15N tracer study of the short-term fate of particulate organic nitrogen at the surface of coastal marine sediments

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Abstract
15N tracer experiments were conducted to examine the fate of particulate organic N at the surface of an intact silty sediment community from Narragansett Bay, Rhode Island (USA). 15N-labeled particulate organic matter (POM, 82.5 atom % 15N excess), obtained from cultured marine diatoms (Skeletonema costatum), was applied to the surface of 10-12 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.degree. C) and fall (16.degree. C). Small amounts of tracer-N were recovered in all sediment and free water compartments after 1.5 d [days] 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 15N in cores incubated for < 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 .times. 10-6 cm2 s-1. Net release of NH4+ from the cores was suppressed for .apprx. 24 h following application of labeled-POM to the sediment surface. This was probably caused by immobilization of N in a rapidly growing microbial population at the sediment surface. Subsequently, the net rates of 15NH4+ production in the cores averaged 13 (SD 5) .mu.mol m-2 h-1 in spring and 32 (SD 12) .mu.mol m-2 h-1 in the fall. The observed rates of NH4+ release suggest that 10-50% of the NH4+ flux from the sediment was due to rapid N 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 < 1/1. Rate constant (% h-1, base e) for the decomposition of the labeled organic-N were 0.075 (SD 0.030) in spring and 0.14 (SD 0.05) in fall. These rates suggest that the half-life of organic N at the surface of coastal marine sediments is in the order of 1-2 mo. in spring and of 2-3 wk in fall.