The Influence of Changes in Carbon and Nitrate Supply on Nitrogen Cycling in Marine Sediments.

Human activity has increased dissolved nitrogen (nitrate, nitrite, and ammonium) inputs to coastal and shelf waters. Because nitrogen is often a limiting nutrient in aquatic systems the resulting increase in primary production and subsequent deposition can lead to O₂ depletion at depth. Sediment microbes can ameliorate nutrient loading by converting bio-available nitrate (nitrogen that other organisms such as phytoplankton can utilize) to biologically unavailable nitrogen gas (N₂), which is subsequently lost to the atmosphere. However this process of denitrification is not the only nitrogen cycling process that occurs in sediments. Nitrate can also undergo dissimilatory nitrate reduction to ammonium (DNRA), and because ammonium remains bioavailable, nitrogen is retained in the system and can further contribute to eutrophication. An understanding of the factors regulating these processes is necessary to evaluate how coastal and shelf sediments will respond to continued nutrient loading under changing environmental conditions. Here we use a combination of numerical modeling and manipulated laboratory experiments to examine how the sediment nitrogen cycle responds changes in the coupled delivery of carbon and nitrogen to the sediment surface, and how this effects the return of nutrients to the overlying water. This work suggests that the ratio of carbon to nitrogen supply may control which nitrate reduction pathway dominates, with denitrification dominating when the ratio of carbon to nitrogen supply is low, and DNRA dominating when the ratio of carbon to nitrogen availability is high. These results suggests that DNRA may a play larger role in coastal and shelf sediments then previously thought and may limit the ability of sediments to act as a sink for bioavailable nitrogen.