NUTRIENT-LOSS TRENDS FOR VEGETABLE AND CITRUS FIELDS IN WEST-CENTRAL FLORIDA .1. NITRATE

Vegetable and citrus production in west-central Florida is reportedly of considerable eutrophication hazard to local groundwater and surface-water bodies, including a 33 000-ha drinking-water supply reservoir (Lake Manatee) near the coast of the Gulf of Mexico. Nitrate-N levels were assessed for three vegetable-production seasons during 1990 and 1991 using a combination of multilevel samplers into the shallow (surficial) aquifer beneath selected vegetable fields and citrus groves, coupled with piezometric wells around each field's periphery to assess direction and rate of groundwater flow. The NO3-N concentrations beneath vegetable sites showed sizeable spikes (up to 130 mg L(-1) NO3-N), especially early in the season and at season's end, though almost exclusively at <1-m depth. These concentrations did not persist, however, with the overwhelming majority of samples from most vegetable sites evidencing NO3-N concentrations below 1 mg L(-1). At citrus sites from the same area, NO3-N concentrations in the surficial water table (located 2-4 m below the soil surface in this case) have evidenced high (20-40 mg NO3-N L(-1)) and persistent levels throughout the entire sampling period. This zone of NO3-N enrichment commonly extends 2 to 3 m into the surficial aquifer before NO3-N concentrations decline to <10 mg L(-1) once more. Rates of lateral movement off-site averaged 20 to 40 m yr(-1) for the vegetable sites and 200 to 400 m yr(-1) for the more undulating citrus sites. It is postulated that gaseous dentrification losses naturally remediate the periodic spikes in NO3-N concentrations beneath the vegetable sites because of the high water tables maintained at such locations, whereas denitrification In the deep sands beneath typical citrus sites proceeds much more slowly due to such limitations as a soluble-C energy source and/or a suitable microbial population. The results have implication with respect to shallow water table vs. deeply rooted crop-production systems from the same geographic area, wherever such pairings may occur.