Until recently, it was believed that biological assimilation and gaseous nitrogen (N) loss through denitrification were the two major fates of nitrate entering or produced within most coastal ecosystems. Denitrification is often viewed as an important ecosystem service that removes reactive N from the ecosystem. However, there is a competing nitrate reduction process, dissimilatory nitrate reduction to ammonium (DNRA), that conserves N within the ecosystem. The recent application of nitrogen stable isotopes as tracers has generated growing evidence that DNRA is a major nitrogen pathway that cannot be ignored. Measurements comparing the importance of denitrification vs. DNRA in 55 coastal sites found that DNRA accounted for more than 30% of the nitrate reduction at 26 sites. DNRA was the dominant pathway at more than one-third of the sites. Understanding what controls the relative importance of denitrification and DNRA, and how the balance changes with increased nitrogen loading, is of critical importance for predicting eutrophication trajectories. Recent improvements in methods for assessing rates of DNRA have helped refine our understanding of the rates and controls of this process, but accurate measurements in vegetated sediment still remain a challenge

[1] The effects of salinity intrusion on the anaerobic microbial and geochemical dynamics of tidal freshwater sediments were investigated using flow-through sediment reactors. In freshwater control sediments, organic matter mineralization was dominated by methanogenesis (62%), followed by sulfate reduction (18%), denitrification (10%), and iron reduction (10%). Upon salinity intrusion, nutrient (ammonium, silicate, phosphate) concentrations increased and rates of methanogenesis declined. Iron-oxide bioavailability increased and microbial iron reduction appeared to account for >60% of organic matter oxidation for several days after salinity intrusion. However, sulfate reduction was the dominant pathway (>50%) of organic matter oxidation within 2 weeks of salinity intrusion, and accounted for >95% of total organic matter mineralization after 4 weeks. Total in situ sediment organic matter mineralization doubled following salinity intrusion. Increased nutrient release, decreased methanogenesis and a rapid shift to sulfate reduction, with a coincident increase overall organic matter mineralization, accompanied salinity intrusion into previously freshwater riverine sediments.

The impact of salt-water intrusion on microbial organic carbon (C) mineralization in tidal freshwater marsh (TFM) soils was investigated in a year-long laboratory experiment in which intact soils were exposed to a simulated tidal cycle of freshwater or dilute salt-water. Gas fluxes [carbon dioxide (CO 2 ) and methane (CH 4 )], rates of microbial processes (sulfate reduction and methanogenesis), and porewater and solid phase biogeochemistry were measured throughout the experiment. Flux rates of CO 2 and, surprisingly, CH 4 increased significantly following salt-water intrusion, and remained elevated relative to freshwater cores for 6 and 5 months, respectively. Following saltwater intrusion, rates of sulfate reduction increased significantly and remained higher than rates in the freshwater controls throughout the experiment. Rates of acetoclastic methanogenesis were higher than rates of hydrogenotrophic methanogenesis, but the rates did not differ by salinity treatment. Soil organic C content decreased significantly in soils experiencing salt-water intrusion. Estimates of total organic C mineralized in freshwater and salt-water amended soils over the 1-year experiment using gas flux measurements (18.2 and 24.9 mol C m -2 , respectively) were similar to estimates obtained from microbial rates (37.8 and 56.2 mol C m -2 , respectively), and to losses in soil organic C content (0 and 44.1 mol C m -2 , respectively). These findings indicate that salt-water intrusion stimulates microbial decomposition, accelerates the loss of organic C from TFM soils, and may put TFMs at risk of permanent inundation.

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Benthic respiration, sediment-water nutrient fluxes, denitrification and dissimilatory nitrate reduction to ammonium (DNRA) were measured in the upper section of the Parker River Estuary from 1993 to 2006. This site experiences large changes in salinity over both short and long time scales. Sediment respiration ranged from 6 to 52 mmol m −2 day −1 and was largely controlled by temperature. Nutrient fluxes were dominated by ammonium fluxes, which ranged from a small uptake of −0.3 to an efflux of over 8.2 mmol N m −2 day −1 . Ammonium fluxes were most highly correlated with salinity and laboratory experiments demonstrated that ammonium fluxes increased when salinity increased. The seasonal pattern of DNRA closely followed salinity. DNRA rates were extremely low in March, less than 0.1 mmol m −2 day −1 , but increased to 2.0 mmol m −2 day −1 in August. In contrast, denitrification rates were inversely related to salinity, ranging from 1 mmol m −2 day −1 during the spring and fall to less than 0.2 mmol m −2 day −1 in late summer. Salinity appears to exert a major control on the nitrogen cycle at this site, and partially decouples sediment ammonium fluxes from organic matter decomposition.