The influence of macrofaunal burrow spacing and diffusive scaling on sedimentary nitrification and denitrification: An experimental simulation and model approach

  • Gilbert F
  • Aller R
  • Hulth S
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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 denitrication 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 denitrication/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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  • Franck Gilbert

  • Robert C. Aller

  • Stefan Hulth

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