The role of denitrification in nitrogen removal and carbon mineralization in Mid-Atlantic Bight sediments

Abstract

Benthic chambers were used to measure in situ fluxes of dissolved gases (N 2, O 2, and N 2O) and inorganic nutrients (NO 3 −, NH 4 +, and PO 4 3−) in continental shelf sediments of the Mid-Atlantic Bight during spring through autumn (May–November). Denitrification was determined by the rate of increase in N 2 relative to Ar, measured using membrane-inlet mass spectrometry. Although sediments were a source of recycled (mineralized) inorganic nitrogen (primarily as NH 4 +) and phosphorus, they only supplied ≈1% of total nitrogen required by water column primary production. Overall, sediments were a net sink for total nitrogen due to denitrification. On average denitrification removed 1.7 mmol N m −2 d −1 and was related to sediment oxygen consumption (SOC), suggesting that nitrogen regenerated from organic matter in the sediment and subsequently nitrified was an important nitrate source for benthic denitrifying bacteria. Coupled nitrification–denitrification was estimated to support 91–100% of total denitrification in LEO-15 sediments. Denitrification in these sediments was an important pathway for carbon mineralization, on average accounting for 13% of total mineralization (2–62%). Denitrification was not related to the organic content of the sediment suggesting that quality of organic matter, rather than simply quantity, was important in controlling denitrification in continental shelf sediments. Denitrification, SOC, and nutrient fluxes showed no strong seasonal patterns. The pattern of N 2O flux was related to bottom water N 2O concentration. Nitrous oxide supersaturation during the summer resulted in flux into sediments, whereas near equilibrium N 2O concentrations during the spring and autumn resulted in flux out of sediments.