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 during the dry season in the Ao Nam Bor mangrove forest at the island of Phuket, Thailand. Dark fluxes of NH sub(4) super(+) 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 mu mol m super(-2) d super(-1)) and out of the sediment in an adjacent unvegetated area (80 mu mol m super(-2) d super(-1)). NO sub(3) super(-) appeared to be taken up by the sediment in both areas, although at rates not significantly different from zero (134 and 85 mu mol m super(-2) d super(-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 super(15)N isotope-pairing technique, were also very low (12 to 74 mu mol m super(-2) d super(-1)).