-irrigated crops in the container experiment and a smaller impact of Cmax in N-transport models. The simulations show that, for summer-irrigated crops, maintaining fertigation at C < Cmax has a significant effect reducing deep leachate concentrations, whereas for the winter annual crops the simulations revealed no threshold effect. It is suggested that for summer-irrigated crops, fertigation below Cmax robustly serves the co-sustainability of intensive agriculture and aquifer water quality; this is also suggested for winter crops, but the benefits are not robust. For short-season, small root-system crops (e.g., lettuce), efforts should be made to detach the crop from the soil.