FIELD ESTIMATION OF NITRATE LOAD FROM VEGETABLE FIELDS IN THE CARIBBEAN BASIN

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The development of best management practices (BMP) and the alloca-tion of pollution among land users in a watershed (TMDL) requires an understanding of the effect of cultural practices on both yields and nutrient leaching below the root zone. `Florida 47' tomato and `La Estrella' tropical pumpkin were grown on plasticulture using combinations of UF–IFAS recommended N and irrigation rates in a 1-ha field in 2004. Average N and irrigation rates over the whole field were 100% and 80%, respectively. Nitrate movement was assessed with twenty-four 1-m-long drainage lysimeters in each plot and ten 7-m-deep wells in and around the field Lysimeters and wells were sampled every 2 and 3 weeks throughout the year, respectively. Leachate volume and concentration in the drainage lysimeters were highly variable. Except shortly after the 25-cm rain due to hurricane Jeanne, most leachate volumes were <1 L·m–1. Annual NO3-N mean treatment load ranged between 7 and 15 kg·ha–1, but these differences were not significant due to high variability (CV = 175%). Single-lysimeter annual highest load was 39 kg·ha–1 of NO3-N (17.5% of N applied). In 2004, NO3-N concentration in well water was <1, ranged between 15 and 35, ranged between 0 and 10 mg·L–1 NO3-N in the up-stream control, inside, and perimeter wells, respectively. These concentrations are in the same ranges as those observed in this field in previous years (1997–2003) and often exceeded the 10 mg/L drinking water standard. Because NO3-N discharge into the environment may occur after the growing season, BMPs should be implemented on a year round basis. The methodology used in load measurement should be improved to better account for spatial variability.