Maintaining intensive agriculture overlying aquifers using the threshold nitrate root‐uptake phenomenon
Daniel Kurtzman, Beeri Kanner, Y. Levy, Ido Nitsan, Asher Bar‐Tal
Abstract
Abstract 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 ( C max), which the leachate concentration increases at higher rates with increasing root‐zone nitrate concentration. The value of C max 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, C max ranged between 20 and 45 mg L −1 of NO 3 –N derived from experiments and models. However, for lettuce ( Lactuca sativa L.), which was irrigated with a large leaching fraction, C max 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 NO 3 –N concentration to C max. 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 C max that is less dramatic than that of the summer‐irrigated crops in the container experiment and a smaller impact of C max in N‐transport models. The simulations show that, for summer‐irrigated crops, maintaining fertigation at C < C max 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 C max 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.