Contribution of denitrification to nitrogen, carbon, and oxygen cycling in tidal creek sediments of a New England salt marsh

The contribution of denitrification to sediment metabolism was studied at 2 sites (muddy and sandy) in unvegetated tidal creek sediments from a small Cape Cod, USA, salt marsh receiving nitrate-enriched groundwater flows (32 mmol m -2 d -1 ). Simultaneous measurements of sediment N 2 , CO 2 , O 2 , and dissolved inorganic N fluxes were made over annual cycles. A total of 46% of the ammonium remineralized within the sediments was transformed to N 2 by coupled nitrificationdenitrification (D n ). Denitrifying and nitrifying bacteria contributed 15 and 18% to total sediment C and O cycling, respectively. C, N, and O 2 cycling rates were limited by both temperature and the availability of labile organic matter. Muddy sediment C content was twice that of sandy sediments, but was half as labile, resulting in similar mean metabolic rates between sediment types (mean muddy and sandy O 2 consumption rates were 62 and 58 mmol m -2 d -1 , respectively; CO 2 production was 58 and 46 mmol m -2 d -1 ; and D n was 5.4 and 4.9 mmol N m -2 d -1 ). Sediment 13 C (-18.5 and -20.8 ) and the molar CO 2 :N flux ratio (6.1) at both sites are consistent with a sediment metabolism based on algal rather than macrophytic biomass, and groundwater nitrate was the dominant source of N supporting algal growth. Annually, D n accounted for 72% of total denitrification, with the remainder accounted for by water column-supported denitrification. Since all the denitrified N originated from groundwater nitrate, algal uptake must have initially out-competed denitrification for water column nitrate, but nearly half of this algal N was subsequently remineralized and denitrified.