Inorganic nitrogen transformations within permeable carbonate sands

• Quantifying inorganic nitrogen sinks in tropical permeable carbonate sands.
• Rapid and significant removal and storage of inorganic N in the permeable sediments.
• Labile organic material did increase DIN uptake and not DIN release.
• DNRA was active but ammonium produced was adsorbed on the sediments.
• Permeable carbonate sands are reservoirs of N under the influence of advective flow.

A combination of in-situ push pull tests and a flow through reactor trial were used to quantify the inorganic nitrogen sinks in the permeable carbonate sands of a tropical coral cay (Heron Island – Great Barrier Reef). Addition of dissolved inorganic nitrogen (DIN in the form of nitrate – NO3−, and ammonium – NH4+) directly into sediment porewater resulted in uptake of up to 97% and 60% of added DIN respectively. The initial push pull experiment qualitatively showed that dissimilatory nitrate reduction to ammonia (DNRA), denitrification and nitrification were all active in the sediments. A flow through reactor experiment provided a more detailed approach to quantify these processes and showed that both denitrification and DNRA occurred within the sands at rates of 7.3 and 5.5 µmol N cm−3 d−1, respectively. Unexpectedly the addition of labile organic material (fresh coral spawn) to the permeable sands did not result in the release of DIN from the reactors, on the contrary it resulted in the increased uptake of both NO3− and NH4+. This was most likely because of the stimulated N uptake associated with the addition of high C:N coral spawn material. The bulk of NH4+ produced via DNRA was found to be adsorbed to sediments within the reactor and was not released with the outlet water. A mass balance over the entire experimental period showed that more inorganic N was retained within the sediments than lost as gaseous products. Our results point to permeable carbonate sands acting as reservoirs of N under the influence of advective flow, even during sudden enrichment periods such as those following coral mass spawning. This implies that permeable carbonate sands may help to buffer coral reefs during periods of extreme oligotrophy.