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Sediment denitrification rates seem to be lower in tropical environments than in temperate environments. Using the isotope pairing technique, we measured actual denitrification rates in the sediment of tropical coastal lagoons. To explain the low denitrification rates observed at all study sites (<5 μmol N 2  m -2  h -1 ), we also evaluated potential oxygen (O 2 ) consumption, potential nitrification, potential denitrification, potential anammox, and estimated dissimilatory nitrate (NO 3  - ) reduction to ammonium (NH 4  + ; DNRA) in the sediment.  15 NO 3  -  and  15 NH 4  +  conversion was measured in oxic and anoxic slurries from the sediment surface. Sediment potential O 2  consumption was used as a proxy for overall mineralization activity. Actual denitrification rates and different potential nitrogen (N) oxidation and reduction processes were significantly correlated with potential O 2  consumption. The contribution of potential nitrification to total O 2  consumption decreased from contributing 9% at sites with the lowest sediment mineralization rates to less than 0.1% at sites with the highest rates. NO 3  -  reduction switched completely from potential denitrification to estimated DNRA. Ammonium oxidation and nitrite (NO 2  - ) reduction by potential anammox contributed up to 3% in sediments with the lowest sediment mineralization rates. The majority of these patterns could be explained by variations in the microbial environments from stable and largely oxic conditions at low sediment mineralization sites to more variable conditions and the prevalences of anaerobic microorganisms at high sediment mineralization sites. Furthermore, the presence of algal and microbial mats on the sediment had a significant effect on all studied processes. We propose a theoretical model based on low and high sediment mineralization rates to explain the growth, activity, and distribution of microorganisms carrying out denitrification and DNRA in sediments that can explain the dominance or coexistence of DNRA and denitrification processes. The results presented here show that the potential activity of anaerobic nitrate-reducing organisms is not dependent on the availability of environmental NO 3  - .

Controls of Sediment Nitrogen Dynamics in Tropical Coastal Lagoons