An isotope dilution method to measure nitrification rates in the northern Gulf of Mexico and other eutrophic waters

  • Carini S
  • McCarthy M
  • Gardner W
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An isotope dilution method is described to determine nitrification rates in the northern Gulf of Mexico (NGOMEX) and other eutrophic waters using high performance liquid chromatography (HPLC). A N15H4+ isotope tracer method was not successful for NGOMEX waters due to excessive levels of unlabeled nitrogen (N) captured on sample filters. Nitrification rates were estimated by measuring the dilution of added N15O3- by N14O2-+N14O3-(=14NOX) generated from in situ N14H4+ during sample incubations. NOXin pre- and post-incubation samples was reduced to NH4+with zinc (Zn) dust following sample acidification. Samples were analyzed by HPLC for NH4+ concentration and atom%15N composition. Results were corrected for background natural abundance NH4+ concentrations measured in sub-samples collected before the reduction step. Nitrification rates were calculated using the difference between initial and final corrected NOXisotopic ratios and concentrations before and after the incubations. Laboratory experiments demonstrated that Zn reduction of NOXto NH4+ averaged 102% (±0.2-0.8%) within 30min. Different levels of N14O3-were added to sub-samples of coastal sea water containing a pool of added N15O3- to simulate a range of nitrification rates. Analysis of the resulting solutions produced a linear relationship (R2=0.9895; p

Author-supplied keywords

  • 15Nitrate
  • Eutrophication
  • HPLC
  • Isotope dilution
  • Isotope tracer
  • Nitrification

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  • Stephen A. Carini

  • Mark J. McCarthy

  • Wayne S. Gardner

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