Journal
ENVIRONMENTAL SCIENCE & TECHNOLOGY
Volume 48, Issue 16, Pages 9555-9562Publisher
AMER CHEMICAL SOC
DOI: 10.1021/es501261s
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Funding
- National Natural Science Foundations [41130525, 41322002, 41271114, 41071135]
- State Key Laboratory of Estuarine and Coastal Research [2010RCDW07]
- NSF [1240798]
- Program for New Century Excellent Talents in University (NCET)
- Marine Scientific Research Project for Public Interest [200905007]
- Direct For Biological Sciences
- Division Of Environmental Biology [1240798] Funding Source: National Science Foundation
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Nitrogen (N) pollution in aquatic ecosystems has attracted much attention over the past decades, but the dynamics of this bioreactive element are difficult to measure in aquatic oxygen-transition environments. Nitrogen-transformation experiments often require measurement of N-15-ammonium ((NH4+)-N-15) ratios in small-volume N-15-enriched samples. Published methods to determine N isotope ratios of dissolved ammonium require large samples and/or costly equipment and effort. We present a novel (OX/MIMS) method to determine N isotope ratios for (NH4+)-N-15 in experimental waters previously enriched with N-15 compounds. Dissolved reduced N-15 (dominated by (NH4+)-N-15) is oxidized with hypobromite iodine to nitrogen gas (N-29(2) and/or N-30(2)) and analyzed by membrane inlet mass spectrometry (MIMS) to quantify (NH4+)-N-15 concentrations. The N isotope ratios, obtained by comparing the (NH4+)-N-15 to total ammonium (via autoanalyzer) concentrations, are compared to the ratios of prepared standards. The OX/MIMS method requires only small sample volumes of water (ca. 12 mL) or sediment slurries and is rapid, convenient, accurate, and precise (R-2 = 0.9994, p < 0.0001) over a range of salinities and is N-15/N-14 ratios. It can provide data needed to quantify rates of ammonium regeneration, potential ammonium uptake, and dissimilatory nitrate reduction to ammonium (DNRA). Isotope ratio results agreed closely (R = 0.998, P = 0.001) with those determined independently by isotope ratio mass spectrometry for DNRA measurements or by ammonium isotope retention time shift liquid chromatography for water-column N-cycling experiments. Application of OX/MIMS should simplify experimental approaches and improve understanding of N-cycling rates and fate in a variety of freshwater and marine environments.
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