Direct denitrification in mangrove sediments in Terminos Lagoon, Mexico
Rates of direct denitrification were measured using 15 N isotope tchniques in intact sediment cores from fringe and basin mangroves in Terminos Lagoon, Mexico. Sediments were injected with 15 NO 3 - and the distribution of 15 N 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 NO 3 - concentrations (25, 100, 200 umol core -1 15 N-KNO 3 - ) on denitrification rates and was performed in the fringe mangrove during the 'Norte' season (January 1992). Highest 15 N-N 2 fluxes were measured in the fringe mangrove at 9.4 umol m -2 h -1 , while denitrification rates in the basin mangrove ranged from 1.9 to 4.5 umol m -2 h -1 . 15 N-N 2 fluxes in sediment cores from the fringe mangrove were significantly higher (4.5 to 7.7 umol m -2 h -1 ) in cores enriched with 200 umol core -1 15 N-KNO 3 - compared to cores enriched with 25 and 100 umol core -1 15 N-KNO 3 - (<1 umol m -2 h -1 ). Most of the applied 15 N was recovered as particulate nitrogen in the sediment and a small fraction reduced to NH 4 + 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 NO 3 - availability. Previous nutrient exchange studies concluded that the fringe mangrove was a 'sink' of NO 3 - since sediment uptake of NO 3 - was assumed lost through denitrification. Results from this study show <10% of sediment NO 3 - uptake in fringe mangroves may be lost to denitrification; the remainder being immobilized in the sediment.