Rates of direct denitrification were measured using N-15 isotope techniques in intact sediment cores from fringe and basin mangroves in Terminos Lagoon, Mexico. Sediments were injected with (NO3-)-N-15 and the distribution of N-15 was measured over time in the head space, overlying water, and sediments. Experiments included an investigation of spatial variation in denitrification rates by comparing results from the fringe and basin mangroves in the rainy season (July 1991)
a second experiment was to determine the effect of 3 different NO3- concentrations (25, 100, 200 mu mol core(-1) N-15-KNO3-) on denitrification rates and was performed in the fringe mangrove during the 'Norte' season (January 1992). Highest N-15-N-2 fluxes were measured in the fringe mangrove at 9.4 mu mol m(-2) h(-1), while denitrification rates in the basin mangrove ranged from 1.9 to 4.5 mu mol m(-2) h(-1). N-15-N-2 fluxes in sediment ment cores from the fringe mangrove were significantly higher (4.5 to 7.7 mu mol m(-2) h(-1)) in cores enriched with 200 mu mol core(-1) N-15-KNO3- compared to cores enriched with 25 and 100 mu mol core(-1) N-15-KNO3- (<1 mu mol m(-2) h(-1)). Most of the applied N-15 was recovered as particulate nitrogen in the sediment and a small fraction reduced to NH4+ in both experiments. The low denitrification rates observed in the fringe and basin mangroves indicate that the capacity for sediment denitrification is limited by low NO3- availability. Previous nutrient exchange studies concluded that the fringe mangrove was a 'sink' of NO3- since sediment uptake of NO3- was assumed lost through denitrification. Results from this study show <10 % of sediment NO3- uptake in fringe mangroves may he lost to denitrification
the remainder being immobilized in the sediment.
