Influence of porewater advection on denitrification in carbonate sands: evidence from repacked sediment column experiments

Porewater flow enhances mineralization rates in organic-poor permeable sands. Here, a series of sediment column experiments were undertaken to assess the potential effect of advective porewater transport on denitrification in permeable carbonate sands collected from Heron Island (Great Barrier Reef). Experimental conditions (flow path length, advection rate, and temperature) were manipulated to represent conditions similar to near shore tropical environments. HgCl₂-poisoned controls were used to assess whether reactions were microbially mediated. Overall, significant correlations were found between oxygen consumption and N₂ production. The N:O₂ slope of 0.114 implied that about 75% of all the nitrogen mineralized was denitrified. A 4-fold increase in sediment column length (from 10 to 40 cm) resulted in an overall increase in oxygen consumption (1.6-fold), TCO₂ production (1.8-fold), and denitrification (1.9-fold). Oxic respiration increased quickly until advection reached 80 L m⁻² h⁻¹ and then plateaued at higher advection rates. Interestingly, denitrification peaked (up to 336 μmol N₂ m⁻² h⁻¹) at intermediate advection rates (30–80 L m⁻² h⁻¹). We speculate that intermediate advection rates enhance the development of microniches (i.e., steep oxygen gradients) within porous carbonate sands, perhaps providing optimum conditions for denitrification. The denitrification peak fell within the broad range of advection rates (often on scales of 1–100 L m⁻² h⁻¹) typically found on continental shelves implying that carbonate sands may play a major, but as yet unquantified, role in oceanic nitrogen budgets.