Effects of fertilization on tidal creek and tidal flat nitrogen cycling
Vieillard, Amanda Marie
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Since the industrial revolution human activities have more than doubled the amount of bioavailable nitrogen (N) on earth leading to far-reaching ecological consequences for coastal marine ecosystems. Salt marsh systems, including their intertidal creek and mudflat sediments, serve as nutrient filters transforming nitrogen and removing it through denitrification. However, as hotspots of nitrogen transformation, these ecosystems are also thought to be sources of nitrous oxide, a powerful greenhouse gas, to the atmosphere. We investigated the influence of various scales of anthropogenic fertilization on the nitrogen cycling in intertidal creek and mudflat sediments in the salt marsh ecosystem of Plum Island Ecosystem Long Term Ecological Research site in northern Massachusetts, USA. Benthic fluxes from whole core incubations showed that long-term fertilization of tidal creek sediment stimulated net denitrification with significantly higher rates in the fertilized creek compared to the reference (162.7 ± 32 and 0.74 ± 39 μmol N m^-2 hr^-1, respectively). However, fertilization also appeared to stimulate dissimilatory nitrate reduction to ammonium (DNRA) with calculated rates also significantly higher in the fertilized compared to reference creek and representing 45 and 11% of total nitrate uptake, respectively. These results indicate that DNRA may outcompete denitrification at higher nitrate concentrations, thus anthropogenic fertilization may be driving tidal creek sediments toward this N regeneration process and thus inhibiting the overall nitrogen removal capacity of the ecosystem. Conversely, a smaller scale, short-term nitrogen addition experiment had no significant impact on nearby tidal flat sediments likely because the fertilization exposure time on the tidal sediments was too short. Overall benthic flux rates were lower in the tidal flat compared to the tidal creeks. However, the tidal flat was also a net N filter with an average net N2 flux of 5.7 ± 2.6 μmol N N m^-2 hr^-1. Rates of nitrification and therefore coupled nitrification-denitrification appeared to be affected by the active microphytobenthos (MPB) community within the tidal flat sediments with oxygen production from photosynthesis fueling coupled denitrification in the light while N fixation dominated under dark conditions. As in the tidal creeks, we found evidence that DNRA is also an important N transformation process within tidal flat sediments. Finally, sediment microprofiling measurements showed these tidal mudflat sediments to be a net sink of N2O (average -6.9 ± 1.7 μmol N2O N m^-2 hr^-1) with significantly higher rates of uptake the longer sediments were exposed to the atmosphere at low tide. Fluxes were shown to be driven by nutrient supply and nitrate limitation of denitrifiers with tidal pulsing. Additionally, smaller, core scale nutrient additions revealed an increase in N2O flux with dissolved inorganic nitrogen (DIN) addition. Importantly, N2O uptake was found to be phosphorus limited. Again, nutrient enrichment appeared to stimulate DNRA over denitrification indicating that fertilization may not only hamper nitrogen removal capacity, but also increase N2O flux to the atmosphere.