The effect of increased nitrate loading on nitrate reduction via denitrification and DNRA in salt marsh sediments

The effects of increased nitrogen loading on denitrification and dissimilatory nitrate reduction to ammonium (DNRA) in marsh sediments were studied in permanently submerged subtidal creek sediments and on the tidally inundated vegetated marsh platform in Plum Island Sound estuary, Massachusetts. DNRA and denitrification in surface sediments were measured at all sites using whole-core incubations and the isotope pairing technique, which allows distinction between denitrification of water column nitrate and coupled nitrification-denitrification. On the marsh platform, denitrification was also measured at depth in the rhizosphere, using a new approach that combined the push-pull method and the isotope pairing technique. In tidal creek sediments, fertilization increased denitrification of water column nitrate by approximately one order of magnitude, and coupled nitrification-denitrification threefold. Coupled nitrification-denitrification made a significant contribution to the total N-2 production in the unfertilized creek but was of minor importance in the fertilized creek due to increased rates of denitrification of water column nitrate. In the surface sediment of the marsh platform, fertilization increased denitrification of water column nitrate by an order of magnitude during inundation of the marsh platform, about 12% of the day. However, coupled nitrification-denitrification occurring at depth in the rhizosphere was the main denitrification pathway, accounting for more than 50% of the total N-2 production in the fertilized as well as in the reference marsh. DNRA was measured in the surface sediment only, where it was comparable in magnitude to denitrification in the fertilized as well as in the unfertilized marsh.