Microbial nitrogen transformations in unconsolidated coral reef sediments

Major nitrogen (N) pools and bacterial transformations of N were examined in carbonate sediments of 3 reefs in the central area of the Great Barrier Reef, Australia. Depth distributions of nitrate (NO3-) and ammonium (NH,+) and rates of NH4+ production, N2 fixation (nitrogenase activity by CzHz reduction) and denitrification were measured in muddy sediments of an inshore reef and in fine-. mediumand coarse-grained sediments at a midshelf and shelf edge reef. Ammonium efflux was estimated from pore water profiles. Estimates of potential rates of NH,' and NOsutilization were made in the upper 2 cm of sediments at the midshelf and shelf edge reefs. Highest concentrations of NH,' (up to 70 pM at 8 cm) were observed in muddy carbonate sediments of inshore Pandora Reef, with somewhat lower concentrations (up to 20 vM) in fine-grained sands of the other 2 sites. Relatively small NH4+ pools, usually less than 10 btM, typified coarse-grained sediments. Nitrate was generally undetectable in these sediments. Rates of NH,+ efflux among sites ranged from 0 to 4 pm01 N m-2 h-', with highest fluxes associated with muds and fine-grained sands. Ammonification rates in the upper 2 cm ranged from 6 to 26 pm01 N m-' h-' among sites, generally increasing with depth. Nitrogenase activity was detected in all sediments examined, with hlghest rates near the surface. N2 fixation could account for more than 50 % of NH,+ production in the upper 2 cm of sediment at 3 of 4 sites. The potential in the upper 2 cm for NH4+ consumption (nitrification and assimilation) ranged from 10 to 60 pm01 N m-' h-', while NO3reduction potenhal ranged from 10 to 80 pm01 N m-' h-' suggesting these may be quantitatively important pathways. Inhibitor experiments indicated that much of the NH,' uthzation might be by nitrification. Very high nitrification rates [up to 3.8 nmol N (g dry sed.)-' h-' or 70 pm01 N m-' h-'] were confirmed at 1 site by a "N isotope dilution method. Low denitrification rates were also detected in these environments, and in many cases under apparently oxic conditions. However, highest rates noted were less than 5 % of the rate of No3reduction. While shallow carbonate sands may be poor in organic material, they are active sites of bacterial N transformations. The NH,' and NOspools in the upper few cm appear to be highly dynamic, with estimated turnover times of substantially less than 1 d. It is also noteworthy that bacterial N:, fixation appears to account for a much larger fraction of NH4+ turnover than in shallow temperate zone sediments.