| The stable nitrogen isotopic composition of dissolved nitrate was determined at six stations ranging from the oligotrophic North Pacific Subtropical Gyre (NPSG) to the eutrophic Eastern Tropical North Pacific (ETNP). Dissolved inorganic nitrogen, oxygen and phosphate concentrations were determined in water column samples at all six stations. In productive, oxic waters nitrate isotopically enriched in 15N (maximum δ15N-NO 3− value of 12.5‰) was most likely the result of assimilatory nitrate reduction. In contrast, high δ15N-NO 3− values (maximum of 12.3‰) associated with high nitrate deficits and anoxic conditions, supported the interpretation of isotopic fractionation due to denitrification. A one-dimensional vertical advection and diffusion model was used to estimate the fractionation factor for denitrification at two stations in the ETNP. A comparison of modeled to observed δ15N-NO3− provided estimates of the isotopic enrichment factor (ϵ) of 30 at station 4 and 30 to 35 at station 5. Isotopically light nitrate (1.1 and 3.2‰) was observed in the upper 200 m of the water column at stations in the ETNP. Tracer studies of 15NH4 and biogeochemical indicators of nitrogen fixation, such as N/P ratios, δ15N-POM and Trichodesmium abundance, supported the interpretation of nitrification as the most plausible explanation for low δ15N-NO 3− values observed in upper water column samples. Nitrification rates increased across the transect from a maximum rate of 1 nmol 1−1 d−1 at station 1 and 23.7 nmol 1−1 d−1 at station 6.; The relative importance of individual microbial pathways to nitrous oxide production is not well known. The intramolecular distribution of 15 N in nitrous oxide provides a basis for distinguishing biological pathways. Concentrated cell suspensions of M. capsulatus Bath and N. europaea were used to investigate the site preference of nitrous oxide by microbial processes during nitrification. The average site preference of nitrous oxide formed by hydroxylamine oxidation by M. capsulatus Bath (5.5 +/− 3.1‰) and N. europaea (−1.4 +/− 1.7‰) and nitrite reduction by N. europaea (−7.7 +/− 3.1‰) differed significantly (ANOVA, f(2,35) = 247.9, p = 0). These results demonstrate that the mechanisms for hydroxylamine oxidation are distinct in M. capsulatus Bath and N. europaea. The average δ 18O-N2O values of nitrous oxide formed during hydroxylamine oxidation by M. capsulatus Bath (53.1 +/− 2.9‰) and N. europaea (−23.4 +/− 7.2) and nitrite reduction by N. europaea (4.6 +/− 1.4) were also significantly different (ANOVA, f(2,35) = 279.98, p = 0) and suggests although the nitrogen isotope value of the hydroxylamine was similar, the Δ 15N associated with hydroxylamine oxidation by M. capsulatus Bath and N. europaea (−2.3 and 26.0‰ for M. capsulatus Bath and N. europaea respectively) provided evidence that differences in isotopic fractionation were associated with two mechanisms. The site preferences in this study are the first measured values for isolated microbial processes. The differences in site preference are significant and indicate that isotopomers provide a basis for apportioning biological processes of nitrou... |
