Variable Nitrogen Isotope Effects Associated With N2O Isotopologue Production: Towards an Understanding of Denitrification Mechanism
Abstract
There is much current interest in the use of the isotopic composition of N2O, including site-specific δ 15N (i.e., the 15N/14N ratios of the central or terminal positions, expressed as δ 15Nα and δ 15Nβ respectively) and δ 15Nbulk (the integrated N isotope value) to understand biological sources of this important greenhouse gas. However, mechanisms driving the variability of δ 15Nbulk, δ 15Nα , and δ 15Nβ values of biologically produced N2O need to be better understood for the effective interpretation of field observations. In denitrification, a major source of N2O, the formation of the N=N bond is generally understood to occur via NO + NO. However evidence suggests that enzyme-bound NO+ + NO2- may also form N2O (Ye et al. 1994), generating potentially significant differences in relative values of δ 15Nbulk, δ 15Nα , and δ 15Nβ depending on production pathways. A useful way to explore reaction mechanism is to determine whether an isotope effect is dependent on substrate concentration, in this case nitrate. Nitrate concentrations are also highly variable in terrestrial and aquatic environments where denitrification occurs. We investigated the δ 15N values of N2O produced by the denitrifier P. aureofaciens at nitrate concentrations ranging from 0.03 mg/L to 2000 mg/L at which the total conversion was < 15 % of the initial nitrate. The bulk 15N enrichment factor (ɛ ) increased with production rate when [NO3-]init varied between 25 mg/L to 844 mg/L, but was constant below this [NO3-]init range (here referred to as low nitrate). The maximum production rate and observed enrichment factors were reached at [NO3-]init = ∼1000 mg/L and remained constant up to 2000 mg/L (here referred to as high nitrate). Results were ɛ = -22.7 ± 2.6 ‰ (n=18) at low nitrate, ɛ = -33.2 ± 3.3 ‰ (n=4) at [NO3-]init = 250 mg/L, and constant at ɛ = -42.3 ± 0.3 ‰ (n=10) at high nitrate. δ 15Nα and δ 15Nβ values relative to tropospheric N2O were respectively -36.6 ± 2.8 ‰ (n=15) and -11.8 ± 5.0 ‰ (n=15) at low nitrate, -50.9 ± 3.2 ‰ (n=4) and -23.2 ± 3.6 ‰ (n=4) at [NO3-]init = 250 mg/L, and -60.0 ± 0.3 ‰ (n=6) and -32.6 ± 1.5 ‰ (n=6) at high nitrate. We found no significant variability of the difference between δ 15Nα and δ 15Nβ with nitrate availability, despite large differences in ɛ . Implications of these results with respect to the mechanism of N2O production will be discussed. Ye R.W., Averill B.A., and Tiedje J.M (1994) Denitrification: Production and Consumption of Nitric Oxide, Appl. Environ. Microbiol. 60: 1053-1058.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2004
- Bibcode:
- 2004AGUFM.B11C..03T
- Keywords:
-
- 4870 Stable isotopes;
- 1040 Isotopic composition/chemistry;
- 1223 Ocean/Earth/atmosphere interactions (3339);
- 0315 Biosphere/atmosphere interactions;
- 0400 Biogeosciences