The presence or absence of air in organic-rich peatland sediments is of profound importance in regulating sediment metabolism. It is not known how much, if any, air entry occurs in salt marsh sediments. We used pressure-volume techniques, dye tracers, and oxygen microelectrodes to determine the amount and distribution of air that enters sediments of Belle Isle Marsh in Boston, Massachusetts, U.S.A., under varying porewater pressures. The porewater pressure threshold for air entry averaged −18 cm (±5.4 cm, n = 5) of water at the marsh surface. Air entry increased approximately linearly with decreasing porewater pressure below this threshold and amounted to about 0.1% of sediment volume per cm H2O of pore-water pressure for cores averaging 10 cm in depth. As near-surface porewater pressure drops to −30 cm of water or lower during neap tides, air entry must periodically occur in Belle Isle Marsh. Field O2 electrode measurements at porewater pressures averaging-26 cm of water also suggested about 1% of air-filled voids existed in the sediments, distributed in occasional “pockets” of millimeter dimensions. Hydraulic conductivity and dye tracer distributions could be explained by macropores in the effective diameter range of 60 to 200 μm. Many of these macropores seem to be associated with plant roots.
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