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I5N tracer experiments were conducted to examine the fate of particulate organic nitrogen at the surface of an intact silty sediment community from Narragansett Bay, R.I. (USA). I5N-labelled particulate organic matter (POM, 82.5 atom % I5N excess), obtained from cultured marine diatoms (Skeletonema costaturn), was applied to the surface of 10 to 12 cm deep sediment cores and the time course distribution of the tracer was determined in inorganic-N and organic-N compartments in sediment and free water. Tracer experiments were conducted in spring (8°C) and fall (16°C). Small amounts of tracer-N were recovered in all sediment and free water compartments after 1.5 d in spring and after 6 h in fall. The initial rates of transport of the tracer downward into the sediment, based on the depth distribution of I5N in cores incubated for less than 2 d, appeared to be anomalously high. Subsequent downward mixing of the tracer in particulate and dissolved forms gave estimates of the sediment vertical mixing coefficient (Dm) of 3 to 5 X 10-6cm2s-2. Net release of NH4+ from the cores was suppressed for about 24 h following application of labelled-POM to the sediment surface. This was probably caused by immobilization of nitrogen in a rapidly growing microbial population at the sediment surface. Subsequently, the net rates of 15NH4+ production in the cores averaged 13 (s.d., 5) μmol m-2 h-1 in spring and 32 (s.d., 12) μmol m-2 h-1 in fall. The observed rates of NH4+ release suggest that 10 to 50 % of the NH4+ flux from the sediment was due to rapid nitrogen remineralization at the sediment-water interface. 15NH4+ produced near the sediment-water interface was partitioned between sediment pore waters and exchange sites on sediment solids in ratios (by atoms) of less than 1/1. Rate constants (% h-1, base e) for the decomposition of the labelled organic-N were 0.075 (s.d., 0.030) in spring and 0.14 (s.d.. 0.05) in fall. These rates suggest that the 'half-life' of organic nitrogen at the surface of coastal marine sediments is in the order of 1 to 2 mo in spring and of 2 to 3 wk in fall.