Isotopologue Measurements Research Articles

Evidence for the predominance of denitrification as a source of N2O in temperate agricultural soils based on isotopologue measurements

Isotopologue values were used to quantify the contribution of denitrification to nitrous oxide (N2O) flux in agricultural and early successional fields in southwest Michigan. Nitrous oxide‐δ15N and δ18O values were poor estimators of microbial origins compared to site preference (SP) (difference in δ15N between the outer and central N atoms of N2O). Site preference was used to evaluate the importance of denitrification (including nitrifier denitrification) in N2O production. Average flux‐weighted SP values for each field ranged between 2.9 and 14.6‰ and, on the basis of SP values for N2O production from denitrification (0‰) and collectively for nitrification and fungal denitrification (37‰), these values indicate that between 61 and 92% of N2O originated from denitrification. Reduction of N2O ranged from undetectable to as much as 50% of production; and because reduction increases SP, our results underestimate the percentage of N2O from bacterial denitrification. The SP values in our study clearly indicate that denitrification is the predominant source of soil‐derived N2O.

A general algorithm for the 17O abundance correction to 13C/ 12C determinations from CO 2 isotopologue measurements, including CO 2 characterised by ‘mass-independent’ oxygen isotope distributions

It is widely recognised that a significant limitation to the ultimate precision of carbon stable isotope ratio measurements, as obtained from dual-inlet mass spectrometric measurements of CO 2 isotopologue ion abundances at m/ z 44, 45, and 46, is the correction for interference from 17O-bearing molecular ions. Two long-established, alternative procedures for determining the magnitude of this correction are in widespread use (although only one has IAEA approval); their differences lead to small but potentially significant discrepancies in the magnitude of the resulting correction. Furthermore, neither approach was designed to accommodate oxygen three-isotope distributions which do not conform to terrestrial mass-dependent behaviour. Stratospheric CO 2, for example, contains a strongly ‘mass-independent’ oxygen isotope composition. A new strategy for determining the 17O-bearing ion correction is presented, for application where the oxygen three-isotope characteristics of the analyte CO 2 are accurately known (or assigned) in terms of the slope λ of the three-isotope fractionation line and the ordinate axis intercept 10 3 ln(1 + k) on a 10 3 ln(1 + δ 17O) versus 10 3 ln(1 + δ 18O) plot. At the heart of the approach is the relationship between 17R, which is the 17O/ 16O ratio of the sample CO 2, and other assigned or empirically determined parameters needed for the δ 13C evaluation: 2 18 R ref 17 R ( 1 + k ) 17 R ref 1 λ - 3 ( 17 R ) 2 + 2 17 R 45 R - 46 R = 0 With 45R and 46R as the respective 45/44 and 46/44 ion abundance ratios of the sample, as obtained by measurement of δ 45(CO 2) and δ 46(CO 2) values reported relative to a reference material (usually VPDB-CO 2), and 17R ref and 18R ref being the respective 17O/ 16O and 18O/ 16O ratios in the same reference material, 17R can readily be obtained by numerical methods, for given λ and k values. The correction procedure involves no approximations, in principle, and is equally applicable to CO 2 of terrestrial, mass-dependent oxygen isotopic composition, as well as to more ‘exotic’ sources. Besides isotopic characterisation of stratospheric CO 2, potential applications include high precision δ 13C measurements of CO 2 derived from the oxidation of tropospheric CO (also characterised by significantly ‘mass-independent’ oxygen isotopic composition); high precision isotopic monitoring of atmospheric CO 2; the metrology of carbonate isotopic reference materials; and the isotopic characterisation of CO 2 which has been equilibrated with waters artificially ‘labelled’ with known enrichments of 17O and 18O.