Samples of sediments from Australian seagrass (Zostera capricorni Aschers.) beds were taken in June to August 1983 (forN experiments) and November 1982 to January 1983 (N experiments). The ammonium pool turned-over every 0.4 to 0.8 d, as determined with aN isotope-dilution technique. The ammonium pool in subtidal bare areas turned-over two to three times more slowly than in adjacent seagrass beds. Gross rates of in situ ammonium regeneration equalled those of utilization, and ranged from 0.04 to 0.35 μmol cm d, or from 50 to 490 mg N m d over the upper 10 cm of the sediment. The potential rate of glycine utilization, measured with a large excess of glycine added to anaerobic incubations, ranged from 0.21 to 0.39μmol cm d, but in situ rates were probably much lower. Between 35 and 65% of addedN-glycine was deaminated over 12 h, and the remainder was most likely assimilated by microbes. Evidence for the seagrasses taking up glycine was equivocal, owing to the rapid deamination of the amino acid and the likelihood that they assimilated the labelled ammonium produced from the glycine.