The in uence of burrow spacing on sedimentary nitri cation and denitri cation was simulated experimentally using sediment plugs of different thicknesses immersed in aerated seawater reser- voirs. Different plug thicknessesmimic different distances between oxygenated burrow centers and produce similar changes in aerobic–anaerobic reaction balances as a function of diffusive transport scaling. The thicknessesused were roughly equivalent to transport scales (interburrowspacing) that could be producedby burrow abundancesof;400 to 50,000m22 , dependingon burrowlumen radii (e.g., 0.05–1 cm). Following the exposure of anoxic sediment plugs to aerated water, an ef cient aerobic nitri cation zone was established within the rst ;2–3 millimeters of sediment. At pseudo-steady state, the thinnest plug (2 mm) simulating highest burrow density, was entirely oxic and the denitrication rate nil. Denitri cation was stimulated in anoxic regions of the thicker plugs (5, 10, and 20 mm) compared to the initial value in experimental sediment. Maximum nitri cation rates and the highest denitrication/nitri cation ratio between oxic nitri cation and adjacent denitri cation zones occurred for the intermediate plug thickness of 5 mm. Of the oxic/anoxic composites, the thickest plug showed the least ef cient coupling between nitri cation/denitri cation zones (lowest denitri cation/nitri cation ratio). Both the thickness of the oxic layer and the total net remineralization of dissolved inorganicN varied inversely with plug thickness. A set of diffusion– reaction models was formulated assuming a range of possible nitri cation kinetic functions. All model forms predicted optimal nitri cation–denitri cation and ammoni cation–denitri cation cou- pling with relative oxic–anoxic zonation scales comparable to intermediate plug thicknesses (5–6 mm). However, none of the commonly assumed kinetic forms for nitri cation could produce the observedNO3 less sensitive to O2 2 pro les in detail, implying that natural sediment populations of nitri ers may be than laboratory strains. Our experimental and model results clearly show that rates of N remineralization and the balance between stimulation/inhibition of denitri cation are highly dependenton sedimentary biogenic structure and the particulargeometriesof irrigatedburrow distributions.
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