CROP NITROGEN-UTILIZATION AND SOIL NITRATE LOSS IN A LETTUCE FIELD

Low N use efficiency and high nitrate (NO3-) pollution potentials are problems in intensive vegetable production systems. The purpose of this study was to quantify N utilization by lettuce (Lactuca sativa L. cv Salinas), and identify periods of NO3- loss in an on-farm study in the Salinas Valley in coastal California. During autumn and winter, surface moisture remained low, and NO3- concentrations increased, reflecting high net mineralizable N, as determined by anaerobic incubation, and nitrification potential, as determined by the chlorate inhibition method. At the onset of a large winter storm, tracer levels of (NO3-)-N-15 were injected in the top 5 mm of soil in 30 cm-deep cylinders. After two weeks, most of the N-15 was present as (NO3-)-N-15 at 10-30 cm depth. By difference, losses to denitrification accounted for approximately 25% of the surface-applied N-15. Leaching below 30 cm did not occur, since no N-15 enrichment of NO3--N was measured in anion-exchange resin membranes placed at the base of each cylinder. During the crop period, NO3- losses were most pronounced after irrigation events. Uptake of N by two crops of lettuce (above- and belowground material) was approximately equal to fertilizer inputs, yet simulation of N fates by the Erosion/Productivity Impact Calculator (EPIC) model indicated losses of 14.6 g-N m-2 by leaching and 2.5 g-N m-2 by denitrification during the 6-month crop period. The large NO3- losses can be attributed to accumulation of soil NO3- during winter that was leached or denitrified during the irrigated crop period.