A re-evaluation of the magnitude and impacts of anthropogenic atmospheric nitrogen inputs on the ocean

We report a new synthesis of best estimates of the inputs of fixed nitrogen to the world ocean via atmospheric deposition, and compare this to fluvial inputs and di-nitrogen fixation. We evaluate the scale of human perturbation of these fluxes. Fluvial inputs dominate inputs to the continental shelf, and we estimate about 75% of this fluvial nitrogen escapes from the shelf to the open ocean. Biological di-nitrogen fixation is the main external source of nitrogen to the open ocean, i.e. beyond the continental shelf. Atmospheric deposition is the primary mechanism by which land based nitrogen inputs, and hence human perturbations of the nitrogen cycle, reach the open ocean. We estimate that anthropogenic inputs are currently leading to an increase in overall ocean carbon sequestration of ~0.4% (equivalent to an uptake of 0.15 Pg C yr-1 and less than the Duce et al., 2008 estimate). The resulting reduction in climate change forcing from this ocean CO2 uptake is offset to a small extent by an increase in ocean N2O emissions. We identify four important feedbacks in the ocean atmosphere nitrogen system that need to be better quantified to improve our understanding of the perturbation of ocean biogeochemistry by atmospheric nitrogen inputs. These feedbacks are recycling of (1) ammonia and (2) organic nitrogen from the ocean to the atmosphere and back, (3) the suppression of nitrogen fixation by increased nitrogen concentrations in surface waters from atmospheric deposition, and (4) increased loss of nitrogen from the ocean by denitrification due to increased productivity stimulated by atmospheric inputs

The contamination of commercial 15N2 gas stocks with 15N-labeled nitrate and ammonium and consequences for nitrogen fixation measurements.

We report on the contamination of commercial 15-nitrogen (15N) N2 gas stocks with 15N-enriched ammonium, nitrate and/or nitrite, and nitrous oxide. 15N2 gas is used to estimate N2 fixation rates from incubations of environmental samples by monitoring the incorporation of isotopically labeled 15N2 into organic matter. However, the microbial assimilation of bioavailable 15N-labeled N2 gas contaminants, nitrate, nitrite, and ammonium, is liable to lead to the inflation or false detection of N2 fixation rates. 15N2 gas procured from three major suppliers was analyzed for the presence of these 15N-contaminants. Substantial concentrations of 15N-contaminants were detected in four Sigma-Aldrich 15N2 lecture bottles from two discrete batch syntheses. Per mole of 15N2 gas, 34 to 1900 µmoles of 15N-ammonium, 1.8 to 420 µmoles of 15N-nitrate/nitrite, and ≥21 µmoles of 15N-nitrous oxide were detected. One 15N2 lecture bottle from Campro Scientific contained ≥11 µmoles of 15N-nitrous oxide per mole of 15N2 gas, and no detected 15N-nitrate/nitrite at the given experimental 15N2 tracer dilutions. Two Cambridge Isotopes lecture bottles from discrete batch syntheses contained ≥0.81 µmoles 15N-nitrous oxide per mole 15N2, and trace concentrations of 15N-ammonium and 15N-nitrate/nitrite. 15N2 gas equilibrated cultures of the green algae Dunaliella tertiolecta confirmed that the 15N-contaminants are assimilable. A finite-differencing model parameterized using oceanic field conditions typical of N2 fixation assays suggests that the degree of detected 15N-ammonium contamination could yield inferred N2 fixation rates ranging from undetectable, <0.01 nmoles N L(-1) d(-1), to 530 nmoles N L(-1) d(-1), contingent on experimental conditions. These rates are comparable to, or greater than, N2 fixation rates commonly detected in field assays. These results indicate that past reports of N2 fixation should be interpreted with caution, and demonstrate that the purity of commercial 15N2 gas must be ensured prior to use in future N2 fixation rate determinations

(<b>a</b>) δ¹⁵N_NO3+NO2 (log scale) of nitrate solutions (10–300 µmol L⁻¹) following equilibration with 0.1 mL ¹⁵N₂ gas from lecture bottles procured from three distributors.

<p>Solutions were 40 mL for Sigma-Aldrich and Campro Scientific equilibrations, and 100 mL for Cambridge Isotopes equilibrations. The solid line corresponds to the δ¹⁵N_NO3 of the control solutions for Sigma-Aldrich and Cambridge Isotopes experiments (δ¹⁵N_NO3 = 23.5±0.5‰); the dashed line corresponds to controls for Campro Scientific experiments (δ¹⁵N_NO3 = 14.15±0.1‰). Paired symbols identify replicate experimental treatments. (<b>b</b>) Corresponding apparent δ¹⁸O_NO3+NO2 of the experimental nitrate solutions. The solid line corresponds to the δ¹⁸O_NO3 of control solutions for the Sigma-Aldrich and Cambridge Isotope experiments (δ¹⁸O_NO3 = 18.9±0.3‰); the dashed line corresponds to controls for Campro Scientific experiments (25.4±0.3‰).</p

The quantity of ¹⁵N-labeled contaminants detected relative to ¹⁵N₂ additions.

<p>The µmoles of ¹⁵N contaminants (NO₃⁻+NO₂⁻, NH₄⁺, and N₂O) detected per mole of ¹⁵N₂ gas from lecture bottles provided by different suppliers. N/A = not available; n.d. = not detected.</p><p>*Not explicitly tested in high sensitivity ¹⁵N₂ dilutions.</p>†<p>Moles of ¹⁵N₂ estimated from the injection volume rather than direct measurements.</p>§<p>⁴⁶N2O measured directly.</p><p>The quantity of ¹⁵N-labeled contaminants detected relative to ¹⁵N₂ additions.</p

The δ¹⁵N of particulate nitrogen (δ¹⁵N_PN) of <i>D. tertiolecta</i> harvested in stationary phase following growth in media containing sodium nitrate (and no ammonium) and equilibrated with ¹⁵N₂ gas from Sigma Aldrich or Cambridge Isotopes.

<p>n = the number of experimental replicates.</p

Inferred N₂ fixation rates (nmoles N L⁻¹d⁻¹) resulting from ¹⁵N-labeled contaminants.

<p>N₂ fixation rates that would be inferred from 24-h field N₂ fixation assays conducted with ¹⁵N₂ stocks containing the respective concentrations ¹⁵N-ammonium contaminants detected in Sigma-Aldrich and Cambridge Isotopes ¹⁵N₂ gas. In the simulations, microbial plankton assimilate ¹⁵N-ammonium rather than fix ¹⁵N₂. Incubations are simulated in volumes of 0.25 L or 4.5 L equilibrated with 0.1 mL of ¹⁵N₂ gas, with 2.0×10⁻⁷ µmol L⁻¹ of plankton nitrogen (with a δ¹⁵N = 0‰) assimilating at a range of specific growth rates, µ (d⁻¹), countered by equivalent recycling rates, at incremental concentrations of ambient ammonium (δ¹⁵N_NH4 = −2‰). Inferred rates of <0.01 nmoles N L⁻¹d⁻¹ are considered undetectable (n.d.).</p><p>Inferred N₂ fixation rates (nmoles N L⁻¹d⁻¹) resulting from ¹⁵N-labeled contaminants.</p

(a) δ¹⁵N_NO3+NO2 (log scale) of higher sensitivity equilibrations of 10 µmol L⁻¹ nitrate solutions (10 mL) with 2 mL of ¹⁵N₂ gas from a Cambridge Isotopes or a Sigma-Aldrich bottle.

<p>(<b>b</b>) Corresponding apparent δ¹⁸O_NO3 (log scale) of higher sensitivity equilibrations of the two stocks. n = the number of experimental replicates.</p

(a) δ¹⁵N_NH4 (log scale) of 5 µmol L⁻¹ ammonium solutions after equilibration with 0.1 mL ¹⁵N₂ gas from respective Sigma-Aldrich and Cambridge Isotopes lecture bottles <i>vs.</i> control solutions.

<p>Sigma-Aldrich treatments utilized 40 mL ammonium solutions, whereas Cambridge Isotopes treatments utilized 100 mL ammonium solutions. (<b>b</b>) δ¹⁵N_NH4 of higher sensitivity equilibrations of 5 µmol L⁻¹ ammonium solutions (10 mL) with 2.0 mL ¹⁵N₂ gas from Cambridge Isotopes lecture bottles <i>vs.</i> control solutions. n = the number of experimental replicates.</p