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

Abstract

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 nitrificationdenitrification 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.

Impacts of long-term fertilization on salt marsh tidal creek benthic nutrient and N<sub>2 gas fluxes

“…Aziz & Nedwell 1986, Koch et al 1992). Additionally, our fertilized creek rates were within the reported range of other New England studies (Kaplan et al 1979, Koop-Jakobsen & Giblin 2010 as well as an Italian study (Eriksson et al 2003). However, our rates were a great deal lower than those reported in tidal creek sediments on the Georgia coast, USA (3517 µmol N 2 -N m −2 h −1…”

“…Salt marsh tidal creeks have often been found to have considerably higher rates of denitrification than any other portion of the vegetated marsh (e.g. Anderson et al 1997, Thomas & Christian 2001, Koop-Jakobsen & Giblin 2010. In fact, although these areas often comprise < 20% of total marsh area, they may be responsible for as much as 60% of total marsh denitrification (Kaplan et al 1979).…”

“…Therefore, these sediments are exposed to this excess N for much longer periods of time than the rest of the marsh. Consequently, there are increased opportunities for transformation of anthropogenic N within the tidal creek sediments (Teal & Howes 2000, Koop-Jakobsen & Giblin 2010. As a result, these environments have been found to be important contributors to overall salt marsh N cycling (Kaplan et al 1979, Abd.…”

“…By taking advantage of a unique site of long-term tidal water fertilization, we evaluated the summer response of salt marsh tidal creeks to increasing N loads. In addition, we compared these directly measured fluxes with isotope pairing rates measured in the same creeks by Koop-Jakobsen & Giblin (2010).…”

The Dynamics of Plant-Mediated Sediment Oxygenation in Spartina anglica Rhizospheres—a Planar Optode Study

“…In order to calculate the diffusive flux of oxygen from the roots, obtaining depth-specific measures of oxygen consumption rates is essential (Meysman et al 2010). The push-pull method was previously successfully applied for investigations in marsh rhizospheres (Addy et al 2002;Koop-Jakobsen and Giblin 2010) and here in combination with planar optode technology, the combined method was very useful for determining depth-specific sediment oxygen demand inside the rhizobox (Fig. 2) The images are overlays of transparent optode images on bw images of the roots.…”