Is denitrification driven by elevation or plant type at a Gulf coast Juncus roemerianus and Spartina alterniflora mixed saltmarsh?

Wetlands provide a crucial ecosystem service by reducing anthropogenic nitrogen released from industrial and agricultural sources. Understanding the mechanisms controlling nitrogen removal in marshes is critical as human populations increase and as marsh areas decrease. Marshes in the U.S. Gulf coast are primarily populated by Spartina alterniflora or Juncus roemerianus. Previous research has indicated that sulfide concentrations are lower in J roemerianus than in S alterniflora marshes. Higher sulfide concentrations could inhibit nitrogen removal by reducing nitrification-denitrification. However, it has yet to be determined if variability in sulfide concentration is a result of differences in elevation and inundation that impact redox conditions, or higher belowground biomass allocation by J roemerianus that ultimately results in more oxygen release to anoxic sediments. We, therefore, measured denitrification rates within an S alterniflora dominated marsh that is interspersed with J roemerianus to determine if variability in sulfide concentrations impact denitrification. We quantified denitrification with intact cores and sediment slurries and examined pore water geochemistry. J roemerianus sediment sulfide concentrations (3.6-419.4 μmol) were consistently lower than those measured in S alterniflora sediments (325.1-2246.6 μmol). NH₄ flux was higher in J roemerianus cores and both NH₄ and PO₄ were present in higher concentration in pore water. Denitrification measured with IPT was higher in J roemerianus (21.0-81.2 μmol N₂ m^-2 hr^-1) than in S alterniflora (14.7-52.1 μmol N₂ m^-2 hr^-1). Potential denitrification in J roemerianus (0.5-128.1 nmole N cm^-3 hr^-1) was generally higher than in S alterniflora (4.1-34.2 nmole N cm^-3 hr^-1). By affecting sediment redox conditions, plant type rather than elevation appears to drive patterns of denitrification in this marsh. The imapct of sea level rise on vegetation distribution and nitrogen removal capacity for these marshes will be discussed.