THE RELATIVE ROLE OF DENITRIFICATION AND IMMOBILIZATION IN THE FATE OF INORGANIC NITROGEN IN MANGROVE SEDIMENTS (TERMINOS LAGOON, MEXICO)

Rates of direct and coupled nitrification-denitrification were measure d with N-15 isotope techniques in intact sediment cores from fringe, b asin, and riverine mangroves in Terminos Lagoon, Mexico. We used intac t sediment cores to evaluate differences in denitrification rates. The lack of N-15(2) production in all cores amended with 200 (or 100) mu mol core(-1 15)NH(4)(+) and the high recovery of N-15 in the sediment in five of eight such experiments indicate that coupled nitrification- denitrification was not important to nitrogen transformation during th e 8-10-d incubations. However, when we added 450 mu mol core(-1 15)NH( 4)(+) and (NO3-)-N-15 to cores from the riverine mangrove, N-2 product ion rates ranged from 28.9 to 221.1 mu mol N m(-2) h(-1) for each nutr ient. We compare coupled nitrification-denitrification rates among dif ferent types of mangrove forests under natural conditions in the neotr opics. Our results with nitrogen isotopes suggest that the uptake of i norganic N from tidal waters within mangrove ecosystems does not neces sarily represent a nitrogen sink via denitrification, but rather a ret ention of nitrogen in mangrove sediments.