We have developed new methods for natural abundance isotope ratio measurement of several biologically available and typically dissolved forms of N that are common in the environment: the 15N/14N, 18O/16O, and 18O/17O/16O of nitrate, the 15N/14N of total dissolved N (i.e. dissolved organic N in waters lacking nitrate and ammonium), and the 15N/14N of ammonium. For paleoceanographic work, we developed new techniques for the 15N/14N of the microfossil-bound N of diatoms (made of opal), planktonic foraminifera (calcite), and scleractinian corals (aragonite). All of these approaches have as their cornerstone the “denitrifier” method for nitrate isotopic analysis, in which nitrate (NO3-) is converted to nitrous oxide (N2O) gas by a strain of denitrifying bacteria that lacks an active N2O reductase, followed by analysis of the product N2O with a stable isotope ratio mass spectrometer.
Coupling of the denitrifier method, which yields the 15N/14N and 18O/16O of nitrate (NO3-)(1,2), with other steps to yield the 15N/14N of dissolved organic N (DON, 3), microfossil-bound organic N (4), and dissolved ammonium (NH4+, 5). “MS” indicate mass spectrometric analysis.
References
We have adapted the ammonia diffusion method of nitrate extraction for natural-abundance level nitrogen isotopic measurement of oceanic nitrate. The method involves: (1) sample concentration (by boiling or evaporation); (2) conversion of nitrate to ammonia using Devarda s alloy; and (3) the gas-phase diffusion of ammonia onto an acidified glass…
We present a method for measuring 15N-NH4+ in marine, estuarine and fresh waters. The advantage of this method is that it is broadly applicable to all types of water and it allows measurements in samples with lower ammonium concentrations than has previously been possible. The procedure is a modification of the ammonia diffusion method and uses…
We report a new method for measurement of the isotopic composition of nitrate (NO3-) at the natural-abundance level in both seawater and freshwater. The method is based on the isotopic analysis of nitrous oxide (N2O) generated from nitrate by denitrifying bacteria that lack N2O-reductase activity. The isotopic composition of both nitrogen and…
We report a novel method for measurement of the oxygen isotopic composition (18O/16O) of nitrate (NO3-) from both seawater and freshwater. The denitrifier method, based on the isotope ratio analysis of nitrous oxide generated from sample nitrate by cultured denitrifying bacteria, has been described elsewhere for its use in nitrogen isotope…
To better constrain the dynamics of the dissolved organic nitrogen (DON) pool and the role of N2 fixation in the nitrogen cycle at the Bermuda Atlantic Time-series Study (BATS) site, we measured the 15N/14N ratio of total nongaseous nitrogen (TN) in the upper 250 m and of nitrate in the upper 1000 m of monthly water column profiles from June…
We describe a method to remove nitrite from seawater and freshwater samples to determine the nitrogen (N) and oxygen (O) isotopic composition of nitrate in aqueous samples (15N/14N and 18O/16O, respectively). This method is simple, inexpensive, and effective in the removal of nitrite without compromising the natural N and O isotopic composition…
Interaction with ozone transfers its anomalous (non-mass-dependent) 17O enrichment to atmospheric nitrogen oxides and nitrate. The 17O anomaly (Δ17O) in nitrate can be used to identify atmospheric nitrate inputs into terrestrial and aquatic environments as well as to study the role of ozone in the atmosphere s reactive nitrogen cycle. We report…
In environmental water samples that contain both nitrate (NO 3-) and nitrite (NO2-), isotopic analysis of nitrate alone by all currently available methods requires pretreatment to remove nitrite. Sulfamic acid addition, used previously for this purpose (Wu JP, Calvert SE, Wong CS. Deep-Sea Research Part I - Oceanographie Research Papers 1997;…