Transformation and transport of inorganic nitrogen in sediments of a southeast Asian mangrove forest

The most important inorganic nitrogen transformations (nitrification, denitrification and N fixation) and DIN fluxes in darkened and inundated sediments were studied d u n n g the dry season in the Ao Nam Bor mangrove forest at the island of Phuket, Thailand Dark fluxes of NH,' were low (not sigruficantly different from zero) and tended to be directed into the sediment in the area vegetated by the tree Rhizophora ap~culata (204 pm01 m d-') and out of the sediment in an adjacent unvegetated area (80 pm01 md l ) NO, appeared to be taken up by the sediment In both areas, although at rates not significantly diffeient from zero (134 and 85 pin01 m-' d-l respectively) The trend for higher uptake of DIN in the vegetated area may be related to microbial assimilation d u n n g degradation of nitrogen-poor tree litter Nitr~fication rates, estimated from potent~al assays uslng oxygen penetration depth or measured as coupled nitrification-denitrification using a 15N isotope-painng technique, were also very low (12 to 74 pm01 m del) Nitr~fication rates appeared higher and penetrated deeper in the vegetated than the unvegetated sedlments as substantiated by higher concentrations of porewater NO3-Den~tnficat~on rates were 3 5 times highei in the vegetated (46 pm01 m-' d-l) than the unvegetated sedlments (13 pm01 m d-') Since mole than 90% of the NO,-needed by denitr~fiers onginated from nltnficatlon (coupled nitrification-denitrification), only 1 to 2 % of the measured NO3-influx from the overlying water was consumed by denitnfication N flxation (284 to 390 pm01 m-2 d-') in the present mangrove sedirnents was estimated to account for about 10% of the net demand by pnmary producers About half of the measured N fixatlon was due to fixation by sulfate reducing bactena The Ao Nam Bor mangrove forest was charactenzed by low concentrations fluxes and rates of microbial transformations of DIN suggest~ng a tight coupling between mineralization and assimilation processes These nitrogen-poor sediinents acted as sinks for nitrogen, due to m~croblal ass~rnilaUon and the presence of trees in the vegetated sedlinent was evident as a 50% higher net retention of DIN KEY WORDS.