Modeling nitrogen cycling in a coastal fresh water sediment

Abstract

Increased nitrogen (N) loading to coastal marine and freshwater systems is occurring worldwide as a result of human activities. Diagenetic processes in sediments can change the N availability in these systems, by supporting removal through denitrification and burial of organic N (Norg) or by enhancing N recycling. In this study, we use a reactive transport model (RTM) to examine N transformations in a coastal fresh water sediment and quantify N removal rates. We also assess the response of the sediment N cycle to environmental changes that may result from increased salinity which is planned to occur at the site as a result of an estuarine restoration project. Field results show that much of the Norg deposited on the sediment is currently remineralized to ammonium. A rapid removal of nitrate is observed in the sediment pore water, with the resulting nitrate reduction rate estimated to be 130 lmol N cm–2 yr–1. A model sensitivity study was conducted altering the distribution of nitrate reduction between dissimilatory nitrate reduction to ammonium (DNRA) and denitrification. These results show a 40% decline in sediment N removal as NO3 – reduction shifts from denitrification to DNRA. This decreased N removal leads to a shift in sediment-water exchange flux of dissolved inorganic nitrogen (DIN) from near zero with denitrification to 133 lmol N cm–2 yr–1 if DNRA is the dominant pathway. The response to salinization includes a shortterm release of adsorbed ammonium. Additional changes expected to result from the estuarine restoration include: lower NO3 – concentrations and greater SO4 2– concentrations in the bottom water, decreased nitrification rates, and increased sediment mixing. The effect of these changes on net DIN flux and N removal vary based on the distribution of DNRA versus denitrification, illustrating the need for a better understanding of factors controlling this competition.