부산대학교 도서관

We examined nitrogen cycling over a 1-yr period in shallow, sandy sediments at two contrasting sites near a barrier island in the northeastern Gulf of Mexico, and we provide here the direct determination of dinitrogen gas ([N.sub.2]) production at ambient nitrate concentrations in permeable marine sediments. Nitrogen-stable isotope tracer techniques were used to quantify [N.sub.2]-production rates and pathways in sediment cores and slurries. To simulate pore-water advection, the dominant transport process in the upper layer of the permeable sand beds, intact sediment cores were perfused with aerated seawater. This perfusion increased denitrification rates up to 2-fold in Apalachicola Bay sands and up to 17-fold in Gulf of Mexico sublittoral sands, respectively, relative to static cores. Seasonal [N.sub.2]-production rates were highest in spring and fall. Denitrified nitrate originated almost entirely from benthic nitrification at the exposed Gulf site, whereas water-column nitrate dominated sedimentary denitrification at the sheltered Bay site. Sediment incubations in stirred chambers were used to determine net fluxes of oxygen ([O.sub.2]), [N.sub.2], nitrate, and ammonium across the sediment water interface during varied degrees of continuous pore-water exchange. Rates of [N.sub.2] efflux correlated with rates of pore-water flow increasing from 0.12 mmol N [m.sup.-2] [d.sup.-1] under diffusion-limited transport conditions up to 0.87 mmol N [m.sup.-2] [d.sup.-1] with pore-water advection. Mineralized nitrogen was completely converted to N2 gas in Gulf of Mexico sediments. Our results demonstrate that advective pore-water circulation will accelerate benthic [N.sub.2] production by coupled nitrification--denitrification and that substantial nitrogen loss occurs from coastal permeable sediments.