2018 ASHS Annual Conference

Nitrate (NO₃) contamination of groundwater is a significant unresolved environmental issue worldwide. Any Nitrogen (N) fertilizers not taken up by shallow rooted vegetable crops with high demand for N, such as lettuce, can leach to the groundwater. Adoption of a preventative strategy to mitigate nitrate contamination of groundwater is critical for controlling the amount of nitrate below the root zone while optimizing crop yield. When combined with the appropriate fertilizers, surface drip irrigation can be a useful tool to help mitigate the nitrate leaching potential of a lettuce crop. The overall goal of our on-going research is to optimize nitrogen use efficiency (NUE) and water use efficiency (WUE) in vegetable production. In this phase, the objective was to assess the potential for nitrate leaching for a lettuce crop subjected to three irrigation regimes and four fertilizer N rates. Butterhead lettuce (Lactuca sativa cv Optima) was planted on a sandy loam soil on 150 cm beds with three rows per bed and intra-row spacing of 30 cm. The experimental design was a strip block with four replicates of three irrigation treatments (Manual, ET-based, and Soil Sensor-based) and four rates of N fertilizers (0, 67, 134, and 202 kg N /ha of CAN17). At harvest, both irrigation (P=0.02) and fertilizer (P=0.08) had a significant effect on above ground biomass. The plots irrigated manually and with the 100% ET-based technique yielded relatively higher biomass than the plots in which irrigation scheduling was based on soil moisture sensors. There was a positive fertilizer response, best described by the polynomial equation y = 1E-06x³ - 0.0003x² + 0.0303x + 1.658 (r² = 1), with an overall 46% increase in biomass with the addition of 202 kg N/ha. Both fertilizer and irrigation treatments had a significant effect on petiole nitrate levels, with the manually irrigated plants having the highest nitrate content followed by soil sensor based and ET-based scheduling, respectively. In the top 12 inches of soil, fertilizer rates had a significant (P=0.0002) effect on post-harvest nitrate levels, with values in excess of 40 ppm in the plots receiving 134 and 202 kg N/ha, and 17ppm in the 67 kg N/ plots, compared to less than 2ppm in the unfertilized plots. These preliminary results further validate the need to quantify biomass yield and nitrate concentrations in plant and soil, if an N- balance approach is to be adopted in an effort to mitigate nitrate leaching.

Keywords: Nitrate leaching, Irrigation scheduling, Butterhead lettuce, N Leaching potential.