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

Abstract

The most important inorganic nitrogen transformations (nitrification, denitrification and N fixation) and DIN fluxes in darkened and inundated sediments were studied during the dry season in the Ao Nam Bor mangrove forest at the island of Phuket, Thailand. Dark fluxes of NH4+ were low (not significantly different from zero) and tended to be directed into the sediment in the area vegetated by the tree Rhizophora apiculata (204 μmol m-2 d-1) and out of the sediment in an adjacent unvegetated area (80 μmol m-2 d-1), NO3- appeared to be taken up by the sediment in both areas, although at rates not significantly different from zero (134 and 85 μmol m-2 d-1, respectively). The trend for higher uptake of DIN in the vegetated area may be related to microbial assimilation during degradation of nitrogen-poor tree litter. Nitrification rates, estimated from potential assays using oxygen penetration depth or measured as coupled nitrification-denitrification using a 15N isotope-pairing technique, were also very low (12 to 74 μmol m-2 d-1). Nitrification rates appeared higher and penetrated deeper in the vegetated than the unvegetated sediments as substantiated by higher concentrations of porewater NO3-. Denitrification rates were 3.5 times higher in the vegetated (46 μmol m-2 d-1) than the unvegetated sediments (13 μmol m-2 d-1). Since more than 90% of the NO3- needed by denitrifiers originated from nitrification (coupled nitrification-denitrification), only 1 to 2% of the measured NO3- influx from the overlying water was consumed by denitrification. N fixation (284 to 390 μmol m-2 d-1) in the present mangrove sediments was estimated to account for about 10% of the net demand by primary producers. About half of the measured N fixation was due to fixation by sulfate reducing bacteria. The Ao Nam Bor mangrove forest was characterized by low concentrations, fluxes and rates of microbial transformations of DIN, suggesting a tight coupling between mineralization and assimilation processes. These nitrogen-poor sediments acted as sinks for nitrogen, due to microbial assimilation, and the presence of trees in the vegetated sediment was evident as a 50% higher net retention of DIN.